Fast and efficient charge breeding of the Californium rare isotope breeder upgradeelectron beam ion sourceP N Ostroumov A Barcikowski C A Dickerson A Perry A I Pikin S I Sharamentov R C Vondrasekand G P Zinkann Citation Review of Scientific Instruments 86 083311 (2015) doi 10106314929464 View online httpdxdoiorg10106314929464 View Table of Contents httpscitationaiporgcontentaipjournalrsi868ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improved charge breeding efficiency of light ions with an electron cyclotron resonance ion source Rev Sci Instrum 83 113303 (2012) 10106314768533 Development of electron beam ion source charge breeder for rare isotopes at Californium Rare IsotopeBreeder Upgradea) Rev Sci Instrum 83 02A902 (2012) 10106313660823 Performance of the Argonne National Laboratory electron cyclotron resonance charge breeder Rev Sci Instrum 82 053301 (2011) 10106313586765 The Brookhaven National Laboratory electron beam ion source for RHICa) Rev Sci Instrum 81 02A509 (2010) 10106313292937 Status of the electron cyclotron resonance charge breeder for the Cf252 fission source project at ATLASa) Rev Sci Instrum 79 02A901 (2008) 10106312802580

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REVIEW OF SCIENTIFIC INSTRUMENTS 86 083311 (2015)

Fast and efficient charge breeding of the Californium rare isotope breederupgrade electron beam ion source

P N Ostroumov1a) A Barcikowski1 C A Dickerson1 A Perry1 A I Pikin2S I Sharamentov1 R C Vondrasek1 and G P Zinkann11Argonne National Laboratory Argonne Illinois 60439 USA2Brookhaven National Laboratory Upton New York 11973 USA

(Received 3 April 2015 accepted 11 August 2015 published online 28 August 2015)

The Electron Beam Ion Source (EBIS) developed to breed Californium Rare Isotope Breeder Up-grade (CARIBU) radioactive beams at Argonne Tandem Linac Accelerator System (ATLAS) is beingtested off-line A unique property of the EBIS is a combination of short breeding times high repetitionrates and a large acceptance Overall we have implemented many innovative features during thedesign and construction of the CARIBU EBIS as compared to the existing EBIS breeders The off-linecharge breeding tests are being performed using a surface ionization source that produces singlycharged cesium ions The main goal of the off-line commissioning is to demonstrate stable operationof the EBIS at a 10 Hz repetition rate and a breeding efficiency into single charge state higher than15 These goals have been successfully achieved and exceeded We have measured (20 plusmn 07)breeding efficiency into the single charge state of 28+ cesium ions with the breeding time of 28 ms Ingeneral the current CARIBU EBIS operational parameters can provide charge breeding of any ionsin the full mass range of periodic table with high efficiency short breeding times and sufficiently lowcharge-to-mass ratio 163 for the heaviest masses for further acceleration in ATLAS In this paperwe discuss the parameters of the EBIS and the charge breeding results in a pulsed injection modewith repetition rates up to 10 Hz C 2015 AIP Publishing LLC [httpdxdoiorg10106314929464]

I INTRODUCTION

The Californium Rare Isotope Breeder Upgrade(CARIBU) for the Argonne National Laboratory Argonne Tan-dem Linac Accelerator System (ATLAS) provides neutron-rich fission fragments from a 1 Curie (Ci) 252Cf source1 Theions are thermalized and collected into a low-energy ion beamby a helium gas catcher mass selected by an isobar separatorand charge bred to higher charge states for acceleration inATLAS To reach energies EA sim 10 MeVu one should injections with a charge-to-mass ratio (qA) ge 17 into ATLAS Atpresent the existing Electron Cyclotron Resonance (ECR) ionsource is used as a charge breeder (CB)2 Higher efficiencyshorter breeding times and higher purity of charge-bred radio-active ion beams can be achieved by using an electron beamion source (EBIS) as a CB instead of an ECR Thereforefive years ago we started development and construction of theEBIS CB The design parameters of the EBIS were reportedin Ref 3 CARIBU generates a wide range of radioactive ionswith masses from 80 to 160 amu and intensities up to 107 ionsper second The efficiency of EBIS-CB in the pulsed injectionmode will be significantly higher than in the continuous (DC)injection mode so a Radio Frequency Quadruple Cooler-Buncher (RFQ-CB) will be used to collect ions downstreamof the mass separator and create short pulses of high qualityradioactive beams with the full transverse normalized emit-tance of sim0003π timesmm timesmrad4 To avoid ion beam intensitylimitations due to space charge the collection time for the

a)ostroumovanlgov

most abundant radioactive ions will be limited to about 33 msDuring collection in the RFQ-CB the EBIS will breed aninjected ion bunch so breeding must occur in le33 ms Thisprocedure is repeated at a frequency of 30 Hz At presentdue to lower intensity beams from CARIBU lower repetitionrates can be used in the RFQ-CB The EBIS includes DC andpulsed power supplies A pulsed electron beam created bypulsing the electron gun anode with a high voltage amplifieris used rather than a DC beam to avoid possible discharges andmaintain high vacuum Many EBIS electrodes require pulsedoperation to facilitate injection trapping and extraction of theion bunch The power supplies timing and control systems forthe CARIBU EBIS were described in Ref 5

The breeding of radioactive ions prior the injection intothe Linac is already in use at ISOLDE (CERN)6 ISAC (TRI-UMF)7 and at NSCL8 The EBIS CB project for CARIBUis utilizing state-of-the-art EBIS technology recently devel-oped at Brookhaven National Laboratory9 as an injector forstable ion beams During the design and construction of theCARIBU EBIS we implemented many innovative modifica-tions with respect to the existing charge breeders of radioac-tive beams and the BNL EBIS Many additional novel designfeatures of the CARIBU EBIS CB are described in Sec II andmore details are given in our previous publications1011 Off-line commissioning of sub-systems and first charge breedingmeasurements were already reported1213 This paper discussesresults of charge breeding experiments with the external singlycharged cesium source Due to the limited space we reportonly major accomplishments related to the availability of theCARIBU EBIS as a new scientific tool for fast breeding ofradioactive beams with high efficiency Results of optimization

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083311-2 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

of both the electron and ion beam characteristics as a functionof various EBIS ion source and beam transport parameterswill be reported in future publications

II DESIGN FEATURES OF THE CARIBU EBIS

The parameters of the electron gun potential distributionin the ion trap region electron collector and injectionextrac-tion beam lines were substantially modified compared to theBNL EBIS CB to obtain a short breeding time the high-est acceptance and breeding efficiency of low intensity rareisotope beams

A E-gun and drift tube structure

The CARIBU EBIS was designed to operate at higherrepetition rate and higher electron beam duty cycle than theBNL EBIS However the electron beam current density insidethe superconducting solenoid is similar to that in the BNLEBIS allowing fast charge breeding of radioactive ion beamsThese improvements were achieved by choosing a 42 mmdiameter single crystal IrCe cathode which is capable toprovide up tosim2 A electron beam current with high perveanceThis electron gun allows us fast breeding (lt30 ms) withreasonably high breeding efficiency into the most abundantcharge state gt25 We have also developed a low-currente-gun to utilize ionization process involving the so-calledshell closures effect14 which is a straightforward and effectivemethod to narrow the charge state distribution and improveefficiency of the breeding process A low-energy electronbeam (sim2 keV) will be used for the ldquoshell closuresrdquo operationmode of the EBIS for breeding of ions with moderate chargestates (zA sim 15) An accurate adjustment of the electronbeam energy will be done to be just below the ionizationenergy of atomic shell closures in order to collect most of theions in the corresponding charge state Such an approach hasbeen experimentally proven for very high charge states14 butnot for the moderate charge states required for the CARIBUradioactive beams To avoid a formation of the virtual cathodefor sim2 keV electron beam a small diameter 08 mm singlecrystal IrCe cathode was designed and built to generate 02A electron beam For the operation of the CARIBU EBISin shell closuresrsquo breeding mode only electron gun will bereplaced no other parts of the CARIBU EBIS need to bemodified This paper discusses the results obtained with the42 mm diameter cathode while breeding with low-currentcathode will be implemented in future as operational schedulepermits

An electron gun beam transport system and collectorwere designed for a 2 A 25 kW DC electron beam An electronbeam inside the EBIS has three different perveance valuesin different areas the e-gun the drift tube structure and thecollector In our case the most challenging task is transportingthe electron beam through the drift tube structure where theelectron beam energy should be low enough to provide highefficiency for the ion charge breeding The perveance inside thedrift tube structure defines the maximum current of the EBIS-CB for a given electron beam energy An electron beam currentthat can be transported without space charge compensation

through a conducting tube with radius rt in vacuum is limitedby the Bursian current which can be written in practical unitsas15

IB = 254 times 10minus6 U32t

1 + 2 ln(rtrb) = P(rbrt) timesU32t (1)

where rb is the radius of the electron beam Ut is the potentialdifference between the tube and cathode (Ut is in volts) IB isthe electron beam current in amperes and P is the perveanceof the electron beam The electron beam current density inEquation (1) is assumed to be homogeneous The higher theperveance is the higher current can be transported at lowerenergies As one can see from Equation (1) the maximumperveance of the electron beam inside the drift tube structureis achieved when the ratio of the electron beam radius to thedrift tube radius is close to 1 In reality the drift tube radiusshould be significantly larger than the electron beam radius toprovide the electron beam transport along the EBIS axis withnegligible beam losses Other advantages of larger diameterdrift tubes are better pumping of the internal drift tube volumeand lower probability of parasitic RF field generation As areasonable compromise we have selected 10 mm drift tuberadius for the CARIBU EBIS

At present the CARIBU EBIS operates with an electronbeam current up to 16 A which is a factor of 2 higher thanany other operational EBIS-based charge breeders for radio-active beams The high current electron beam transmission isachieved with relatively low electron beam energy in the trap65 keV

Special attention was paid to the design of the vacuum sys-tem to enable high purity charge-bred radioactive ion beamsThe ion trap includes 11 drift tubes installed on a pipe scaffoldwhich is coated by non-evaporable getter (NEG) using thetechnology recently developed at CERN16 All drift tubes havelongitudinal slots to facilitate efficient pumping of the ion trapvolume as shown in Fig 1 The location and orientation ofthe slots have been selected to avoid possible discharges in thepresence of a high magnetic field and RF instabilities inducedby the high current electron beam In the environment ofhigh stray magnetic fields both turbo- and cryopumps requireappropriate magnetic shielding for reliable operation We de-signed the vacuum pump shielding to minimize the effect ofthe magnetic field distortion along the electron beam pathIn addition eight dipole steering coils in the horizontal andvertical planes have been implemented into the EBIS system

Complete assembling of 11 drift tubes was performedinside a class 100 clean room to avoid contamination of thesurfaces with oils and particulates No fine adjustment wasforeseen in the design of drift tube structure so the alignmentrelies on the manufacturing accuracy of all elements Afterthe assembly was completed the alignment was verified usinga FaroArm Platinum series 3D scanner with an accuracy ofplusmn25 microm The alignment of drift tubes from 1 to 9 was foundto be within plusmn100 microm in the vertical direction and withinplusmn200 microm in the horizontal direction Drift tubes are numberedstarting from the output of the e-gun anode downstream to thecollector entrance Alignment of the e-gun and collector waswithin plusmn100 microm to the rest of drift tube structure Finally thewhole EBIS assembly from e-gun to collector was aligned to

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083311-3 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 1 Set of individual drift tubes prior the installation (left) and a view (from the e-gun side) of the scaffold with installed drift tubes (right)

the magnetic axis of a 6 T superconducting solenoid withinplusmn50 microm

B Ion injection and extraction (I-E) lines

The I-E ion beam transport line includes two Einzel lensassemblies two steerers and an acceleration tube as wasdiscussed in Ref 10 Initial beam dynamic studies revealedthat the beam is not axially symmetric due to the not axisym-metric grounded vacuum chambers The aperture diameterof the Einzel lens its gap and the physical size of vacuumchambers were optimized to balance the linear region of thefocusing fields at the reduced effect of the vacuum chamberson the beam quality Three X-Y electrostatic steerer configu-rations (parallel quartered diagonally quartered and two setsof diagonally halved) have been studied in terms of the beamemittance growth Voltages were applied to the electrodes untila deflection of 13 mrad was achieved in both X and Y planesfor each assembly Two sets of diagonally halved steerer wereselected as they provided lowest emittance growth Our resultsare consisted with Ref 17 which reported low beam aberra-tions when the diagonally halved cylinder is used to createdipole electric field as compared to the parallel sectionedconfigurations

Numerical simulations and breeding efficiency measure-ments at BNLrsquos test EBIS revealed that the 30 electrostaticswitchyard for the injection and extraction of ions can intro-duce significant distortions on singly charged beams beinginjected into the EBIS The CARIBU EBIS switchyard em-ploys a single electrostatic deflector to direct the injected ionbeam from the source line to the I-E line and to guide theextracted beam from the I-E line to the diagnostic line Adiagonally halved cylindrical deflector was chosen over otherdeflector configurations because this design can maintain theion beam quality better than others as discussed with respectto the steerer assemblies A schematic of the switchyard isshown in Fig 2 In the final configuration the source beamline will be used for injection of the radioactive beams from theCARIBU and the spare beam line port will be used for injectionof singly charged cesium beam The deflector midplane wasshifted from the intersection of the 15 source beamline axisand the 0 I-E beamline axis by 096 cm as seen in Fig 2The value of this offset was determined by programmaticallyminimizing the deflection and the shift of the injected ion beamfrom the 0 axis in the EM Studio18 This offset will ensure

the injected beam can ideally be deflected exactly on axisThe overall length of the deflector was chosen to keep thenecessary applied voltages less than plusmn10 kV the magnitudefor commonly available and moderately priced high voltageamplifiers

