Deposition of Pure Lead Photo-Cathodes by Deposition of Pure Lead Photo-Cathodes by Means of UHV Cathodic ArcMeans of UHV Cathodic Arc

P. Strzyżewski1), J. Langner1), M.J. Sadowski1), J. Witkowski1), J. Sekutowicz2), T. Rao3), J. Smedley3), P. Kneisel4), L. Cultrera5), G. Gatti5) and F. Tazzioli5)

1) Soltan Institute for Nuclear Studies (IPJ), 05-400 Otwock-Swierk, Polande-mail: p.strzyzewski(at)ipj.gov.pl

2) DESY-MHF, 22-603 Hamburg, Germany3) BNL, Upton, New York, USA

4) TJNAF, Newport News, VA 23606, USA5) INFN/LNF, Frascati (Rome), Italy

The International Workshop on:THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY

Legnaro, Italy, October 9-12, 2006

22

Cathode Options for Cathode Options for SRFSRF Injector Injector

Use the niobium wall as a cathode;Use the niobium wall as a cathode; Simple, but low Nb QE limits current

- J. Smedley, et al., J. Applied Physics 98, 043111 2005

Coat Nb in cathode region with another superconductor, with Coat Nb in cathode region with another superconductor, with better QE;better QE;

Lead (Pb) was chosen as a first candidate

- J. Sekutowicz et al., TTF Meeting, Frascati, June 2003, Phys. Rev. ST- AB, vol. 8, January 2005)

- J. Sekutowicz et al., TESLA-FEL Report 2005-09, DESY, Hamburg, December 2005.

- J. Smedley et al., Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tennessee; WPAPO38

- J. Sekutowicz et al., Proceedings of 2006 European Particle Accelerator Conference, Edinburgh, Scotland; THPLS092

- P. Strzyzewski et al., Proceedings of 2006 European Particle Accelerator Conference, Edinburgh, Scotland; THPCH176

33

Nb-Pb RF-Gun: MotivationsNb-Pb RF-Gun: Motivations

Building a CW operating RF-source of Building a CW operating RF-source of ~0.5-1mA class for an ~0.5-1mA class for an XFEL facility.XFEL facility.

Operation in CW mode with high accelerator gradient on photoOperation in CW mode with high accelerator gradient on photo—cathodes.—cathodes.

Lower power dissipation and excellent thermal stability.Lower power dissipation and excellent thermal stability.

A prealiminary design of 1.6 cell Nb-Pb RF Gun (DESY)

Test cavities; (left) DESY option with closed endplate, (right) JLab option with hole in the endplate and plug (marked in red).

44

Comparison ofComparison of Pb & Nb Pb & Nb

LeadLeadType I SuperconductorType I Superconductor

Used in Ion AcceleratorsUsed in Ion Accelerators

Critical Temperature: 7.2KCritical Temperature: 7.2K

Critical Magnetic Field:Critical Magnetic Field:** 770mT0mT

Photoelectric Work Function: 3.95eVPhotoelectric Work Function: 3.95eV

Expansion Coefficient: 7Expansion Coefficient: 7·10·10-6-61/K1/K

NiobiumNiobiumType II SuperconductorType II Superconductor

Many Accelerator ApplicationsMany Accelerator Applications

Critical Temperature: 9.2KCritical Temperature: 9.2K

Critical Magnetic Field:* Critical Magnetic Field:* 16160mT0mT

Photoelectric Work Function: 4.3eVPhotoelectric Work Function: 4.3eV

Expansion Coefficient: 3·10Expansion Coefficient: 3·10-5-5 1/K 1/K

* Values for 2K, 1.3 GHz(from Basic Principles of RF Superconductivity and Superconducting Cavities, Peter Schmüser)

55

Quantum Efficiency of differentQuantum Efficiency of different l leadead films films

QE of five analyzed Pb samples measured at 300K using setup at BNL

• Arc deposited Pb cathode was chosen for further investigations.

• Poor QE of bulk Pb???

66

Surface uniformitySurface uniformity((Courtesy ofCourtesy of BNL) BNL)

Sputtered

SolidVacuum

Deposited

Arc

Deposited

All cathodes laser cleaned with 0.2 mJ/mm2 of 248nm light

77

Cathodic Vacuum Arc in UHVCathodic Vacuum Arc in UHVPhysical properties:Physical properties:- - full ionisfull ionisationation of the plasma of the plasma,,- - absence of a working gas sustaining theabsence of a working gas sustaining the discharge,discharge,- - presence of multiple-charged ions,presence of multiple-charged ions,- - high kinetic energy of ions (1high kinetic energy of ions (100-1-1550eV)0eV),,- - deposition process can be fully controlled by means of magnetic and deposition process can be fully controlled by means of magnetic and electric fields.electric fields.

Film properties:Film properties:- - very high density and smoothness (if filtered),very high density and smoothness (if filtered),-- high quality – strongly reduce high quality – strongly reducedd film defects (i.e. voids) film defects (i.e. voids),,- possibility of making pure metal films and compoundspossibility of making pure metal films and compounds - possibility of the deposition upon components of sophisticated shapes.possibility of the deposition upon components of sophisticated shapes.

