characterization of the iota proton source
TRANSCRIPT
Characterization of the IOTA Proton SourceLee Teng Internship Program Summer 2017
Samantha Young / Mentor: Kermit CarlsonLoyola University Chicago
August 11, 2017
AbstractThis project focuses on characterizing the IOTA proton source through changing the parameters of four
various components of the Low Energy Beam Transport (LEBT). Because of an inefficient filament, currentwas limited to 2 mA when 40 mA is ultimately desired. Through an investigation of the solenoids and trimsof the LEBT, we sought more knowledge about the optimum settings for running the IOTA proton source.
1 Introduction
The Integral-Optics Test Accelerator (IOTA)proton injector emerged from the setups at theHigh Intensity Neutrino Source (HINS) facilityat the Meson Detector Building (MDB) [1]. TheHINS radio frequency quadrupole (RFQ) wasperformance limited, but the beam was sufficientfor experiments. Once completed, the ion sourcewill include the HINS ion source, a low energybeam transport (LEBT), a RFQ, and a bunchercavity. These will generate a 2.5 MeV protonbeam for injection into the storage ring. Thispaper focuses on the ion source and the LEBT,not the full proton injector. Since the RFQ cav-ity is currently not in use, the beam is character-ized to the Faraday cup located past the sourcemagnet and the two beamline solenoids that aimto sculpt and direct the beam of ions. Once thebeam is characterized, the components will bemoved to the FAST enclosure at the New MuonLab (NML) for full source commissioning. Formore information on the specific components ofthe ion source, see section 2.
Protons and ions are drawn out to be injectedinto the RFQ through a change in potential thatgoes from negative to positive. It has been shown
to be more efficient in energizing than going froma ground to ground state [5]. The priority was toparameterize the three solenoids and the sourcepressure. However, physical changes also oc-curred throughout the course of testing. Theoriginal filament, used to date, was installed as apart of the HINS project and has likely been ex-posed to atmosphere, severely limiting the sourceoutput. Efforts are underway to replace this, butthe following data reflects the original filament.This is alright at this stage, as the goal was tosuccessfully produce beam current and investi-gate operating parameters.
By the end of June 2017, beam current wasformed and the power supply reached 20 kV,which will be the limit for a while due to safetyconcerns. The goal is to eventually bring thepower supply to 30-40 kV. 2 mA of current wasachieved, a significantly higher output than atthe beginning of testing.
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2 Equipment/Methodology
Figure 1: Low Energy Beam Transport(LEBT).
The Faraday cup is located in place of theRFQ depicted above in Figure 1. The Fara-day cup leads the beam through a 10 kΩ re-sistor, allowing the oscilloscope to measure cur-rent. While connected to the ACNET networkand available for added parameters, there is cur-rently no current added to the toroid depicted inthe middle. The LeCroy oscilloscope offers av-eraging techniques, allowing for a cleaner wave-form. These tests used 10-20 pulses per averagedpoint.
Figure 2: Waveform at 2.0 mA. Blue tracerepresents beam current and the red trace
represents current integral (Vs).
Figure 3: The source scope in the high voltagerack. Channel 1 is the arc current, channel 2 isthe arc voltage, and channel 3 is the arc trigger.Any toroid output is below the chart threshold,
which exists if beam current is formed. Thedownward-curved shape of the cyan trace shows
that the extractor is current limited.
2.1 Duoplasmatron Source
Figure 4: Diagram of the ion source.
The ion source components are from the for-mer HINS program, but new power supplies wererequired as well as configuration of the sourceconnections in order to restore operation. Thisincluded copper bands that initially acted as ter-minals on the stages of the acceleration column.The bands initially used for connections alongthe accelerating column were found to be in-sufficient at 20 kV, so aluminum balls with setscrews were fabricated to provide better contactand avoid arcing due to corona or loose connec-tions. These additions were completed later inthe study, and most likely alleviated some of thesparking experienced earlier. A duoplasmatronsource is advantageous as it allows for a largebeam intensity, high gas efficiency, and a capa-bility to produce negative and multicharged ions
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[7]. In the state of initial testing, the source func-tioned properly but lacked more rounded clampsand a newly coated filament. The main purposeof this construction was to see if current was fea-sible. This preliminary construction also allowedfor prediction of future problems, as well as theability to gauge roughly where limits are in re-gards to operation. The choice of 20 kV comesfrom consultation with ESHQ, so it is possibleto increase the high voltage in the future, giventhat communication occurs.