III EBIS LAYOUT AND OPERATIONAL PARAMETERS

About a year ago we started extensive charge breed-ing experiments at the CARIBU EBIS to establish optimaloperational parameters These experiments were interruptedseveral times for extended periods of a month or longer forthe improvement of the EBIS sub-systems The first chargebreeding results of the cesium beam were reported at a recentsymposium on electron beam ion sources and traps19 Signifi-cant performance improvements have been achieved in the pastyear as is discussed throughout the content of this paper The3D model and general view of the EBIS at the off-line locationare shown in Figs 3 and 4 respectively The commissioningof the EBIS sub-systems1213 and beam dynamics studies werediscussed in earlier publications10 Current operational param-eters of the EBIS are listed in Table I together with possible

FIG 2 The electrostatic switchyard configuration and deflector positioningnecessary to inject the ion beam on the 0 beamline axis

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083311-4 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 3 3D model of CARIBU EBIS CB in off-line installation (1mdashe-gun 2mdashroom temperature solenoid 3mdashrecently installed NEG pump 4mdashend crossesof the ion trap chambers 5mdash6 T superconducting solenoid 6mdashelectron collector with room temperature coil 7mdashEinzel lens and steerers (EBIS potential)8mdash75 kV accelerating tube 9mdashFaraday cup for cesium beam (FC2) 10mdashpepper pot Multi-Channel Plate (MCP)-based emittance probe 11mdashEinzel lensand steerers (ground potential) 12mdashFaraday cup for cesium beam (FC1) 13mdashsteerers 14mdashquadrupole lenses 15mdashCs+ ion source and accelerating tube16mdashelectrostatic switchyard 17mdashFaraday cup for charge bred beam (FC3) 18mdash70 bending magnet 19mdashslits and Faraday cup (FC5))

ultimate parameters Fig 5 shows the potential distributionalong the EBIS for the charge breeding of residual gas oran externally injected singly charged ion beam The potentialdistribution is shown with respect to the EBIS platform whichis biased to 20 kV with respect to the ground The electrongun is biased to minus575 kV and operates with a 75 kV anodevoltage provided by a high voltage (HV) amplifier The e-gunbias anode voltage cathode heater current and the potentialof the first drift tube have been optimized to extract a 16 Acurrent and minimize electron beam losses along the EBISThe e-gun bias and the trap potential together with the electronbeam current define the electron beam energy in the trap Thetrap potential was optimized to provide the highest extractedcharge of the ionized residual gas

For the off-line charge breeding tests of the CARIBUEBIS we decided to use a commercial cesium ion sourcewhich is a good match with the mass range of CARIBU beamsand readily available from industry20 Trappingextraction ofthe ion pulse takes place by raisinglowering the potential ofa barrier electrode using fast solid state Behlke switches21

The switching time is less than 01 micros In most experimentswe applied sim1 kV barrier potential with respect to the trappotential while a sim300 V barrier potential is sufficient fortrapping For extraction the barrier potential is simminus400 V withrespect to the trap potential

A EBIS vacuum system

The EBIS vacuum system includes a set of turbopumpscoupled to scroll pumps cryopumps and NEG surfaces Thedetails of the EBIS vacuum system were described in Ref 12Efficient vacuum separation of the trap region from the e-gunand collector is provided We incorporated all-metal vacuumvalves from both sides of the trap region to facilitate easyreplacement of e-guns In February 2015 a NEG pump wasinstalled near the e-gun (position 3 in Fig 3) Recently we havedecided to repeat the activation of getter strips and sputteredgetter surfaces in the trap region by in-situ baking up to 450 CAfter completion of baking and several weeks of pumping theion gauge readings were 13 times 10minus10 Torr at e-gun chamber

FIG 4 General view of the EBIS in off-line installation This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

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083311-5 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

TABLE I EBIS parameters

Parameter Operational Possible ultimate Units

Magnetic field 50 60 TElectron beam current Ie 16 20 ACathode radius 21 21 mmMagnetic field on cathode 015 015 TEBIS platform bias voltage 20 50 kVTrap length 05 07 mElectron beam radius in the trap reTRAP

a 364 332 micromElectron beam density in the trapa 385 577 Acm2

Electron beam energy in the trapa 6495 6265 eVSpace charge potential well ∆UeTRAP

a 299 374 VElectron beam velocity ve in the trapa 48times107 47times107 msNormalized full acceptanceb 0024 0024 πtimesmmtimesmradTrap capacitya 23 30 nCRepetition rate Up to 10 30 HzDuty cycle Up to 40 90

aEstimated using general EBIS theory see for example Ref 22bSimulated with tracking codes10

and 45 times 10minus10 Torr near the collector with the electron beamswitched off This is a significant improvement as comparedto the previous data19 The residual pressure depends on theelectron beam current and its duty cycle At lower electronbeam currents below 1 A the duty cycle can be close to 100However at 16 A stable vacuum and stable charge breedingare observed only if the duty cycle is le40 at 10 Hz repetitionrate For the same electron beam current at a 1 Hz repetitionrate the duty cycle is slightly higher Operation at higher dutycycles requires extended vacuum conditioning

B Cesium source

We use a commercial ion gun TB-171 model HWIG-25020 to produce a singly charged cesium ion beam Cesiumions are accelerated to sim235 keV by the biased anode voltageand an 11-gap accelerating tube The cesium source providesa stable DC beam The pulsed cesium beam with the pulselength up to 55 micros is formed by an electrostatic deflector andinjected into the EBIS trap with a 5 ms delay with respect to theelectron beam pulse One electrode of the deflector (chopper)is pulsed up to 55 kV by a HV power supply coupled to a fast

FIG 5 Simplified schematic of the EBIS structure and potential distributionduring the breeding (1mdashe-gun 2mdashthe first drift tube 3mdashthe second and thirddrift tubes 4mdashtrap region 5mdashdrift tube with the barrier potential 6mdashdrifttubes 7mdashelectron collector 8mdashion extractor)

Behlke switch The full normalized acceptance of the EBISwas numerically defined for the wide range of parametersin our previous publication10 The normalized acceptancecan be approximated with the expression AFULL[mm timesmrad]= π times 38 times 10minus3reTRAP[mm]∆UeTRAP[V] and equal to0024π timesmm timesmrad for current EBIS parameters It is signif-icantly higher than the emittance of the radioactive beamsexpected from RFQ cooler-buncher which is sim0003π timesmmtimesmrad normalized Therefore for the purpose of EBIS effi-ciency measurements and application of these results to theradioactive beams it is important to form a cesium beam withan emittance comparable to that expected from the CARIBUsource The emittance of the cesium beam is established bythe appropriate focusing with a cesium source Einzel lens anelectrostatic triplet and adjustment of an iris style apertureThe cesium beam is very sensitive to the tuning of the beamoptics due to the non-linear character of the source extractionsystem and some off-centered beam trajectory in the transportsystem In our experiments the intensity of the cesium beamwas defined both by the heater in the ion source and the colli-mator About 80 of the cesium beam was intercepted by thecollimator The beam intensity did exceedsim4 times 107 ionspulsewhich corresponds to 64 pC A typical cesium beam image onthe phosphor screen of the pepper pot emittance meter and aprocessed emittance shape of this beam are shown in Fig 6The measured rms normalized emittance of the cesium beam issim(0006 plusmn 00015)π timesmm timesmrad The full cesium emittanceis slightly larger than the EBIS acceptance but the currentcollimator configuration is unable to reduce the emittancefurther In addition some losses due to beam emittance andbeam center mismatch with the EBIS acceptance may takeplace All this results in a transmission efficiency of the cesiumbeam through the EBIS lower than 100

C Ion beam diagnostics tools

Predominantly we use an oscilloscope to measure the totalcharge of injected and extracted ion beam pulses incident on

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083311-6 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 6 A typical cesium beam image on the phosphor screen of the pepper pot emittance meter (a) and its emittance ((b) and (c)) The distance between thepepper-pot holes is 05 mm The color code corresponds to a linear incremental change of the density from 0 to 10 The central red area corresponds to thedensity higher than 10

Faraday cups There are 3 Faraday cups (FC1 FC2 and FC3)with 46 mm input apertures installed on air cylinders (seepositions 9 12 and 17 in Fig 3) and a stationary cup (FC5at position 19 in Fig 3) installed downstream of adjustableslits after the bending magnet FC1 and FC2 are used forthe measurements of the injected cesium beam charge Basedon our previous experience with the current measurementsof multi-charge ion beams the electron suppression voltageis minus300 V The accuracy of beam charge measurements isplusmn1 As is known multi-charge beams can produce energeticAuger electrons that can escape the FC Our measurementswith ATLAS multi-charge beams show that the contributionof energetic electrons to the measurement error is less than04 if a minus300 V biased voltage is applied Therefore it is ourcommon practice to apply minus300 V to all Faraday cups used forthe measurements of multi-charge ion beams A typical FC2signal from the pulsed cesium beam is shown in Fig 7 Thetotal charge is calculated as Qtot = VpeakCFC where Vpeak isthe peak voltage and CFC is the capacitance of the FC andsignal cables Calculation of the charge by area integrationresults in the same value within a plusmn1 error For chargebreeding efficiency measurements we use FCs with identicalcapacitances to avoid error due to different decay times of thevoltages during the charge accumulation over the pulse length

FIG 7 Cesium beam signal reading on the FC2 prior to injection into theEBIS

A pepper-pot emittance probe based on a Multi-ChannelPlate (MCP) and a phosphor screen (position 10 in Fig 3)23

was used to measure the emittance of the injected cesiumbeam Due to the very high sensitivity of the MCP very lowinjected cesium beam intensities sim01 pC per pulse can beanalyzed by the emittance probe

D Electron beam commissioning

The stability of high current-high duty cycle electronbeam in the EBIS is the key for breeding of ions with highefficiency The set of electron beam diagnostics and results ofthe initial electron beam commissioning at 4 T magnetic fieldwere reported in the previous publication12 The time structureof the electron beam was formed by applying a pulsed highvoltage potential to the e-gun anode Optimal transport of theelectron beam with very low relative losses below 10minus4 fromthe e-gun cathode to the collector was established by tuning oftwo sets of X and Y steering dipole coils installed on vacuumchamber crosses in both sides of the superconducting (SC)solenoid and 4 pairs of magnetic steering coils installed onthe beam tube inside the SC solenoid bore In addition thecollector solenoid field is tuned to minimize the electron beamlosses The availability of steering coils is essential for the100 transmission of the electron beam The tuning of theelectron beam is complete when vacuum gauges do not showany pressure fluctuations Currently we can stably operate 16A electron beam at 40 duty cycle or 115 A electron beam at90 duty cycle Increasing either current or duty cycle of theelectron beam requires long vacuum conditioning

IV RESIDUAL GAS IONIZATION

When the barrier electrode potential is raised (position 5in Fig 5) ionization of residual gas atoms and molecules takesplace in the trap The charge bred ions of the residual gas areextracted and analyzed using 70 bending magnet In Sec Vof this paper we discuss experimental data taken from FC5 formultiple pulses while the dipole current was varied The beamcharge values delivered in each pulse were generally stablebut we used averaging of the signal over several pulses whenthis was not the case Fig 8 shows spectra of the residual gasfor two different confinement times for the EBIS operational

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083311-7 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 8 Residual gas spectrum at 30 ms (a) and 65 ms (b) of confinement time Ion pulse charge is Q[pC]= 019timesV[mV]

parameters shown in Table I To increase mass resolution thebeam was collimated upstream of the FC5 The residual gascontains ions of hydrogen carbon and oxygen The relativecontribution of nitrogen is very low We do not see molecularions in the spectrum The preferable values of charge-to-massratios for CARIBU beams for the extraction separation andfurther acceleration in ATLAS are in the range from 17 to14 These ratios can be achieved with breeding times from10 to 30 ms and appear on FC5 approximately in the range ofmagnet currents from 36 to 47 A By adjusting the breedingtime the most abundant charge state of a rare isotope ionbeam can be located between residual gas peaks and separatedfrom the residual gas contamination in the transport systemdownstream of the EBIS We plan a detailed study of possiblecontaminants from the residual gas in the region between O3+

and O4+with upgraded Faraday cups and amplifiers to measurefemtocoulomb-level charges

The capacity of the trap is calculated as CTRAP =IeLTRAP

ve= 23 nC where Ie is the electron beam current ve is the elect-ron velocity in the trap and LTRAP is the length of the trapFor typical confinement times le30 ms required for CARIBU

beams the neutralization factor is below 2 The total ex-tracted charge of residual gas ions does not exceed sim7 ofthe total charge capacity of the trap for confinement times upto 150 ms

V CESIUM BREEDING EFFICIENCY MEASUREMENTS

The main goal of the off-line commissioning was todemonstrate stable operation of EBIS at a 10 Hz repetitionrate and a breeding efficiency into single charge state higherthan 15 These goals have been successfully achieved andexceeded The breeding efficiency measurements were per-formed according to the procedure described in detail inRef 24 This procedure uses measurements of total chargeon FC3 with and without cesium injection The total numberof extracted cesium ions was calculated as a ratio of cesiumcharge on FC3 to the average charge state of the cesium beamThe latter was found from the measured cesium spectrumobtained on FC5 with 1 or 2 mm wide slits Examples of thecesium spectra for 20 and 28 ms breeding times for the EBISoperational parameters listed in Table I are shown in Fig 9 For

FIG 9 Spectra of charge-bred cesium and residual gas ions for two breeding times 20 ms (a) and 28 ms (b) This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-8 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

these measurements to obtain a good resolution of differentcharge states the slits upstream of the FC5 were closed to2 mm The average charge state z was calculated using theexpression

z =

zmaxz=zmin

Qz

zmaxz=zmin

Qz

z

(2)

where Qz is the total charge of ions at a particular chargestate z zmin and zmax are the minimum and maximum chargestates correspondingly in the spectrum of charge-bred ce-sium ions The total charge Qz was calculated as an areaunder the Gaussian distribution obtained from the fit to themeasured distribution The relative abundance of cesium ionswith different charge states is defined by the expression

Az =Qz

z middotzmax

z=zmin

Qz

z

middot 100 (3)