UltraUltra -- high high vvacuum conditionsacuum conditions

- Pressure below 10- Pressure below 10-10-10 Torr makes possible the practical elimination of Torr makes possible the practical elimination of impurities, like water vapors, nitrogen and Cimpurities, like water vapors, nitrogen and CxxHHYY

88

Planar arc facilityPlanar arc facility

Scheme of a UHV filtered planar cathode arc facility equipped with an RF Gun to be

coated.

Picture of the planar arc facility equipped with the magnetic filter (without external

coils) and the RF Gun.

Shielding tube enables the deposition upon the RF Gun endplate,

however its geometry (length and diameter) strongly decreases deposition rate.

99

Arc Plasma TransportationArc Plasma Transportation

Values of the measured ion-current density along the axis of the investigated shielding tube.

1010

Formation of Pb thin filmsFormation of Pb thin films

Arc discharge current: 25A

Voltage: 17-18V

Base pressure: <10-10mbar

Pressure during arc: ~10-7mbar

Substrates: niobium, copper, sapphire

Deposition rate: 0.5nm/s

Bias: -70V

Film temperature: <100oC

Cathode spot diameter: <4mm

1111

Pb thin films characterization: SEMPb thin films characterization: SEMSEM pictures of Pb films deposited by means of cathodic vacuum arc

(courtesy of BNL)

magn. 250 magn. 3000

1212

Pb thin films characterization: SIMSPb thin films characterization: SIMS

Result of a purity profile measurement of Pb layer deposited on sapphire substrate by means of Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)

1313

Pb thin films characterization: Pb thin films characterization: GD-OESGD-OES

Glow Discharge – Optical Emission Spectrometry (GD-OES) analysis of the Pb layer profile, which was deposited upon

the Cu-substrate under the same conditions as the sapphire ones

1414

First result of Q measurement First result of Q measurement of Nb-Pb RF Gunof Nb-Pb RF Gun (at DESY) (at DESY)

with Pb coating

1.E+08

1.E+09

1.E+10

0 5 10 15 20 25 30 Epeak [MV/m]

Qo

Q of Nb-PB RF Gun measured at DESY (August 2006, Courtesy of J. Sekutowicz)

Low value of Q is probably caused by a bad welding of Niobium.

Renewed Pb deposition and Q measurement are planned in November 2006

1515

First result of Q measurement First result of Q measurement of Nb-Pb RF Gunof Nb-Pb RF Gun (at JLab) (at JLab)

1,0E+09

1,0E+10

0 5 10 15 20 25 30 35 40 45 50

Epeak[MV/m]

Qo

"Nb Plug"

Lead Plug #2

Q of Nb-PB RF Gun measured at JLab (October 2006, Courtesy of P. Kneisel)

1616

ConclusionsConclusions

Niobium, although Niobium, although it is it is a ga goodood superconductor, is a relatively poor superconductor, is a relatively poor photocathodephotocathode..

For moderate average currents, SC lead plating the cathode may be For moderate average currents, SC lead plating the cathode may be an alternative to niobiuman alternative to niobium..

UHV aUHV arc deposited lead film was chosen for further investigationsrc deposited lead film was chosen for further investigations..

QEQE266nm266nm== 0.035%, QE0.035%, QE213nm213nm== 0.27%, QE0.27%, QE190nm190nm== 0.55%0.55%

Very high density and purity of arc deposited Very high density and purity of arc deposited PbPb films were films were confirmedconfirmed..

Pb Pb films deposition upon the RF Gun films deposition upon the RF Gun end-end-plate is a challengeplate is a challenge..

The fThe first results of Qirst results of Qoo measurements are quite satisf measurements are quite satisfactory ones.actory ones.

1717

Future plansFuture plans

Within the Within the collaboration with BNL, QE measurements will be carried collaboration with BNL, QE measurements will be carried out on samples without and with a SRF cavity at cryogenic out on samples without and with a SRF cavity at cryogenic temperaturestemperatures, in order, in order to investigate the behavior of the to investigate the behavior of the SCSC lead lead under laser irradiationunder laser irradiation..

Possible approaches for improving Possible approaches for improving the UHV cathodicthe UHV cathodic arc deposition arc deposition of Pb photocathodesof Pb photocathodes::

- - a a new T-type magnetic filter for better plasma transport new T-type magnetic filter for better plasma transport efficiency,efficiency, - - a a kHz pulse bias and arc discharge current (instead of DC kHz pulse bias and arc discharge current (instead of DC ones) for higher film density ones) for higher film density → higher QE→ higher QE..

The dThe deposition of Mg thin filmseposition of Mg thin films?????? ( (see see L. Cultrera et al., Metal Film L. Cultrera et al., Metal Film Photo-Cathodes For High Brightness Electron InjectorsPhoto-Cathodes For High Brightness Electron Injectors, , Proc. EPAC2006, Edinburg, UK; MOPCH02Proc. EPAC2006, Edinburg, UK; MOPCH02..