The starting point was the observation fromearlier studies that the three main factors foroperation were the geometry of the source, thesource pressure, and the cathode/anode dis-charge [7]. Since the geometry is fixed, thisproject focuses on varying the source pressure,beamline solenoids, and source solenoid. Insidethe duoplasmatron, the cathode is depressed to-20 kV, forming a plasma that flows towards anopen space right before the anode. Electrons arerepelled at the negatively charged anode, whilethe ions flow through the small hole. In the freespace the ion plasma begins to grow in size, un-til it reaches the source solenoid. With propertuning, extraction is maximized, allowing for astrong beam current.
2.2 Filament Coating
Figure 5: Coating a filament to test theprocedure. This filament will temporarily
replace the coated one.
The source filament uses a variety of chemi-cals to facilitate production of a plasma. Ions aredrawn from this plasma, so a large production isdesirable to produce a beam strong enough tomeasure in mA. When increasing the filamentcurrent, it is important to keep a check on thevacuum pressure, as increasing the current tooquickly will ionize the gas around it and increasethe vacuum pressure rapidly. As standard oper-ations suggest 105 mTorr for vacuum pressure, itis suggested to increase filament current slowlyso the PID can regulate the vacuum pressurebelow 125 mTorr. The cause for the vacuumchange is the excess material being burned offof the filament, so that a clean beam can be cre-ated. When current runs through the filament, itheats up, priming the filament coating to ionizethe hydrogen gas in the source. Because of thecomposition, compounds collect on the filamentwhen it is not in use. This results in outgassingfrom the filament as these compounds vaporizewith increasing current. During nominal opera-tion a PID loop keeps the source pressure regu-lated to within 1 mTorr of a set point. Allowingthe vacuum pressure to jump too high can causedamage to the duoplasmatron.
As seen in section 3, a poorly coated filamentgreatly affects proton production and beam cur-rent. The filament is constructed on a founda-tion of nickel mesh, which is then dipped into thecoating mixture [6]. At the base of the mixture isBaCO3 and SrCO3, which combined is known asa ”radio mixture”. The whole coating will be re-ferred to as Radio Mixture 3. The original mixthen has to be added to a liquid hydrocarbon,which in this case is banana oil. This is pairedwith multiple other extra chemicals to create thefinal mixture. The filament must be dipped anddried in this mixture multiple times, as a sin-gle coating leaves unfilled cracks. As of now thecoating is naturally dried, though it is possibleto heat dry. Once a new filament is installed,significantly more beam current is expected andless of an arc current limit (currently 1.98 A) onthe filament. It is expected to produce 40-60 mAof beam current and a full arc current of 25 A.
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2.3 Source Solenoid
Figure 6: Solenoid with open box.
This is located after the filament, serving asa shaper for the ions that are pulled out. A lowemittance is desired, so the magnetic field pro-duced by this solenoid condenses and shapes thebeam so that a ”thin” beam passes through intothe RF cavity. A low starting emittance is desir-able, as the beam will naturally expand once itenters the RFQ.
2.4 Faraday Cup
Figure 7: Faraday cup (thin cylindrical tube)leading to resistor through coax connections.
The Faraday cup is the end of our experi-mental section, attached static at the end of theLEBT. It is made of OFHC copper, measuring1” by 1.75” by 0.5”. The signal is directed outof the UHV system by means of a 0.051” diam-
eter stainless steel wire. The Faraday cup is at-tached remotely via a horizontal pneumatic ac-tuator, and the data pulled from this helps formthe plots and measurements found in the oscillo-scope [2].
2.5 ACL
The Accelerator Controls Network (ACNET)facilitates operation of the proton source. Accel-erator Control Language (ACL) code is writtenso that measurements can be taken automati-cally without having to record individual values.An example of a plot generated by hand can befound in section 3. Two separate codes were cre-ated to take traces and plots from the scope. Theroutines used are:
• Raster scan of the LEBT solenoids andheat map generation
• Continuous loops for faraday cup statistics
• Code to collect scope waveforms
Using these resources while changing source pa-rameters allows the raw data to be saved aswell as plotted so individual parameters could begraphed and measured in Excel appropriately.