We noticed that the calculations of average charge state couldalso be done using a peak value of the signal for each chargestate Qpeak

z with similar results Breeding efficiency into thecharge state with the highest abundance ηzmax is defined as aratio of the number of extracted ions with highest abundance tothe number of injected Cs+ ions and it can be found from theexpression ηzmax = ηtot middot Amax

z where ηtot is the total transmis-sion efficiency of cesium ions The above mentioned methodincludes an error related to the measurements of the totalcharge of residual gas ions with and without cesium injectionDue to the interaction of cesium ions with the ions of residualgas the total extracted charge of residual gas ions is slightlymodified From the comparison of the spectra of the residualgas ions (Fig 8(a)) and cesium beam (Fig 9(b)) it is clear thatthe ions of the residual gas are well separated with respect tothe cesium spectrum Therefore the relative change of the totalcharge of the residual gas ξ extracted with and without cesiuminjection can be approximately estimated as ξ = 2 Σ1minusΣ2

Σ1+Σ2 where

Σ1 and Σ2 are the integrated charges of the residual gas ionsin the spectra with and without cesium injection respectivelyThe calculations show that ξ = 22

In addition we have performed direct measurements ofthe cesium charge in a single charge state after the bendingmagnet This is possible for charge states below sim20 when thefull intensity in the single charge state can be transported to theFC5 with a 10 mm slit opening The breeding efficiency into asingle charge state can be calculated directly by measuring thecharge on FC5 This method results in the breeding efficiencyconsistent with the method described above and in Ref 20within plusmn25 Overall the systematic and measurement er-rors result in plusmn35 relative error of the breeding efficiencyevaluations

After several optimization rounds of cesium beam tun-ing the EBIS parameters and injection-extraction potentialsconsistently resulted in total injection-extraction efficienciesof sim70 and can be increased to 80 with elaborate beamtuning An example of FC3 measurements with and withoutinjection of the cesium beam is shown in Fig 10 Thesedata together with the FC2 reading of the injected cesium

FIG 10 FC3 signal for a single ion pulse without and with injection ofcesium beam

beam charge and average charge state calculations from thebeam spectrum measurements on FC5 were used for the totalbreeding efficiency calculation We found that the relativeabundance of the charge state 28+ was 27 for a 28 msbreeding time as is shown in Fig 11 Therefore the absolutebreeding efficiency of cesium ions into charge state 28+ is20 It is interesting to note that we observed a higher abun-dance of charge state 28+ as compared to the CBSIM simula-tions25 Based on these measurements the EBIS should pro-videge20 breeding efficiency of CARIBU rare isotope beamsbecause the expected CARIBU beam emittance is severaltimes smaller than the cesium beam emittance We should beable to reach an absolute breeding efficiency ofsim27 when theEBIS CB is relocated and coupled to CARIBU These exper-iments clearly demonstrate fast and efficient charge breed-ing of cesium ions As it follows from the CBSIM simu-lations any ion in the full mass range of the periodic tablecan be charge bred with high efficiency within short breed-ing times to a sufficiently high charge-to-mass ratio (ge163for the heaviest masses) for further acceleration in ATLASwith the presently demonstrated parameters of the CARIBUEBIS

Fig 12 shows the average charge state of charge bredions as a function of breeding time The simulations with

FIG 11 Measured and calculated using CBSIM code relative abundancesfor 28 ms breeding time

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083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

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083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

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083311-3 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 1 Set of individual drift tubes prior the installation (left) and a view (from the e-gun side) of the scaffold with installed drift tubes (right)

the magnetic axis of a 6 T superconducting solenoid withinplusmn50 microm

B Ion injection and extraction (I-E) lines

The I-E ion beam transport line includes two Einzel lensassemblies two steerers and an acceleration tube as wasdiscussed in Ref 10 Initial beam dynamic studies revealedthat the beam is not axially symmetric due to the not axisym-metric grounded vacuum chambers The aperture diameterof the Einzel lens its gap and the physical size of vacuumchambers were optimized to balance the linear region of thefocusing fields at the reduced effect of the vacuum chamberson the beam quality Three X-Y electrostatic steerer configu-rations (parallel quartered diagonally quartered and two setsof diagonally halved) have been studied in terms of the beamemittance growth Voltages were applied to the electrodes untila deflection of 13 mrad was achieved in both X and Y planesfor each assembly Two sets of diagonally halved steerer wereselected as they provided lowest emittance growth Our resultsare consisted with Ref 17 which reported low beam aberra-tions when the diagonally halved cylinder is used to createdipole electric field as compared to the parallel sectionedconfigurations

Numerical simulations and breeding efficiency measure-ments at BNLrsquos test EBIS revealed that the 30 electrostaticswitchyard for the injection and extraction of ions can intro-duce significant distortions on singly charged beams beinginjected into the EBIS The CARIBU EBIS switchyard em-ploys a single electrostatic deflector to direct the injected ionbeam from the source line to the I-E line and to guide theextracted beam from the I-E line to the diagnostic line Adiagonally halved cylindrical deflector was chosen over otherdeflector configurations because this design can maintain theion beam quality better than others as discussed with respectto the steerer assemblies A schematic of the switchyard isshown in Fig 2 In the final configuration the source beamline will be used for injection of the radioactive beams from theCARIBU and the spare beam line port will be used for injectionof singly charged cesium beam The deflector midplane wasshifted from the intersection of the 15 source beamline axisand the 0 I-E beamline axis by 096 cm as seen in Fig 2The value of this offset was determined by programmaticallyminimizing the deflection and the shift of the injected ion beamfrom the 0 axis in the EM Studio18 This offset will ensure

the injected beam can ideally be deflected exactly on axisThe overall length of the deflector was chosen to keep thenecessary applied voltages less than plusmn10 kV the magnitudefor commonly available and moderately priced high voltageamplifiers

III EBIS LAYOUT AND OPERATIONAL PARAMETERS

About a year ago we started extensive charge breed-ing experiments at the CARIBU EBIS to establish optimaloperational parameters These experiments were interruptedseveral times for extended periods of a month or longer forthe improvement of the EBIS sub-systems The first chargebreeding results of the cesium beam were reported at a recentsymposium on electron beam ion sources and traps19 Signifi-cant performance improvements have been achieved in the pastyear as is discussed throughout the content of this paper The3D model and general view of the EBIS at the off-line locationare shown in Figs 3 and 4 respectively The commissioningof the EBIS sub-systems1213 and beam dynamics studies werediscussed in earlier publications10 Current operational param-eters of the EBIS are listed in Table I together with possible

FIG 2 The electrostatic switchyard configuration and deflector positioningnecessary to inject the ion beam on the 0 beamline axis

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1283131212 On Mon 25 Jan 2016 162631

083311-4 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 3 3D model of CARIBU EBIS CB in off-line installation (1mdashe-gun 2mdashroom temperature solenoid 3mdashrecently installed NEG pump 4mdashend crossesof the ion trap chambers 5mdash6 T superconducting solenoid 6mdashelectron collector with room temperature coil 7mdashEinzel lens and steerers (EBIS potential)8mdash75 kV accelerating tube 9mdashFaraday cup for cesium beam (FC2) 10mdashpepper pot Multi-Channel Plate (MCP)-based emittance probe 11mdashEinzel lensand steerers (ground potential) 12mdashFaraday cup for cesium beam (FC1) 13mdashsteerers 14mdashquadrupole lenses 15mdashCs+ ion source and accelerating tube16mdashelectrostatic switchyard 17mdashFaraday cup for charge bred beam (FC3) 18mdash70 bending magnet 19mdashslits and Faraday cup (FC5))

ultimate parameters Fig 5 shows the potential distributionalong the EBIS for the charge breeding of residual gas oran externally injected singly charged ion beam The potentialdistribution is shown with respect to the EBIS platform whichis biased to 20 kV with respect to the ground The electrongun is biased to minus575 kV and operates with a 75 kV anodevoltage provided by a high voltage (HV) amplifier The e-gunbias anode voltage cathode heater current and the potentialof the first drift tube have been optimized to extract a 16 Acurrent and minimize electron beam losses along the EBISThe e-gun bias and the trap potential together with the electronbeam current define the electron beam energy in the trap Thetrap potential was optimized to provide the highest extractedcharge of the ionized residual gas

For the off-line charge breeding tests of the CARIBUEBIS we decided to use a commercial cesium ion sourcewhich is a good match with the mass range of CARIBU beamsand readily available from industry20 Trappingextraction ofthe ion pulse takes place by raisinglowering the potential ofa barrier electrode using fast solid state Behlke switches21

The switching time is less than 01 micros In most experimentswe applied sim1 kV barrier potential with respect to the trappotential while a sim300 V barrier potential is sufficient fortrapping For extraction the barrier potential is simminus400 V withrespect to the trap potential

A EBIS vacuum system

The EBIS vacuum system includes a set of turbopumpscoupled to scroll pumps cryopumps and NEG surfaces Thedetails of the EBIS vacuum system were described in Ref 12Efficient vacuum separation of the trap region from the e-gunand collector is provided We incorporated all-metal vacuumvalves from both sides of the trap region to facilitate easyreplacement of e-guns In February 2015 a NEG pump wasinstalled near the e-gun (position 3 in Fig 3) Recently we havedecided to repeat the activation of getter strips and sputteredgetter surfaces in the trap region by in-situ baking up to 450 CAfter completion of baking and several weeks of pumping theion gauge readings were 13 times 10minus10 Torr at e-gun chamber

FIG 4 General view of the EBIS in off-line installation This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-5 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

TABLE I EBIS parameters

Parameter Operational Possible ultimate Units

Magnetic field 50 60 TElectron beam current Ie 16 20 ACathode radius 21 21 mmMagnetic field on cathode 015 015 TEBIS platform bias voltage 20 50 kVTrap length 05 07 mElectron beam radius in the trap reTRAP

a 364 332 micromElectron beam density in the trapa 385 577 Acm2

Electron beam energy in the trapa 6495 6265 eVSpace charge potential well ∆UeTRAP

a 299 374 VElectron beam velocity ve in the trapa 48times107 47times107 msNormalized full acceptanceb 0024 0024 πtimesmmtimesmradTrap capacitya 23 30 nCRepetition rate Up to 10 30 HzDuty cycle Up to 40 90

aEstimated using general EBIS theory see for example Ref 22bSimulated with tracking codes10

and 45 times 10minus10 Torr near the collector with the electron beamswitched off This is a significant improvement as comparedto the previous data19 The residual pressure depends on theelectron beam current and its duty cycle At lower electronbeam currents below 1 A the duty cycle can be close to 100However at 16 A stable vacuum and stable charge breedingare observed only if the duty cycle is le40 at 10 Hz repetitionrate For the same electron beam current at a 1 Hz repetitionrate the duty cycle is slightly higher Operation at higher dutycycles requires extended vacuum conditioning

B Cesium source

We use a commercial ion gun TB-171 model HWIG-25020 to produce a singly charged cesium ion beam Cesiumions are accelerated to sim235 keV by the biased anode voltageand an 11-gap accelerating tube The cesium source providesa stable DC beam The pulsed cesium beam with the pulselength up to 55 micros is formed by an electrostatic deflector andinjected into the EBIS trap with a 5 ms delay with respect to theelectron beam pulse One electrode of the deflector (chopper)is pulsed up to 55 kV by a HV power supply coupled to a fast

FIG 5 Simplified schematic of the EBIS structure and potential distributionduring the breeding (1mdashe-gun 2mdashthe first drift tube 3mdashthe second and thirddrift tubes 4mdashtrap region 5mdashdrift tube with the barrier potential 6mdashdrifttubes 7mdashelectron collector 8mdashion extractor)

Behlke switch The full normalized acceptance of the EBISwas numerically defined for the wide range of parametersin our previous publication10 The normalized acceptancecan be approximated with the expression AFULL[mm timesmrad]= π times 38 times 10minus3reTRAP[mm]∆UeTRAP[V] and equal to0024π timesmm timesmrad for current EBIS parameters It is signif-icantly higher than the emittance of the radioactive beamsexpected from RFQ cooler-buncher which is sim0003π timesmmtimesmrad normalized Therefore for the purpose of EBIS effi-ciency measurements and application of these results to theradioactive beams it is important to form a cesium beam withan emittance comparable to that expected from the CARIBUsource The emittance of the cesium beam is established bythe appropriate focusing with a cesium source Einzel lens anelectrostatic triplet and adjustment of an iris style apertureThe cesium beam is very sensitive to the tuning of the beamoptics due to the non-linear character of the source extractionsystem and some off-centered beam trajectory in the transportsystem In our experiments the intensity of the cesium beamwas defined both by the heater in the ion source and the colli-mator About 80 of the cesium beam was intercepted by thecollimator The beam intensity did exceedsim4 times 107 ionspulsewhich corresponds to 64 pC A typical cesium beam image onthe phosphor screen of the pepper pot emittance meter and aprocessed emittance shape of this beam are shown in Fig 6The measured rms normalized emittance of the cesium beam issim(0006 plusmn 00015)π timesmm timesmrad The full cesium emittanceis slightly larger than the EBIS acceptance but the currentcollimator configuration is unable to reduce the emittancefurther In addition some losses due to beam emittance andbeam center mismatch with the EBIS acceptance may takeplace All this results in a transmission efficiency of the cesiumbeam through the EBIS lower than 100

C Ion beam diagnostics tools

Predominantly we use an oscilloscope to measure the totalcharge of injected and extracted ion beam pulses incident on

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1283131212 On Mon 25 Jan 2016 162631

083311-6 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 6 A typical cesium beam image on the phosphor screen of the pepper pot emittance meter (a) and its emittance ((b) and (c)) The distance between thepepper-pot holes is 05 mm The color code corresponds to a linear incremental change of the density from 0 to 10 The central red area corresponds to thedensity higher than 10

Faraday cups There are 3 Faraday cups (FC1 FC2 and FC3)with 46 mm input apertures installed on air cylinders (seepositions 9 12 and 17 in Fig 3) and a stationary cup (FC5at position 19 in Fig 3) installed downstream of adjustableslits after the bending magnet FC1 and FC2 are used forthe measurements of the injected cesium beam charge Basedon our previous experience with the current measurementsof multi-charge ion beams the electron suppression voltageis minus300 V The accuracy of beam charge measurements isplusmn1 As is known multi-charge beams can produce energeticAuger electrons that can escape the FC Our measurementswith ATLAS multi-charge beams show that the contributionof energetic electrons to the measurement error is less than04 if a minus300 V biased voltage is applied Therefore it is ourcommon practice to apply minus300 V to all Faraday cups used forthe measurements of multi-charge ion beams A typical FC2signal from the pulsed cesium beam is shown in Fig 7 Thetotal charge is calculated as Qtot = VpeakCFC where Vpeak isthe peak voltage and CFC is the capacitance of the FC andsignal cables Calculation of the charge by area integrationresults in the same value within a plusmn1 error For chargebreeding efficiency measurements we use FCs with identicalcapacitances to avoid error due to different decay times of thevoltages during the charge accumulation over the pulse length

FIG 7 Cesium beam signal reading on the FC2 prior to injection into theEBIS

A pepper-pot emittance probe based on a Multi-ChannelPlate (MCP) and a phosphor screen (position 10 in Fig 3)23

was used to measure the emittance of the injected cesiumbeam Due to the very high sensitivity of the MCP very lowinjected cesium beam intensities sim01 pC per pulse can beanalyzed by the emittance probe

D Electron beam commissioning

The stability of high current-high duty cycle electronbeam in the EBIS is the key for breeding of ions with highefficiency The set of electron beam diagnostics and results ofthe initial electron beam commissioning at 4 T magnetic fieldwere reported in the previous publication12 The time structureof the electron beam was formed by applying a pulsed highvoltage potential to the e-gun anode Optimal transport of theelectron beam with very low relative losses below 10minus4 fromthe e-gun cathode to the collector was established by tuning oftwo sets of X and Y steering dipole coils installed on vacuumchamber crosses in both sides of the superconducting (SC)solenoid and 4 pairs of magnetic steering coils installed onthe beam tube inside the SC solenoid bore In addition thecollector solenoid field is tuned to minimize the electron beamlosses The availability of steering coils is essential for the100 transmission of the electron beam The tuning of theelectron beam is complete when vacuum gauges do not showany pressure fluctuations Currently we can stably operate 16A electron beam at 40 duty cycle or 115 A electron beam at90 duty cycle Increasing either current or duty cycle of theelectron beam requires long vacuum conditioning

IV RESIDUAL GAS IONIZATION

When the barrier electrode potential is raised (position 5in Fig 5) ionization of residual gas atoms and molecules takesplace in the trap The charge bred ions of the residual gas areextracted and analyzed using 70 bending magnet In Sec Vof this paper we discuss experimental data taken from FC5 formultiple pulses while the dipole current was varied The beamcharge values delivered in each pulse were generally stablebut we used averaging of the signal over several pulses whenthis was not the case Fig 8 shows spectra of the residual gasfor two different confinement times for the EBIS operational

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

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083311-7 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 8 Residual gas spectrum at 30 ms (a) and 65 ms (b) of confinement time Ion pulse charge is Q[pC]= 019timesV[mV]

parameters shown in Table I To increase mass resolution thebeam was collimated upstream of the FC5 The residual gascontains ions of hydrogen carbon and oxygen The relativecontribution of nitrogen is very low We do not see molecularions in the spectrum The preferable values of charge-to-massratios for CARIBU beams for the extraction separation andfurther acceleration in ATLAS are in the range from 17 to14 These ratios can be achieved with breeding times from10 to 30 ms and appear on FC5 approximately in the range ofmagnet currents from 36 to 47 A By adjusting the breedingtime the most abundant charge state of a rare isotope ionbeam can be located between residual gas peaks and separatedfrom the residual gas contamination in the transport systemdownstream of the EBIS We plan a detailed study of possiblecontaminants from the residual gas in the region between O3+

and O4+with upgraded Faraday cups and amplifiers to measurefemtocoulomb-level charges

The capacity of the trap is calculated as CTRAP =IeLTRAP

ve= 23 nC where Ie is the electron beam current ve is the elect-ron velocity in the trap and LTRAP is the length of the trapFor typical confinement times le30 ms required for CARIBU

beams the neutralization factor is below 2 The total ex-tracted charge of residual gas ions does not exceed sim7 ofthe total charge capacity of the trap for confinement times upto 150 ms

V CESIUM BREEDING EFFICIENCY MEASUREMENTS

The main goal of the off-line commissioning was todemonstrate stable operation of EBIS at a 10 Hz repetitionrate and a breeding efficiency into single charge state higherthan 15 These goals have been successfully achieved andexceeded The breeding efficiency measurements were per-formed according to the procedure described in detail inRef 24 This procedure uses measurements of total chargeon FC3 with and without cesium injection The total numberof extracted cesium ions was calculated as a ratio of cesiumcharge on FC3 to the average charge state of the cesium beamThe latter was found from the measured cesium spectrumobtained on FC5 with 1 or 2 mm wide slits Examples of thecesium spectra for 20 and 28 ms breeding times for the EBISoperational parameters listed in Table I are shown in Fig 9 For

FIG 9 Spectra of charge-bred cesium and residual gas ions for two breeding times 20 ms (a) and 28 ms (b) This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-8 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

these measurements to obtain a good resolution of differentcharge states the slits upstream of the FC5 were closed to2 mm The average charge state z was calculated using theexpression

z =

zmaxz=zmin

Qz

zmaxz=zmin

Qz

z

(2)

where Qz is the total charge of ions at a particular chargestate z zmin and zmax are the minimum and maximum chargestates correspondingly in the spectrum of charge-bred ce-sium ions The total charge Qz was calculated as an areaunder the Gaussian distribution obtained from the fit to themeasured distribution The relative abundance of cesium ionswith different charge states is defined by the expression

Az =Qz

z middotzmax

z=zmin

Qz

z

middot 100 (3)

We noticed that the calculations of average charge state couldalso be done using a peak value of the signal for each chargestate Qpeak

z with similar results Breeding efficiency into thecharge state with the highest abundance ηzmax is defined as aratio of the number of extracted ions with highest abundance tothe number of injected Cs+ ions and it can be found from theexpression ηzmax = ηtot middot Amax

z where ηtot is the total transmis-sion efficiency of cesium ions The above mentioned methodincludes an error related to the measurements of the totalcharge of residual gas ions with and without cesium injectionDue to the interaction of cesium ions with the ions of residualgas the total extracted charge of residual gas ions is slightlymodified From the comparison of the spectra of the residualgas ions (Fig 8(a)) and cesium beam (Fig 9(b)) it is clear thatthe ions of the residual gas are well separated with respect tothe cesium spectrum Therefore the relative change of the totalcharge of the residual gas ξ extracted with and without cesiuminjection can be approximately estimated as ξ = 2 Σ1minusΣ2

Σ1+Σ2 where

Σ1 and Σ2 are the integrated charges of the residual gas ionsin the spectra with and without cesium injection respectivelyThe calculations show that ξ = 22

In addition we have performed direct measurements ofthe cesium charge in a single charge state after the bendingmagnet This is possible for charge states below sim20 when thefull intensity in the single charge state can be transported to theFC5 with a 10 mm slit opening The breeding efficiency into asingle charge state can be calculated directly by measuring thecharge on FC5 This method results in the breeding efficiencyconsistent with the method described above and in Ref 20within plusmn25 Overall the systematic and measurement er-rors result in plusmn35 relative error of the breeding efficiencyevaluations

After several optimization rounds of cesium beam tun-ing the EBIS parameters and injection-extraction potentialsconsistently resulted in total injection-extraction efficienciesof sim70 and can be increased to 80 with elaborate beamtuning An example of FC3 measurements with and withoutinjection of the cesium beam is shown in Fig 10 Thesedata together with the FC2 reading of the injected cesium

FIG 10 FC3 signal for a single ion pulse without and with injection ofcesium beam

beam charge and average charge state calculations from thebeam spectrum measurements on FC5 were used for the totalbreeding efficiency calculation We found that the relativeabundance of the charge state 28+ was 27 for a 28 msbreeding time as is shown in Fig 11 Therefore the absolutebreeding efficiency of cesium ions into charge state 28+ is20 It is interesting to note that we observed a higher abun-dance of charge state 28+ as compared to the CBSIM simula-tions25 Based on these measurements the EBIS should pro-videge20 breeding efficiency of CARIBU rare isotope beamsbecause the expected CARIBU beam emittance is severaltimes smaller than the cesium beam emittance We should beable to reach an absolute breeding efficiency ofsim27 when theEBIS CB is relocated and coupled to CARIBU These exper-iments clearly demonstrate fast and efficient charge breed-ing of cesium ions As it follows from the CBSIM simu-lations any ion in the full mass range of the periodic tablecan be charge bred with high efficiency within short breed-ing times to a sufficiently high charge-to-mass ratio (ge163for the heaviest masses) for further acceleration in ATLASwith the presently demonstrated parameters of the CARIBUEBIS

Fig 12 shows the average charge state of charge bredions as a function of breeding time The simulations with

FIG 11 Measured and calculated using CBSIM code relative abundancesfor 28 ms breeding time

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083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

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083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

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083311-4 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 3 3D model of CARIBU EBIS CB in off-line installation (1mdashe-gun 2mdashroom temperature solenoid 3mdashrecently installed NEG pump 4mdashend crossesof the ion trap chambers 5mdash6 T superconducting solenoid 6mdashelectron collector with room temperature coil 7mdashEinzel lens and steerers (EBIS potential)8mdash75 kV accelerating tube 9mdashFaraday cup for cesium beam (FC2) 10mdashpepper pot Multi-Channel Plate (MCP)-based emittance probe 11mdashEinzel lensand steerers (ground potential) 12mdashFaraday cup for cesium beam (FC1) 13mdashsteerers 14mdashquadrupole lenses 15mdashCs+ ion source and accelerating tube16mdashelectrostatic switchyard 17mdashFaraday cup for charge bred beam (FC3) 18mdash70 bending magnet 19mdashslits and Faraday cup (FC5))

ultimate parameters Fig 5 shows the potential distributionalong the EBIS for the charge breeding of residual gas oran externally injected singly charged ion beam The potentialdistribution is shown with respect to the EBIS platform whichis biased to 20 kV with respect to the ground The electrongun is biased to minus575 kV and operates with a 75 kV anodevoltage provided by a high voltage (HV) amplifier The e-gunbias anode voltage cathode heater current and the potentialof the first drift tube have been optimized to extract a 16 Acurrent and minimize electron beam losses along the EBISThe e-gun bias and the trap potential together with the electronbeam current define the electron beam energy in the trap Thetrap potential was optimized to provide the highest extractedcharge of the ionized residual gas

For the off-line charge breeding tests of the CARIBUEBIS we decided to use a commercial cesium ion sourcewhich is a good match with the mass range of CARIBU beamsand readily available from industry20 Trappingextraction ofthe ion pulse takes place by raisinglowering the potential ofa barrier electrode using fast solid state Behlke switches21

The switching time is less than 01 micros In most experimentswe applied sim1 kV barrier potential with respect to the trappotential while a sim300 V barrier potential is sufficient fortrapping For extraction the barrier potential is simminus400 V withrespect to the trap potential

A EBIS vacuum system

The EBIS vacuum system includes a set of turbopumpscoupled to scroll pumps cryopumps and NEG surfaces Thedetails of the EBIS vacuum system were described in Ref 12Efficient vacuum separation of the trap region from the e-gunand collector is provided We incorporated all-metal vacuumvalves from both sides of the trap region to facilitate easyreplacement of e-guns In February 2015 a NEG pump wasinstalled near the e-gun (position 3 in Fig 3) Recently we havedecided to repeat the activation of getter strips and sputteredgetter surfaces in the trap region by in-situ baking up to 450 CAfter completion of baking and several weeks of pumping theion gauge readings were 13 times 10minus10 Torr at e-gun chamber

FIG 4 General view of the EBIS in off-line installation This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

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083311-5 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

TABLE I EBIS parameters

Parameter Operational Possible ultimate Units

Magnetic field 50 60 TElectron beam current Ie 16 20 ACathode radius 21 21 mmMagnetic field on cathode 015 015 TEBIS platform bias voltage 20 50 kVTrap length 05 07 mElectron beam radius in the trap reTRAP

a 364 332 micromElectron beam density in the trapa 385 577 Acm2

Electron beam energy in the trapa 6495 6265 eVSpace charge potential well ∆UeTRAP

a 299 374 VElectron beam velocity ve in the trapa 48times107 47times107 msNormalized full acceptanceb 0024 0024 πtimesmmtimesmradTrap capacitya 23 30 nCRepetition rate Up to 10 30 HzDuty cycle Up to 40 90

aEstimated using general EBIS theory see for example Ref 22bSimulated with tracking codes10

and 45 times 10minus10 Torr near the collector with the electron beamswitched off This is a significant improvement as comparedto the previous data19 The residual pressure depends on theelectron beam current and its duty cycle At lower electronbeam currents below 1 A the duty cycle can be close to 100However at 16 A stable vacuum and stable charge breedingare observed only if the duty cycle is le40 at 10 Hz repetitionrate For the same electron beam current at a 1 Hz repetitionrate the duty cycle is slightly higher Operation at higher dutycycles requires extended vacuum conditioning

B Cesium source

We use a commercial ion gun TB-171 model HWIG-25020 to produce a singly charged cesium ion beam Cesiumions are accelerated to sim235 keV by the biased anode voltageand an 11-gap accelerating tube The cesium source providesa stable DC beam The pulsed cesium beam with the pulselength up to 55 micros is formed by an electrostatic deflector andinjected into the EBIS trap with a 5 ms delay with respect to theelectron beam pulse One electrode of the deflector (chopper)is pulsed up to 55 kV by a HV power supply coupled to a fast

FIG 5 Simplified schematic of the EBIS structure and potential distributionduring the breeding (1mdashe-gun 2mdashthe first drift tube 3mdashthe second and thirddrift tubes 4mdashtrap region 5mdashdrift tube with the barrier potential 6mdashdrifttubes 7mdashelectron collector 8mdashion extractor)