2.6 Experimentation
The independent variables studied weresource solenoid current, beamline solenoid cur-rent, and source pressure. One parameter is var-ied at a time while leaving the rest of the pa-rameters at standard (see section 3). The peakof current is measured for this one parameter,which is considered the optimum setting. Theoptimum setting is applied for testing the nextvariable. The overall current peak for all opti-mum settings is the ideal conditions for operat-ing the source. These conditions could changewhen the source is rebuilt and the filament isrecoated.
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3 Results
3.1 Initial Scans
The data found below reflects tests run dur-ing week five. These were recorded by hand togain a level of comfort with operating the ionsource. Currents were calculated from Ohm’slaw with a voltage output and a 10 kΩ resistor.Current originally only reached to singular μA,and was now seeing values around .20-.25 mA.Readings are estimated based on the range ofoscillation, limiting accuracy.
Figure 8: Beam current as a result of changinghorizontal trims. Beamline solenoids turned off.
Figure 9: Changing horizontal trims. Beamlinesolenoids turned on.
Figure 10: Changing source solenoid current.
In week seven, the ion source was in the pro-cess of being rebooted after a ”spark” (see figure11). The spark caused all power sources to shutoff, and a PLC card and arc modulator bulk sup-ply had to be replaced as smoke could be smelledfrom the high voltage rack. Vacuum is seen torise drastically due to the reboot of the VATvalve. Since the pressure is regulated, it beganto decrease back to the 105 mTorr. There was adip in the high voltage power supply. The sourcepressure tripped the filament supply, and one in-stance of smoke caused the detector to trip allother supplies.
Figure 11: Device Readings at Time of Spark.
Connections were changed to prevent cur-rent running through the valve, which proba-bly contributed to the sparking. The aluminumballs were ready for installation, preventing fu-ture sparking. After alleviating these issues, a
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consistent beam was achieved with different ex-tractor behavior. While extractor current inter-feres with the strength of the beam current, itcan be accounted for through proper parameter-ization. At a less optimal setting, the extractorcurrent is seen to vary over a large time frame.The goal was to alter the pressure to see if thisvariation can be eliminated. Vacuum pressurewas varied by 10 mTorr while at 0.5A incrementsof the solenoid current. Source pressure is main-tained by a leak valve to the H2 bottle controlledthrough a PID controller. The change is not in-stantaneous so the resulting beam current shouldalways be compared to the source pressure readback rather than the set point. Below are theconditions where the highest beam current wasachieved.
Figure 12: Extractor current stabilizing (blue).
The extractor current, represented by thecyan line varies and then drops below the chartboundaries after optimizing. However, once thesource reached 125 mTorr on the vacuum pres-sure, the beam disappeared and there was a dipin the voltage. There was a failure in operatingthe modulator bulk supply, and this preventedfurther vacuum testing that day. The concernwas that continuing to use the old filament couldcause permanent damage to the duoplasmatron.Replacing the current filament with a freshlycoated one will allow for a cleaner beam anda controllable pulse that isn’t emission limited.However, steps are being taken to redo some ofthe electrical connections, as it is possible thatthe high voltage is not being distributed prop-erly.
3.2 Final Scans
These tests took place during week nine. Anew modulator bulk supply had been added, andthe source was more stable than during the previ-ous scans. It was possible to leave the high volt-age and filament current on so that the filamentremained outgassed. A simple tuning with thesource occurred where parameters were tunedwithout measurement, and this occurred witheach device to see the highest current possible.The source reached .7 mA of beam current, com-pared to the .25 mA achieved in the first weeks ofoperation. Since the source was stable, datalog-ging techniques were applied to perform scans onsingle parameters. First, the beamline solenoidswere investigated. Since the two solenoids workin tandem, a heat map was applied to find theoptimal combination of solenoid currents. In theheat map, the highlighted square represents thehighest beam current achieved, and the respec-tive solenoid currents can be traced back via theplot axes.