Behlke switch The full normalized acceptance of the EBISwas numerically defined for the wide range of parametersin our previous publication10 The normalized acceptancecan be approximated with the expression AFULL[mm timesmrad]= π times 38 times 10minus3reTRAP[mm]∆UeTRAP[V] and equal to0024π timesmm timesmrad for current EBIS parameters It is signif-icantly higher than the emittance of the radioactive beamsexpected from RFQ cooler-buncher which is sim0003π timesmmtimesmrad normalized Therefore for the purpose of EBIS effi-ciency measurements and application of these results to theradioactive beams it is important to form a cesium beam withan emittance comparable to that expected from the CARIBUsource The emittance of the cesium beam is established bythe appropriate focusing with a cesium source Einzel lens anelectrostatic triplet and adjustment of an iris style apertureThe cesium beam is very sensitive to the tuning of the beamoptics due to the non-linear character of the source extractionsystem and some off-centered beam trajectory in the transportsystem In our experiments the intensity of the cesium beamwas defined both by the heater in the ion source and the colli-mator About 80 of the cesium beam was intercepted by thecollimator The beam intensity did exceedsim4 times 107 ionspulsewhich corresponds to 64 pC A typical cesium beam image onthe phosphor screen of the pepper pot emittance meter and aprocessed emittance shape of this beam are shown in Fig 6The measured rms normalized emittance of the cesium beam issim(0006 plusmn 00015)π timesmm timesmrad The full cesium emittanceis slightly larger than the EBIS acceptance but the currentcollimator configuration is unable to reduce the emittancefurther In addition some losses due to beam emittance andbeam center mismatch with the EBIS acceptance may takeplace All this results in a transmission efficiency of the cesiumbeam through the EBIS lower than 100

C Ion beam diagnostics tools

Predominantly we use an oscilloscope to measure the totalcharge of injected and extracted ion beam pulses incident on

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083311-6 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 6 A typical cesium beam image on the phosphor screen of the pepper pot emittance meter (a) and its emittance ((b) and (c)) The distance between thepepper-pot holes is 05 mm The color code corresponds to a linear incremental change of the density from 0 to 10 The central red area corresponds to thedensity higher than 10

Faraday cups There are 3 Faraday cups (FC1 FC2 and FC3)with 46 mm input apertures installed on air cylinders (seepositions 9 12 and 17 in Fig 3) and a stationary cup (FC5at position 19 in Fig 3) installed downstream of adjustableslits after the bending magnet FC1 and FC2 are used forthe measurements of the injected cesium beam charge Basedon our previous experience with the current measurementsof multi-charge ion beams the electron suppression voltageis minus300 V The accuracy of beam charge measurements isplusmn1 As is known multi-charge beams can produce energeticAuger electrons that can escape the FC Our measurementswith ATLAS multi-charge beams show that the contributionof energetic electrons to the measurement error is less than04 if a minus300 V biased voltage is applied Therefore it is ourcommon practice to apply minus300 V to all Faraday cups used forthe measurements of multi-charge ion beams A typical FC2signal from the pulsed cesium beam is shown in Fig 7 Thetotal charge is calculated as Qtot = VpeakCFC where Vpeak isthe peak voltage and CFC is the capacitance of the FC andsignal cables Calculation of the charge by area integrationresults in the same value within a plusmn1 error For chargebreeding efficiency measurements we use FCs with identicalcapacitances to avoid error due to different decay times of thevoltages during the charge accumulation over the pulse length

FIG 7 Cesium beam signal reading on the FC2 prior to injection into theEBIS

A pepper-pot emittance probe based on a Multi-ChannelPlate (MCP) and a phosphor screen (position 10 in Fig 3)23

was used to measure the emittance of the injected cesiumbeam Due to the very high sensitivity of the MCP very lowinjected cesium beam intensities sim01 pC per pulse can beanalyzed by the emittance probe

D Electron beam commissioning

The stability of high current-high duty cycle electronbeam in the EBIS is the key for breeding of ions with highefficiency The set of electron beam diagnostics and results ofthe initial electron beam commissioning at 4 T magnetic fieldwere reported in the previous publication12 The time structureof the electron beam was formed by applying a pulsed highvoltage potential to the e-gun anode Optimal transport of theelectron beam with very low relative losses below 10minus4 fromthe e-gun cathode to the collector was established by tuning oftwo sets of X and Y steering dipole coils installed on vacuumchamber crosses in both sides of the superconducting (SC)solenoid and 4 pairs of magnetic steering coils installed onthe beam tube inside the SC solenoid bore In addition thecollector solenoid field is tuned to minimize the electron beamlosses The availability of steering coils is essential for the100 transmission of the electron beam The tuning of theelectron beam is complete when vacuum gauges do not showany pressure fluctuations Currently we can stably operate 16A electron beam at 40 duty cycle or 115 A electron beam at90 duty cycle Increasing either current or duty cycle of theelectron beam requires long vacuum conditioning

IV RESIDUAL GAS IONIZATION

When the barrier electrode potential is raised (position 5in Fig 5) ionization of residual gas atoms and molecules takesplace in the trap The charge bred ions of the residual gas areextracted and analyzed using 70 bending magnet In Sec Vof this paper we discuss experimental data taken from FC5 formultiple pulses while the dipole current was varied The beamcharge values delivered in each pulse were generally stablebut we used averaging of the signal over several pulses whenthis was not the case Fig 8 shows spectra of the residual gasfor two different confinement times for the EBIS operational

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083311-7 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 8 Residual gas spectrum at 30 ms (a) and 65 ms (b) of confinement time Ion pulse charge is Q[pC]= 019timesV[mV]

parameters shown in Table I To increase mass resolution thebeam was collimated upstream of the FC5 The residual gascontains ions of hydrogen carbon and oxygen The relativecontribution of nitrogen is very low We do not see molecularions in the spectrum The preferable values of charge-to-massratios for CARIBU beams for the extraction separation andfurther acceleration in ATLAS are in the range from 17 to14 These ratios can be achieved with breeding times from10 to 30 ms and appear on FC5 approximately in the range ofmagnet currents from 36 to 47 A By adjusting the breedingtime the most abundant charge state of a rare isotope ionbeam can be located between residual gas peaks and separatedfrom the residual gas contamination in the transport systemdownstream of the EBIS We plan a detailed study of possiblecontaminants from the residual gas in the region between O3+

and O4+with upgraded Faraday cups and amplifiers to measurefemtocoulomb-level charges

The capacity of the trap is calculated as CTRAP =IeLTRAP

ve= 23 nC where Ie is the electron beam current ve is the elect-ron velocity in the trap and LTRAP is the length of the trapFor typical confinement times le30 ms required for CARIBU

beams the neutralization factor is below 2 The total ex-tracted charge of residual gas ions does not exceed sim7 ofthe total charge capacity of the trap for confinement times upto 150 ms

V CESIUM BREEDING EFFICIENCY MEASUREMENTS

The main goal of the off-line commissioning was todemonstrate stable operation of EBIS at a 10 Hz repetitionrate and a breeding efficiency into single charge state higherthan 15 These goals have been successfully achieved andexceeded The breeding efficiency measurements were per-formed according to the procedure described in detail inRef 24 This procedure uses measurements of total chargeon FC3 with and without cesium injection The total numberof extracted cesium ions was calculated as a ratio of cesiumcharge on FC3 to the average charge state of the cesium beamThe latter was found from the measured cesium spectrumobtained on FC5 with 1 or 2 mm wide slits Examples of thecesium spectra for 20 and 28 ms breeding times for the EBISoperational parameters listed in Table I are shown in Fig 9 For

FIG 9 Spectra of charge-bred cesium and residual gas ions for two breeding times 20 ms (a) and 28 ms (b) This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-8 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

these measurements to obtain a good resolution of differentcharge states the slits upstream of the FC5 were closed to2 mm The average charge state z was calculated using theexpression

z =

zmaxz=zmin

Qz

zmaxz=zmin

Qz

z

(2)

where Qz is the total charge of ions at a particular chargestate z zmin and zmax are the minimum and maximum chargestates correspondingly in the spectrum of charge-bred ce-sium ions The total charge Qz was calculated as an areaunder the Gaussian distribution obtained from the fit to themeasured distribution The relative abundance of cesium ionswith different charge states is defined by the expression

Az =Qz

z middotzmax

z=zmin

Qz

z

middot 100 (3)

We noticed that the calculations of average charge state couldalso be done using a peak value of the signal for each chargestate Qpeak

z with similar results Breeding efficiency into thecharge state with the highest abundance ηzmax is defined as aratio of the number of extracted ions with highest abundance tothe number of injected Cs+ ions and it can be found from theexpression ηzmax = ηtot middot Amax

z where ηtot is the total transmis-sion efficiency of cesium ions The above mentioned methodincludes an error related to the measurements of the totalcharge of residual gas ions with and without cesium injectionDue to the interaction of cesium ions with the ions of residualgas the total extracted charge of residual gas ions is slightlymodified From the comparison of the spectra of the residualgas ions (Fig 8(a)) and cesium beam (Fig 9(b)) it is clear thatthe ions of the residual gas are well separated with respect tothe cesium spectrum Therefore the relative change of the totalcharge of the residual gas ξ extracted with and without cesiuminjection can be approximately estimated as ξ = 2 Σ1minusΣ2

Σ1+Σ2 where

Σ1 and Σ2 are the integrated charges of the residual gas ionsin the spectra with and without cesium injection respectivelyThe calculations show that ξ = 22

In addition we have performed direct measurements ofthe cesium charge in a single charge state after the bendingmagnet This is possible for charge states below sim20 when thefull intensity in the single charge state can be transported to theFC5 with a 10 mm slit opening The breeding efficiency into asingle charge state can be calculated directly by measuring thecharge on FC5 This method results in the breeding efficiencyconsistent with the method described above and in Ref 20within plusmn25 Overall the systematic and measurement er-rors result in plusmn35 relative error of the breeding efficiencyevaluations

After several optimization rounds of cesium beam tun-ing the EBIS parameters and injection-extraction potentialsconsistently resulted in total injection-extraction efficienciesof sim70 and can be increased to 80 with elaborate beamtuning An example of FC3 measurements with and withoutinjection of the cesium beam is shown in Fig 10 Thesedata together with the FC2 reading of the injected cesium

FIG 10 FC3 signal for a single ion pulse without and with injection ofcesium beam

beam charge and average charge state calculations from thebeam spectrum measurements on FC5 were used for the totalbreeding efficiency calculation We found that the relativeabundance of the charge state 28+ was 27 for a 28 msbreeding time as is shown in Fig 11 Therefore the absolutebreeding efficiency of cesium ions into charge state 28+ is20 It is interesting to note that we observed a higher abun-dance of charge state 28+ as compared to the CBSIM simula-tions25 Based on these measurements the EBIS should pro-videge20 breeding efficiency of CARIBU rare isotope beamsbecause the expected CARIBU beam emittance is severaltimes smaller than the cesium beam emittance We should beable to reach an absolute breeding efficiency ofsim27 when theEBIS CB is relocated and coupled to CARIBU These exper-iments clearly demonstrate fast and efficient charge breed-ing of cesium ions As it follows from the CBSIM simu-lations any ion in the full mass range of the periodic tablecan be charge bred with high efficiency within short breed-ing times to a sufficiently high charge-to-mass ratio (ge163for the heaviest masses) for further acceleration in ATLASwith the presently demonstrated parameters of the CARIBUEBIS

Fig 12 shows the average charge state of charge bredions as a function of breeding time The simulations with

FIG 11 Measured and calculated using CBSIM code relative abundancesfor 28 ms breeding time

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1283131212 On Mon 25 Jan 2016 162631

083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

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1283131212 On Mon 25 Jan 2016 162631

083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

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1283131212 On Mon 25 Jan 2016 162631

083311-5 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

TABLE I EBIS parameters

Parameter Operational Possible ultimate Units

Magnetic field 50 60 TElectron beam current Ie 16 20 ACathode radius 21 21 mmMagnetic field on cathode 015 015 TEBIS platform bias voltage 20 50 kVTrap length 05 07 mElectron beam radius in the trap reTRAP

a 364 332 micromElectron beam density in the trapa 385 577 Acm2

Electron beam energy in the trapa 6495 6265 eVSpace charge potential well ∆UeTRAP

a 299 374 VElectron beam velocity ve in the trapa 48times107 47times107 msNormalized full acceptanceb 0024 0024 πtimesmmtimesmradTrap capacitya 23 30 nCRepetition rate Up to 10 30 HzDuty cycle Up to 40 90

aEstimated using general EBIS theory see for example Ref 22bSimulated with tracking codes10

and 45 times 10minus10 Torr near the collector with the electron beamswitched off This is a significant improvement as comparedto the previous data19 The residual pressure depends on theelectron beam current and its duty cycle At lower electronbeam currents below 1 A the duty cycle can be close to 100However at 16 A stable vacuum and stable charge breedingare observed only if the duty cycle is le40 at 10 Hz repetitionrate For the same electron beam current at a 1 Hz repetitionrate the duty cycle is slightly higher Operation at higher dutycycles requires extended vacuum conditioning

B Cesium source

We use a commercial ion gun TB-171 model HWIG-25020 to produce a singly charged cesium ion beam Cesiumions are accelerated to sim235 keV by the biased anode voltageand an 11-gap accelerating tube The cesium source providesa stable DC beam The pulsed cesium beam with the pulselength up to 55 micros is formed by an electrostatic deflector andinjected into the EBIS trap with a 5 ms delay with respect to theelectron beam pulse One electrode of the deflector (chopper)is pulsed up to 55 kV by a HV power supply coupled to a fast

FIG 5 Simplified schematic of the EBIS structure and potential distributionduring the breeding (1mdashe-gun 2mdashthe first drift tube 3mdashthe second and thirddrift tubes 4mdashtrap region 5mdashdrift tube with the barrier potential 6mdashdrifttubes 7mdashelectron collector 8mdashion extractor)

Behlke switch The full normalized acceptance of the EBISwas numerically defined for the wide range of parametersin our previous publication10 The normalized acceptancecan be approximated with the expression AFULL[mm timesmrad]= π times 38 times 10minus3reTRAP[mm]∆UeTRAP[V] and equal to0024π timesmm timesmrad for current EBIS parameters It is signif-icantly higher than the emittance of the radioactive beamsexpected from RFQ cooler-buncher which is sim0003π timesmmtimesmrad normalized Therefore for the purpose of EBIS effi-ciency measurements and application of these results to theradioactive beams it is important to form a cesium beam withan emittance comparable to that expected from the CARIBUsource The emittance of the cesium beam is established bythe appropriate focusing with a cesium source Einzel lens anelectrostatic triplet and adjustment of an iris style apertureThe cesium beam is very sensitive to the tuning of the beamoptics due to the non-linear character of the source extractionsystem and some off-centered beam trajectory in the transportsystem In our experiments the intensity of the cesium beamwas defined both by the heater in the ion source and the colli-mator About 80 of the cesium beam was intercepted by thecollimator The beam intensity did exceedsim4 times 107 ionspulsewhich corresponds to 64 pC A typical cesium beam image onthe phosphor screen of the pepper pot emittance meter and aprocessed emittance shape of this beam are shown in Fig 6The measured rms normalized emittance of the cesium beam issim(0006 plusmn 00015)π timesmm timesmrad The full cesium emittanceis slightly larger than the EBIS acceptance but the currentcollimator configuration is unable to reduce the emittancefurther In addition some losses due to beam emittance andbeam center mismatch with the EBIS acceptance may takeplace All this results in a transmission efficiency of the cesiumbeam through the EBIS lower than 100

C Ion beam diagnostics tools

Predominantly we use an oscilloscope to measure the totalcharge of injected and extracted ion beam pulses incident on

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1283131212 On Mon 25 Jan 2016 162631

083311-6 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 6 A typical cesium beam image on the phosphor screen of the pepper pot emittance meter (a) and its emittance ((b) and (c)) The distance between thepepper-pot holes is 05 mm The color code corresponds to a linear incremental change of the density from 0 to 10 The central red area corresponds to thedensity higher than 10

Faraday cups There are 3 Faraday cups (FC1 FC2 and FC3)with 46 mm input apertures installed on air cylinders (seepositions 9 12 and 17 in Fig 3) and a stationary cup (FC5at position 19 in Fig 3) installed downstream of adjustableslits after the bending magnet FC1 and FC2 are used forthe measurements of the injected cesium beam charge Basedon our previous experience with the current measurementsof multi-charge ion beams the electron suppression voltageis minus300 V The accuracy of beam charge measurements isplusmn1 As is known multi-charge beams can produce energeticAuger electrons that can escape the FC Our measurementswith ATLAS multi-charge beams show that the contributionof energetic electrons to the measurement error is less than04 if a minus300 V biased voltage is applied Therefore it is ourcommon practice to apply minus300 V to all Faraday cups used forthe measurements of multi-charge ion beams A typical FC2signal from the pulsed cesium beam is shown in Fig 7 Thetotal charge is calculated as Qtot = VpeakCFC where Vpeak isthe peak voltage and CFC is the capacitance of the FC andsignal cables Calculation of the charge by area integrationresults in the same value within a plusmn1 error For chargebreeding efficiency measurements we use FCs with identicalcapacitances to avoid error due to different decay times of thevoltages during the charge accumulation over the pulse length

FIG 7 Cesium beam signal reading on the FC2 prior to injection into theEBIS

A pepper-pot emittance probe based on a Multi-ChannelPlate (MCP) and a phosphor screen (position 10 in Fig 3)23

was used to measure the emittance of the injected cesiumbeam Due to the very high sensitivity of the MCP very lowinjected cesium beam intensities sim01 pC per pulse can beanalyzed by the emittance probe

D Electron beam commissioning

The stability of high current-high duty cycle electronbeam in the EBIS is the key for breeding of ions with highefficiency The set of electron beam diagnostics and results ofthe initial electron beam commissioning at 4 T magnetic fieldwere reported in the previous publication12 The time structureof the electron beam was formed by applying a pulsed highvoltage potential to the e-gun anode Optimal transport of theelectron beam with very low relative losses below 10minus4 fromthe e-gun cathode to the collector was established by tuning oftwo sets of X and Y steering dipole coils installed on vacuumchamber crosses in both sides of the superconducting (SC)solenoid and 4 pairs of magnetic steering coils installed onthe beam tube inside the SC solenoid bore In addition thecollector solenoid field is tuned to minimize the electron beamlosses The availability of steering coils is essential for the100 transmission of the electron beam The tuning of theelectron beam is complete when vacuum gauges do not showany pressure fluctuations Currently we can stably operate 16A electron beam at 40 duty cycle or 115 A electron beam at90 duty cycle Increasing either current or duty cycle of theelectron beam requires long vacuum conditioning

IV RESIDUAL GAS IONIZATION

When the barrier electrode potential is raised (position 5in Fig 5) ionization of residual gas atoms and molecules takesplace in the trap The charge bred ions of the residual gas areextracted and analyzed using 70 bending magnet In Sec Vof this paper we discuss experimental data taken from FC5 formultiple pulses while the dipole current was varied The beamcharge values delivered in each pulse were generally stablebut we used averaging of the signal over several pulses whenthis was not the case Fig 8 shows spectra of the residual gasfor two different confinement times for the EBIS operational

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1283131212 On Mon 25 Jan 2016 162631

083311-7 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 8 Residual gas spectrum at 30 ms (a) and 65 ms (b) of confinement time Ion pulse charge is Q[pC]= 019timesV[mV]

parameters shown in Table I To increase mass resolution thebeam was collimated upstream of the FC5 The residual gascontains ions of hydrogen carbon and oxygen The relativecontribution of nitrogen is very low We do not see molecularions in the spectrum The preferable values of charge-to-massratios for CARIBU beams for the extraction separation andfurther acceleration in ATLAS are in the range from 17 to14 These ratios can be achieved with breeding times from10 to 30 ms and appear on FC5 approximately in the range ofmagnet currents from 36 to 47 A By adjusting the breedingtime the most abundant charge state of a rare isotope ionbeam can be located between residual gas peaks and separatedfrom the residual gas contamination in the transport systemdownstream of the EBIS We plan a detailed study of possiblecontaminants from the residual gas in the region between O3+

and O4+with upgraded Faraday cups and amplifiers to measurefemtocoulomb-level charges

The capacity of the trap is calculated as CTRAP =IeLTRAP

ve= 23 nC where Ie is the electron beam current ve is the elect-ron velocity in the trap and LTRAP is the length of the trapFor typical confinement times le30 ms required for CARIBU

beams the neutralization factor is below 2 The total ex-tracted charge of residual gas ions does not exceed sim7 ofthe total charge capacity of the trap for confinement times upto 150 ms

V CESIUM BREEDING EFFICIENCY MEASUREMENTS

The main goal of the off-line commissioning was todemonstrate stable operation of EBIS at a 10 Hz repetitionrate and a breeding efficiency into single charge state higherthan 15 These goals have been successfully achieved andexceeded The breeding efficiency measurements were per-formed according to the procedure described in detail inRef 24 This procedure uses measurements of total chargeon FC3 with and without cesium injection The total numberof extracted cesium ions was calculated as a ratio of cesiumcharge on FC3 to the average charge state of the cesium beamThe latter was found from the measured cesium spectrumobtained on FC5 with 1 or 2 mm wide slits Examples of thecesium spectra for 20 and 28 ms breeding times for the EBISoperational parameters listed in Table I are shown in Fig 9 For

FIG 9 Spectra of charge-bred cesium and residual gas ions for two breeding times 20 ms (a) and 28 ms (b) This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-8 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

these measurements to obtain a good resolution of differentcharge states the slits upstream of the FC5 were closed to2 mm The average charge state z was calculated using theexpression

z =

zmaxz=zmin

Qz

zmaxz=zmin

Qz

z

(2)

where Qz is the total charge of ions at a particular chargestate z zmin and zmax are the minimum and maximum chargestates correspondingly in the spectrum of charge-bred ce-sium ions The total charge Qz was calculated as an areaunder the Gaussian distribution obtained from the fit to themeasured distribution The relative abundance of cesium ionswith different charge states is defined by the expression

Az =Qz

z middotzmax

z=zmin

Qz

z

middot 100 (3)

We noticed that the calculations of average charge state couldalso be done using a peak value of the signal for each chargestate Qpeak

z with similar results Breeding efficiency into thecharge state with the highest abundance ηzmax is defined as aratio of the number of extracted ions with highest abundance tothe number of injected Cs+ ions and it can be found from theexpression ηzmax = ηtot middot Amax

z where ηtot is the total transmis-sion efficiency of cesium ions The above mentioned methodincludes an error related to the measurements of the totalcharge of residual gas ions with and without cesium injectionDue to the interaction of cesium ions with the ions of residualgas the total extracted charge of residual gas ions is slightlymodified From the comparison of the spectra of the residualgas ions (Fig 8(a)) and cesium beam (Fig 9(b)) it is clear thatthe ions of the residual gas are well separated with respect tothe cesium spectrum Therefore the relative change of the totalcharge of the residual gas ξ extracted with and without cesiuminjection can be approximately estimated as ξ = 2 Σ1minusΣ2

Σ1+Σ2 where

Σ1 and Σ2 are the integrated charges of the residual gas ionsin the spectra with and without cesium injection respectivelyThe calculations show that ξ = 22

In addition we have performed direct measurements ofthe cesium charge in a single charge state after the bendingmagnet This is possible for charge states below sim20 when thefull intensity in the single charge state can be transported to theFC5 with a 10 mm slit opening The breeding efficiency into asingle charge state can be calculated directly by measuring thecharge on FC5 This method results in the breeding efficiencyconsistent with the method described above and in Ref 20within plusmn25 Overall the systematic and measurement er-rors result in plusmn35 relative error of the breeding efficiencyevaluations

After several optimization rounds of cesium beam tun-ing the EBIS parameters and injection-extraction potentialsconsistently resulted in total injection-extraction efficienciesof sim70 and can be increased to 80 with elaborate beamtuning An example of FC3 measurements with and withoutinjection of the cesium beam is shown in Fig 10 Thesedata together with the FC2 reading of the injected cesium

FIG 10 FC3 signal for a single ion pulse without and with injection ofcesium beam

beam charge and average charge state calculations from thebeam spectrum measurements on FC5 were used for the totalbreeding efficiency calculation We found that the relativeabundance of the charge state 28+ was 27 for a 28 msbreeding time as is shown in Fig 11 Therefore the absolutebreeding efficiency of cesium ions into charge state 28+ is20 It is interesting to note that we observed a higher abun-dance of charge state 28+ as compared to the CBSIM simula-tions25 Based on these measurements the EBIS should pro-videge20 breeding efficiency of CARIBU rare isotope beamsbecause the expected CARIBU beam emittance is severaltimes smaller than the cesium beam emittance We should beable to reach an absolute breeding efficiency ofsim27 when theEBIS CB is relocated and coupled to CARIBU These exper-iments clearly demonstrate fast and efficient charge breed-ing of cesium ions As it follows from the CBSIM simu-lations any ion in the full mass range of the periodic tablecan be charge bred with high efficiency within short breed-ing times to a sufficiently high charge-to-mass ratio (ge163for the heaviest masses) for further acceleration in ATLASwith the presently demonstrated parameters of the CARIBUEBIS

Fig 12 shows the average charge state of charge bredions as a function of breeding time The simulations with

FIG 11 Measured and calculated using CBSIM code relative abundancesfor 28 ms breeding time

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-6 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 6 A typical cesium beam image on the phosphor screen of the pepper pot emittance meter (a) and its emittance ((b) and (c)) The distance between thepepper-pot holes is 05 mm The color code corresponds to a linear incremental change of the density from 0 to 10 The central red area corresponds to thedensity higher than 10

Faraday cups There are 3 Faraday cups (FC1 FC2 and FC3)with 46 mm input apertures installed on air cylinders (seepositions 9 12 and 17 in Fig 3) and a stationary cup (FC5at position 19 in Fig 3) installed downstream of adjustableslits after the bending magnet FC1 and FC2 are used forthe measurements of the injected cesium beam charge Basedon our previous experience with the current measurementsof multi-charge ion beams the electron suppression voltageis minus300 V The accuracy of beam charge measurements isplusmn1 As is known multi-charge beams can produce energeticAuger electrons that can escape the FC Our measurementswith ATLAS multi-charge beams show that the contributionof energetic electrons to the measurement error is less than04 if a minus300 V biased voltage is applied Therefore it is ourcommon practice to apply minus300 V to all Faraday cups used forthe measurements of multi-charge ion beams A typical FC2signal from the pulsed cesium beam is shown in Fig 7 Thetotal charge is calculated as Qtot = VpeakCFC where Vpeak isthe peak voltage and CFC is the capacitance of the FC andsignal cables Calculation of the charge by area integrationresults in the same value within a plusmn1 error For chargebreeding efficiency measurements we use FCs with identicalcapacitances to avoid error due to different decay times of thevoltages during the charge accumulation over the pulse length

FIG 7 Cesium beam signal reading on the FC2 prior to injection into theEBIS

A pepper-pot emittance probe based on a Multi-ChannelPlate (MCP) and a phosphor screen (position 10 in Fig 3)23

was used to measure the emittance of the injected cesiumbeam Due to the very high sensitivity of the MCP very lowinjected cesium beam intensities sim01 pC per pulse can beanalyzed by the emittance probe