Figure 13: Heat map of beamline solenoids.
The first solenoid was then set to 395 A andthe second was set to 120 A. Current ran consis-tently between 1.5-1.6 mA. Leaving these param-eters, the source solenoid was scanned through atime plot where values were linearly decreased,starting from 2.5 A. There was a peak observedat 2.45 A, in which beam current reached 1.96mA. There were instances after the scan thatthe beam current reached over 2 mA at 2.45 Aof source magnet current. However, leaving the
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source solenoid at 2.4 A is not suggested as watertemperatures were reaching 40 C. Until heatingcapabilities are improved, the source should berun at less than 2 A. Source pressure was inves-tigated, and while current increased as pressureincreased, it is dangerous to increase the pres-sure, as apparent from the previous failure at125 mTorr. Current reached approximately 2.0mA during these tests.
Figure 14: Varying source solenoid currents(magnet current v. Faraday cup output).
Figure 15: Varying source pressure.
Maximum beam currents were found for thefollowing settings: source solenoid magnet cur-rent of 2.4 A (Fig. 14), source pressure of 138mTorr (Fig. 15), and LEBT solenoids 1 and 2settings of 395 A and 120 A respectively (Fig.13). The solenoid magnet should not be runabove 2 A to avoid damaging the source solenoid,
and the steady increase of source pressure be-tween 90 and 125 mTorr only resulted in a .2mA improvement over the selected nominal of105 mTorr. These conditions yielded a maxi-mum 2 mA of beam current on the Faraday cup(Fig. 2).
4 Conclusions
There are future improvements planned forthe IOTA source, as a replacement filament isbeing prepared for installation. The oscilloscopeoutputted 2.0 mA of current, and the new fil-ament will meet the 40 mA requirement. Thedesign for the source calls for 25 A of arc cur-rent, and the low achievable arc of 1.98 A limitsthe number of ions that can be injected. Witha newly coated cathode, a full arc current of 25A can be achieved. Using the 2.0 mA of beam,I was able to demonstrate that the output cur-rent is dependent on the independent variables ofsource pressure and solenoid current. A bendingdipole magnet will be installed after the faradaycup to create a momentum study on the beamconstituents.
5 Acknowledgements
I would like to thank my mentor, KermitCarlson for showing me not only the workingsof the IOTA source, but the activity going onthroughout NML as a whole. I was able to gethands on experience in multiple areas to enrichmy knowledge of the FAST division at Fermilab.Thank you for guiding me through the physicsI needed to fully understand the process of in-jecting protons, and for your patience through-out the various problems encountered through-out the project. I would also like to thank DeanEdstrom for teaching me about ACNET controlsand how to efficiently collect data using ACLcode. I learned a lot about how accelerator con-trols work, and this allowed me to carry out ex-periments confidently and feel comfortable turn-ing on and working with the ion source. Finally,I would like to thank my supervisor, Eric Prebys,for helping make my learning experience as a Lee
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Teng intern the best it could be. I was also luckythat he is a major part of the IOTA program, soI was able to use him as resource when needed.
6 References
[1] M. Church et al., ASTA: Proposal for anAccelerator RD User Facility at Fermilab’sAdvances Superconducting Test Accelerator,FERMILAB-TM-2568 (2013).[2] A. Valishev, IOTA - A Brief Parametric Pro-file, Focused Workshop on Scientific Opportuni-ties for IOTA (2015).[3] S. Antipov et al., IOTA: facility and exper-imental beam physics program, IOP Publishingfor Sissa Medialab (2017).[4] G. Ostrovskaya and A. Frank, A.G. Tech.Phys. 57: 495, 101134 (2012).[5] L. Alvarez, Energy Doubling in dc Acceler-ators, Review of Scientific Instruments 22:705(1951).[6] C. Schmidt and M. Popovic, Duoplasma-tron Source Modifications for 3He+ Operation,FERMILAB-Conf-97/374.[7] C. Lejeune, Theoretical and ExperimentalStudy of the Duoplasmatron Ion Source. NuclearInstruments and Methods 116, North-HollandPublishing (1974) p. 417-428.
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