D Electron beam commissioning

The stability of high current-high duty cycle electronbeam in the EBIS is the key for breeding of ions with highefficiency The set of electron beam diagnostics and results ofthe initial electron beam commissioning at 4 T magnetic fieldwere reported in the previous publication12 The time structureof the electron beam was formed by applying a pulsed highvoltage potential to the e-gun anode Optimal transport of theelectron beam with very low relative losses below 10minus4 fromthe e-gun cathode to the collector was established by tuning oftwo sets of X and Y steering dipole coils installed on vacuumchamber crosses in both sides of the superconducting (SC)solenoid and 4 pairs of magnetic steering coils installed onthe beam tube inside the SC solenoid bore In addition thecollector solenoid field is tuned to minimize the electron beamlosses The availability of steering coils is essential for the100 transmission of the electron beam The tuning of theelectron beam is complete when vacuum gauges do not showany pressure fluctuations Currently we can stably operate 16A electron beam at 40 duty cycle or 115 A electron beam at90 duty cycle Increasing either current or duty cycle of theelectron beam requires long vacuum conditioning

IV RESIDUAL GAS IONIZATION

When the barrier electrode potential is raised (position 5in Fig 5) ionization of residual gas atoms and molecules takesplace in the trap The charge bred ions of the residual gas areextracted and analyzed using 70 bending magnet In Sec Vof this paper we discuss experimental data taken from FC5 formultiple pulses while the dipole current was varied The beamcharge values delivered in each pulse were generally stablebut we used averaging of the signal over several pulses whenthis was not the case Fig 8 shows spectra of the residual gasfor two different confinement times for the EBIS operational

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-7 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 8 Residual gas spectrum at 30 ms (a) and 65 ms (b) of confinement time Ion pulse charge is Q[pC]= 019timesV[mV]

parameters shown in Table I To increase mass resolution thebeam was collimated upstream of the FC5 The residual gascontains ions of hydrogen carbon and oxygen The relativecontribution of nitrogen is very low We do not see molecularions in the spectrum The preferable values of charge-to-massratios for CARIBU beams for the extraction separation andfurther acceleration in ATLAS are in the range from 17 to14 These ratios can be achieved with breeding times from10 to 30 ms and appear on FC5 approximately in the range ofmagnet currents from 36 to 47 A By adjusting the breedingtime the most abundant charge state of a rare isotope ionbeam can be located between residual gas peaks and separatedfrom the residual gas contamination in the transport systemdownstream of the EBIS We plan a detailed study of possiblecontaminants from the residual gas in the region between O3+

and O4+with upgraded Faraday cups and amplifiers to measurefemtocoulomb-level charges

The capacity of the trap is calculated as CTRAP =IeLTRAP

ve= 23 nC where Ie is the electron beam current ve is the elect-ron velocity in the trap and LTRAP is the length of the trapFor typical confinement times le30 ms required for CARIBU

beams the neutralization factor is below 2 The total ex-tracted charge of residual gas ions does not exceed sim7 ofthe total charge capacity of the trap for confinement times upto 150 ms

V CESIUM BREEDING EFFICIENCY MEASUREMENTS

The main goal of the off-line commissioning was todemonstrate stable operation of EBIS at a 10 Hz repetitionrate and a breeding efficiency into single charge state higherthan 15 These goals have been successfully achieved andexceeded The breeding efficiency measurements were per-formed according to the procedure described in detail inRef 24 This procedure uses measurements of total chargeon FC3 with and without cesium injection The total numberof extracted cesium ions was calculated as a ratio of cesiumcharge on FC3 to the average charge state of the cesium beamThe latter was found from the measured cesium spectrumobtained on FC5 with 1 or 2 mm wide slits Examples of thecesium spectra for 20 and 28 ms breeding times for the EBISoperational parameters listed in Table I are shown in Fig 9 For

FIG 9 Spectra of charge-bred cesium and residual gas ions for two breeding times 20 ms (a) and 28 ms (b) This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-8 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

these measurements to obtain a good resolution of differentcharge states the slits upstream of the FC5 were closed to2 mm The average charge state z was calculated using theexpression

z =

zmaxz=zmin

Qz

zmaxz=zmin

Qz

z

(2)

where Qz is the total charge of ions at a particular chargestate z zmin and zmax are the minimum and maximum chargestates correspondingly in the spectrum of charge-bred ce-sium ions The total charge Qz was calculated as an areaunder the Gaussian distribution obtained from the fit to themeasured distribution The relative abundance of cesium ionswith different charge states is defined by the expression

Az =Qz

z middotzmax

z=zmin

Qz

z

middot 100 (3)

We noticed that the calculations of average charge state couldalso be done using a peak value of the signal for each chargestate Qpeak

z with similar results Breeding efficiency into thecharge state with the highest abundance ηzmax is defined as aratio of the number of extracted ions with highest abundance tothe number of injected Cs+ ions and it can be found from theexpression ηzmax = ηtot middot Amax

z where ηtot is the total transmis-sion efficiency of cesium ions The above mentioned methodincludes an error related to the measurements of the totalcharge of residual gas ions with and without cesium injectionDue to the interaction of cesium ions with the ions of residualgas the total extracted charge of residual gas ions is slightlymodified From the comparison of the spectra of the residualgas ions (Fig 8(a)) and cesium beam (Fig 9(b)) it is clear thatthe ions of the residual gas are well separated with respect tothe cesium spectrum Therefore the relative change of the totalcharge of the residual gas ξ extracted with and without cesiuminjection can be approximately estimated as ξ = 2 Σ1minusΣ2

Σ1+Σ2 where

Σ1 and Σ2 are the integrated charges of the residual gas ionsin the spectra with and without cesium injection respectivelyThe calculations show that ξ = 22

In addition we have performed direct measurements ofthe cesium charge in a single charge state after the bendingmagnet This is possible for charge states below sim20 when thefull intensity in the single charge state can be transported to theFC5 with a 10 mm slit opening The breeding efficiency into asingle charge state can be calculated directly by measuring thecharge on FC5 This method results in the breeding efficiencyconsistent with the method described above and in Ref 20within plusmn25 Overall the systematic and measurement er-rors result in plusmn35 relative error of the breeding efficiencyevaluations

After several optimization rounds of cesium beam tun-ing the EBIS parameters and injection-extraction potentialsconsistently resulted in total injection-extraction efficienciesof sim70 and can be increased to 80 with elaborate beamtuning An example of FC3 measurements with and withoutinjection of the cesium beam is shown in Fig 10 Thesedata together with the FC2 reading of the injected cesium

FIG 10 FC3 signal for a single ion pulse without and with injection ofcesium beam

beam charge and average charge state calculations from thebeam spectrum measurements on FC5 were used for the totalbreeding efficiency calculation We found that the relativeabundance of the charge state 28+ was 27 for a 28 msbreeding time as is shown in Fig 11 Therefore the absolutebreeding efficiency of cesium ions into charge state 28+ is20 It is interesting to note that we observed a higher abun-dance of charge state 28+ as compared to the CBSIM simula-tions25 Based on these measurements the EBIS should pro-videge20 breeding efficiency of CARIBU rare isotope beamsbecause the expected CARIBU beam emittance is severaltimes smaller than the cesium beam emittance We should beable to reach an absolute breeding efficiency ofsim27 when theEBIS CB is relocated and coupled to CARIBU These exper-iments clearly demonstrate fast and efficient charge breed-ing of cesium ions As it follows from the CBSIM simu-lations any ion in the full mass range of the periodic tablecan be charge bred with high efficiency within short breed-ing times to a sufficiently high charge-to-mass ratio (ge163for the heaviest masses) for further acceleration in ATLASwith the presently demonstrated parameters of the CARIBUEBIS

Fig 12 shows the average charge state of charge bredions as a function of breeding time The simulations with

FIG 11 Measured and calculated using CBSIM code relative abundancesfor 28 ms breeding time

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-7 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 8 Residual gas spectrum at 30 ms (a) and 65 ms (b) of confinement time Ion pulse charge is Q[pC]= 019timesV[mV]

parameters shown in Table I To increase mass resolution thebeam was collimated upstream of the FC5 The residual gascontains ions of hydrogen carbon and oxygen The relativecontribution of nitrogen is very low We do not see molecularions in the spectrum The preferable values of charge-to-massratios for CARIBU beams for the extraction separation andfurther acceleration in ATLAS are in the range from 17 to14 These ratios can be achieved with breeding times from10 to 30 ms and appear on FC5 approximately in the range ofmagnet currents from 36 to 47 A By adjusting the breedingtime the most abundant charge state of a rare isotope ionbeam can be located between residual gas peaks and separatedfrom the residual gas contamination in the transport systemdownstream of the EBIS We plan a detailed study of possiblecontaminants from the residual gas in the region between O3+

and O4+with upgraded Faraday cups and amplifiers to measurefemtocoulomb-level charges

The capacity of the trap is calculated as CTRAP =IeLTRAP

ve= 23 nC where Ie is the electron beam current ve is the elect-ron velocity in the trap and LTRAP is the length of the trapFor typical confinement times le30 ms required for CARIBU

beams the neutralization factor is below 2 The total ex-tracted charge of residual gas ions does not exceed sim7 ofthe total charge capacity of the trap for confinement times upto 150 ms

V CESIUM BREEDING EFFICIENCY MEASUREMENTS

The main goal of the off-line commissioning was todemonstrate stable operation of EBIS at a 10 Hz repetitionrate and a breeding efficiency into single charge state higherthan 15 These goals have been successfully achieved andexceeded The breeding efficiency measurements were per-formed according to the procedure described in detail inRef 24 This procedure uses measurements of total chargeon FC3 with and without cesium injection The total numberof extracted cesium ions was calculated as a ratio of cesiumcharge on FC3 to the average charge state of the cesium beamThe latter was found from the measured cesium spectrumobtained on FC5 with 1 or 2 mm wide slits Examples of thecesium spectra for 20 and 28 ms breeding times for the EBISoperational parameters listed in Table I are shown in Fig 9 For

FIG 9 Spectra of charge-bred cesium and residual gas ions for two breeding times 20 ms (a) and 28 ms (b) This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-8 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

these measurements to obtain a good resolution of differentcharge states the slits upstream of the FC5 were closed to2 mm The average charge state z was calculated using theexpression

z =

zmaxz=zmin

Qz

zmaxz=zmin

Qz

z

(2)

where Qz is the total charge of ions at a particular chargestate z zmin and zmax are the minimum and maximum chargestates correspondingly in the spectrum of charge-bred ce-sium ions The total charge Qz was calculated as an areaunder the Gaussian distribution obtained from the fit to themeasured distribution The relative abundance of cesium ionswith different charge states is defined by the expression

Az =Qz

z middotzmax

z=zmin

Qz

z

middot 100 (3)

We noticed that the calculations of average charge state couldalso be done using a peak value of the signal for each chargestate Qpeak

z with similar results Breeding efficiency into thecharge state with the highest abundance ηzmax is defined as aratio of the number of extracted ions with highest abundance tothe number of injected Cs+ ions and it can be found from theexpression ηzmax = ηtot middot Amax

z where ηtot is the total transmis-sion efficiency of cesium ions The above mentioned methodincludes an error related to the measurements of the totalcharge of residual gas ions with and without cesium injectionDue to the interaction of cesium ions with the ions of residualgas the total extracted charge of residual gas ions is slightlymodified From the comparison of the spectra of the residualgas ions (Fig 8(a)) and cesium beam (Fig 9(b)) it is clear thatthe ions of the residual gas are well separated with respect tothe cesium spectrum Therefore the relative change of the totalcharge of the residual gas ξ extracted with and without cesiuminjection can be approximately estimated as ξ = 2 Σ1minusΣ2

Σ1+Σ2 where

Σ1 and Σ2 are the integrated charges of the residual gas ionsin the spectra with and without cesium injection respectivelyThe calculations show that ξ = 22

In addition we have performed direct measurements ofthe cesium charge in a single charge state after the bendingmagnet This is possible for charge states below sim20 when thefull intensity in the single charge state can be transported to theFC5 with a 10 mm slit opening The breeding efficiency into asingle charge state can be calculated directly by measuring thecharge on FC5 This method results in the breeding efficiencyconsistent with the method described above and in Ref 20within plusmn25 Overall the systematic and measurement er-rors result in plusmn35 relative error of the breeding efficiencyevaluations

After several optimization rounds of cesium beam tun-ing the EBIS parameters and injection-extraction potentialsconsistently resulted in total injection-extraction efficienciesof sim70 and can be increased to 80 with elaborate beamtuning An example of FC3 measurements with and withoutinjection of the cesium beam is shown in Fig 10 Thesedata together with the FC2 reading of the injected cesium

FIG 10 FC3 signal for a single ion pulse without and with injection ofcesium beam

beam charge and average charge state calculations from thebeam spectrum measurements on FC5 were used for the totalbreeding efficiency calculation We found that the relativeabundance of the charge state 28+ was 27 for a 28 msbreeding time as is shown in Fig 11 Therefore the absolutebreeding efficiency of cesium ions into charge state 28+ is20 It is interesting to note that we observed a higher abun-dance of charge state 28+ as compared to the CBSIM simula-tions25 Based on these measurements the EBIS should pro-videge20 breeding efficiency of CARIBU rare isotope beamsbecause the expected CARIBU beam emittance is severaltimes smaller than the cesium beam emittance We should beable to reach an absolute breeding efficiency ofsim27 when theEBIS CB is relocated and coupled to CARIBU These exper-iments clearly demonstrate fast and efficient charge breed-ing of cesium ions As it follows from the CBSIM simu-lations any ion in the full mass range of the periodic tablecan be charge bred with high efficiency within short breed-ing times to a sufficiently high charge-to-mass ratio (ge163for the heaviest masses) for further acceleration in ATLASwith the presently demonstrated parameters of the CARIBUEBIS

Fig 12 shows the average charge state of charge bredions as a function of breeding time The simulations with

FIG 11 Measured and calculated using CBSIM code relative abundancesfor 28 ms breeding time

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-8 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

these measurements to obtain a good resolution of differentcharge states the slits upstream of the FC5 were closed to2 mm The average charge state z was calculated using theexpression

z =

zmaxz=zmin

Qz

zmaxz=zmin

Qz

z

(2)

where Qz is the total charge of ions at a particular chargestate z zmin and zmax are the minimum and maximum chargestates correspondingly in the spectrum of charge-bred ce-sium ions The total charge Qz was calculated as an areaunder the Gaussian distribution obtained from the fit to themeasured distribution The relative abundance of cesium ionswith different charge states is defined by the expression

Az =Qz

z middotzmax

z=zmin

Qz

z

middot 100 (3)

We noticed that the calculations of average charge state couldalso be done using a peak value of the signal for each chargestate Qpeak

z with similar results Breeding efficiency into thecharge state with the highest abundance ηzmax is defined as aratio of the number of extracted ions with highest abundance tothe number of injected Cs+ ions and it can be found from theexpression ηzmax = ηtot middot Amax

z where ηtot is the total transmis-sion efficiency of cesium ions The above mentioned methodincludes an error related to the measurements of the totalcharge of residual gas ions with and without cesium injectionDue to the interaction of cesium ions with the ions of residualgas the total extracted charge of residual gas ions is slightlymodified From the comparison of the spectra of the residualgas ions (Fig 8(a)) and cesium beam (Fig 9(b)) it is clear thatthe ions of the residual gas are well separated with respect tothe cesium spectrum Therefore the relative change of the totalcharge of the residual gas ξ extracted with and without cesiuminjection can be approximately estimated as ξ = 2 Σ1minusΣ2

Σ1+Σ2 where

Σ1 and Σ2 are the integrated charges of the residual gas ionsin the spectra with and without cesium injection respectivelyThe calculations show that ξ = 22

In addition we have performed direct measurements ofthe cesium charge in a single charge state after the bendingmagnet This is possible for charge states below sim20 when thefull intensity in the single charge state can be transported to theFC5 with a 10 mm slit opening The breeding efficiency into asingle charge state can be calculated directly by measuring thecharge on FC5 This method results in the breeding efficiencyconsistent with the method described above and in Ref 20within plusmn25 Overall the systematic and measurement er-rors result in plusmn35 relative error of the breeding efficiencyevaluations

After several optimization rounds of cesium beam tun-ing the EBIS parameters and injection-extraction potentialsconsistently resulted in total injection-extraction efficienciesof sim70 and can be increased to 80 with elaborate beamtuning An example of FC3 measurements with and withoutinjection of the cesium beam is shown in Fig 10 Thesedata together with the FC2 reading of the injected cesium

FIG 10 FC3 signal for a single ion pulse without and with injection ofcesium beam

beam charge and average charge state calculations from thebeam spectrum measurements on FC5 were used for the totalbreeding efficiency calculation We found that the relativeabundance of the charge state 28+ was 27 for a 28 msbreeding time as is shown in Fig 11 Therefore the absolutebreeding efficiency of cesium ions into charge state 28+ is20 It is interesting to note that we observed a higher abun-dance of charge state 28+ as compared to the CBSIM simula-tions25 Based on these measurements the EBIS should pro-videge20 breeding efficiency of CARIBU rare isotope beamsbecause the expected CARIBU beam emittance is severaltimes smaller than the cesium beam emittance We should beable to reach an absolute breeding efficiency ofsim27 when theEBIS CB is relocated and coupled to CARIBU These exper-iments clearly demonstrate fast and efficient charge breed-ing of cesium ions As it follows from the CBSIM simu-lations any ion in the full mass range of the periodic tablecan be charge bred with high efficiency within short breed-ing times to a sufficiently high charge-to-mass ratio (ge163for the heaviest masses) for further acceleration in ATLASwith the presently demonstrated parameters of the CARIBUEBIS

Fig 12 shows the average charge state of charge bredions as a function of breeding time The simulations with

FIG 11 Measured and calculated using CBSIM code relative abundancesfor 28 ms breeding time

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-9 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 12 Average charge state as a function of breeding time

CBSIM code25 are also shown in this plot The CBSIM simula-tions describe the experimental data well with some tendencyof higher average charge states at longer breeding times ascompared to the measurements

Another practical parameter is the relative abundance ofthe ions in the charge state distribution Obviously we arelooking for the highest abundance into a single charge stateFig 13 shows the measured and simulated highest abundancein the charge state distribution as a function of breeding timeFor short breeding times le15 ms we observe lower breed-ing efficiency for the most abundant charge states while theaverage charge state is in agreement with the CBSIM simula-tions Possible explanation for this is the injection of a slightlymisaligned ion beam with respect to the electron beam Anexplanation for higher abundance in charge states 27+ and 28+as compared to the CBSIM simulations requires additionalstudies

Emittance and energy spread of the extracted and chargeselected ion beam are important parameters for the followingtransport and matching to ATLAS In the off-line EBIS setupwe do not plan to measure the emittance of the extractedbeam due to the lack of space for charge selection The emit-tance of the extracted beam in EBIS has been measured atthe BNL high-current EBIS26 and REX-ISOLDE EBIS22 atCERN Emittance studies at REX-ISOLDE have shown thatthe emittance is a weak function of the magnetic field in non-neutralized beams which is the case of CARIBU EBIS Alsothe emittance is a strong function of the charge-to-mass ratio

FIG 13 Measured (dots) and simulated (solid line) highest relative abun-dance as a function of breeding time

FIG 14 Cs17+ beam intensity distribution as a function of magnet currentBreeding time is 5 ms

the lower this ratio the lower the emittance On the basis ofREX-ISOLDErsquos simulations we have approximated the fullnormalized CARIBU EBIS emittance as 005π timesmm timesmradfor zA = 17133 With an approximate transverse emittanceof the extracted beam the energy spread can be estimated byfitting simulated data to the cesium beam spectrum and beamprofile after the magnet and slits For short breeding times(lower charge states) individual charge states can be fullyseparated after the slits as is shown in Fig 14 A 10 mm slitopening allowed us to select an individual charge state at fullintensity The intensity of the beam for a given charge state onFC5 was measured as the slit width was varied The differen-tiation of this curve produces the horizontal beam profile asis shown in Fig 15 By varying the beam energy spread inthe TRACK simulation code27 we fit simulated data to themeasured intensity distribution in a single charge state as isshown in Fig 14 Also a simulated beam profile was fit to themeasured one (Fig 15) These fits gave us a beam rms energyspread as σE asymp 184 eV per charge state z This results in afull energy spread of 6σE = 110 eVz As is well-known theupper estimate of the full energy spread is equal to the depthof the potential well inside the electron beam2228 However inour measurements for a low breeding time of 5 ms we observea factor of sim3 lower energy spread than the potential well of299 V shown in Table I

Currently the CARIBU beams at ATLAS are charge bredusing an ECR charge breeder The breeding efficiency of the

FIG 15 Cs17+ beam profile downstream of the magnet Breeding time is5 ms

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-10 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

FIG 16 The breeding efficiency of the cesium beam as a function of the biasvoltage deviation from the optimal voltage

ECR is quite sensitive to the voltage difference between theCARIBU gas catcher and ECR charge breeder29 To find asimilar sensitivity of the EBIS to the energy of the injectedbeam we have measured the cesium beam intensity on FC5while the cesium source bias was varied The results are shownin Fig 16 In the range of plusmn15V there is no change in breed-ing efficiency Therefore the sensitivity of the EBIS to theenergy of the injection singly charged ions is several timesless than in the current ECR charge breeder This is anotheradvantage of using the EBIS as a charge breeder instead of theECR

VI CONCLUSIONS

The CARIBU EBIS CB has been successfully commis-sioned off-line with the external singly charged cesium ionsource The performance of the EBIS fully meets specifica-tions to breed rare isotope beams delivered from CARIBUThe EBIS can provide charge-to-mass ratios ge17 for allCARIBU beams with low breeding times in the range from5 to 30 ms Up to a 20 breeding efficiency into singlecharge state of cesium has been demonstrated The over-all transmission of cesium beam through the EBIS is 70routinely The transport efficiency of the charge bred cesiumbeam is lower than 100 primarily due to the larger emit-tance of the cesium beam than the EBIS acceptance Evengreater breeding efficiencies ge25 are expected for the smallemittance beams from CARIBU The EBIS is ready to berelocated and integrated into ATLAS and CARIBU Breedingof CARIBU beams at a 10 Hz repetition rate is availableimmediately Tuning and conditioning of the EBIS at dutycycles higher than 50 and repetition rates up to 30 Hz is underway

Overall success of the CARIBU EBIS commissioningis the result of implementation of many innovative designfeatures together with application of the state-of-the-art tech-niques and procedures during the design and mechanical as-sembly The major precursors to the successful commission-ing were ultra-high vacuum high electron beam current with

relatively low energy in the trap optimized linear focusing andsteering fields in the injection and extraction region excel-lent alignment of the electron and ion beam transport systemcomponents availability of the 8 pairs of dipole steering coilsfor the transport of electron beam and availability of thetracking codes for the study of ion beam optics in a wide rangeof parameter space The energy spread of charge-bred ions hasbeen determined by using original beam tracking code andfitting of the simulated ion beam distribution to the measureddata

ACKNOWLEDGMENTS

This work was supported by the US Department of En-ergy Office of Nuclear Physics under Contract No DE-AC02-06CH11357 We express our gratitude to Dr S A Kondra-shev A Levand G Cherry and W Jansma for the significantcontribution during the design and assembly of the CARIBUEBIS

1G Savard S Baker C Davids A F Levand E F Moore R C Pardo RVondrasek B J Zabransky and G Zinkann Nucl Instrum Methods PhysRes Sect B 266 4086 (2008)

2R Vondrasek A Kolomiets A Levand R Pardo G Savard and R ScottRev Sci Instrum 82 053301 (2011)

3P N Ostroumov S Kondrashev R Pardo G Savard R Vondrasek JAlessi E Beebe and A Pikin J Instrum 5 C07004 (2010)

4R C Pardo G Savard S Baker C Davids E F Moore R Vondrasekand G Zinkann Nucl Instrum Methods Phys Res Sect B 261 965(2007)

5C Dickerson and C Peters AIP Conf Proc 1640 136 (2015)6F J C Wenander Nucl Instrum Methods Phys Res Sect B 2664346ndash4353 (2008)

7A Lapierre M Brodeur T Brunnea S Ettenauer A T Gallant V SimonM Good M W Froese J R Crespo Loacutepez-Urrutia P Delheij S Epp RRingle S Schwarz J Ullrich and J Dilling Nucl Instrum Methods PhysRes Sect A 624(1) 54ndash64 (2010)

8A Lapierre S Schwarz T M Baumann K Cooper K Kittimanapun AJ Rodriguez C Sumithrarachchi S J Williams W Wittmer D Leitnerand G Bollen Rev Sci Instrum 85 02B701 (2014)

9A Pikin J G Alessi E N Beebe A Kponou R Lambiase R LockeyD Raparia J Ritter L Snydstrup and Y Tan J Instrum 5 C09003(2010)

10C Dickerson B Mustapha A Pikin S Kondrashev P Ostroumov ALevand and R Fischer Phys Rev Spec TopndashAccel Beams 16 024201(2013)

11S Kondrashev C Dickerson A Levand P N Ostroumov R C Pardo GSavard R Vondrasek J Alessi E Beebe A Pikin G I Kuznetsov and MA Batazova Rev Sci Instrum 83 02A902 (2012)

12S Kondrashev A Barcikowski C Dickerson R Fischer P N OstroumovR Vondrasek and A Pikin Rev Sci Instrum 85 02B901 (2014)

13S Kondrashev C Dickerson A Levand P N Ostroumov R VondrasekA Pikin G I Kuznetsov and M A Batazova in Proceedings of the 12thHeavy Ion Accelerator Technology Conference Chicago 18-21 June 2012(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2012)pp 165ndash169

14RE Marrs et al ldquoMeasurement of electron-impact-excitation cross sec-tions for very highly charged ionsrdquo Phys Rev Lett 60 1715ndash1718(1988)

15M V Nezlin Physics of Intense Beams in Plasmas (IOP Publishing Ltd1993)

16C Benvenuti in Proceedings of the 6th European Particle AcceleratorConference Stockholm Sweden (IOP Publishing 1998) p 200

17P Mandal G Sikler and M Mukherjee J Instrum 6 P02004 (2011)18See httpwwwcstde for CST EM- MW-Studio CST GmbH Darmstadt

Germany19S A Kondrashev A Barcikowski C Dickerson P N Ostroumov S

Sharamentov R Vondrasek and A Pikin AIP Conf Proc 1640 54(2015)

This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at httpscitationnewaiporgtermsconditions Downloaded to IP

1283131212 On Mon 25 Jan 2016 162631

083311-11 Ostroumov et al Rev Sci Instrum 86 083311 (2015)

20See httpwwwcathodecom for HeatWave Labs Inc21See httpwww behlkecom for Behlke Power Electronics GmbH22F J C Wenander ldquoCharge breeding and production of multiply charged

ions in EBIS and ECRISrdquo Thesis for the degree of Doctor of PhilosophyChalmers University of Technology Goumlteborg Sweden 2001

23S Kondrashev A Barcikowski A Levand P N Ostroumov R PardoG Savard R H Scott T Sun R Vondrasek and G Zinkann in Pro-ceedings of Linear Accelerator Conference LINAC2010 Tsukuba Japan(Joint Accelerator Conferences Website (JACoW) wwwjacoworg 2010)

24S Kondrashev J G Alessi E N Beebe C Dickerson P N OstroumovA Pikin and G Savard Nucl Instrum Methods Phys Res Sect A 64218ndash24 (2011)

25R Becker O Kester and Th Stoehlker J Phys Conf Ser 58 443ndash446(2007)

26A Pikin J G Alessi E Beebe O Gould D Graham A Kponou KPrelec J Ritter and V Zajic in Proceedings of the 8th European ParticleAccelerator Conference Paris France (Joint Accelerator ConferencesWebsite (JACoW) wwwjacoworg 2002) pp 1732ndash1734

27P N Ostroumov V Aseev and B Mustapha Phys Rev Spec TopndashAccelBeams 7 090101 (2004)

28A Pikin J G Alessi E N Beebe A Kponou and K Prelec Rev SciInstrum 77 03A910 (2006)

29R Vondrasek A Levand R Pardo G Savard and R Scott Rev SciInstrum 83 02A913 (2012)

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