1. Deposition of Pure Lead Photo-Cathodes by Means 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), Otwock-Swierk, Poland
p.strzyzewski(at)ipj.gov.pl
2) DESY-MHF, Hamburg, Germany
3) BNL, Upton, New York, USA
4) TJNAF, Newport News, VA 23606, USA
5) 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

2. Cathode Options for SRF Injector
Use the niobium wall as a cathode;
Simple, but low Nb QE limits current
- J. Smedley, et al., J. Applied Physics 98,
Coat Nb in cathode region with another superconductor, with 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 , 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

3. Nb-Pb RF-Gun: Motivations
Building a CW operating RF-source of ~0.5-1mA class for an XFEL facility.
Operation in CW mode with high accelerator gradient on photo—cathodes.
Lower power dissipation and excellent thermal stability.
Test cavities; (left) DESY option with closed endplate, (right) JLab option with hole in the endplate and plug (marked in red).
A prealiminary design of 1.6 cell Nb-Pb RF Gun (DESY)

4. Comparison of Pb & Nb Lead Niobium Type I Superconductor
Used in Ion Accelerators
Critical Temperature: 7.2K
Critical Magnetic Field:* 70mT
Photoelectric Work Function: 3.95eV
Expansion Coefficient: 7·10-61/K
Niobium
Type II Superconductor
Many Accelerator Applications
Critical Temperature: 9.2K
Critical Magnetic Field:* 160mT
Photoelectric Work Function: 4.3eV
Expansion Coefficient: 3·10-5 1/K
* Values for 2K, 1.3 GHz
(from Basic Principles of RF Superconductivity and Superconducting Cavities, Peter Schmüser)

5. Quantum Efficiency of different lead 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???

6. Surface uniformity
(Courtesy of BNL)
Arc
Deposited
Sputtered
Vacuum
Deposited
Solid
All cathodes laser cleaned with 0.2 mJ/mm2 of 248nm light

7. Cathodic Vacuum Arc in UHV
Physical properties:
- full ionisation of the plasma,
- absence of a working gas sustaining the discharge,
- presence of multiple-charged ions,
- high kinetic energy of ions (10-150eV),
- deposition process can be fully controlled by means of magnetic and
electric fields.
Film properties:
- very high density and smoothness (if filtered),
- high quality – strongly reduced film defects (i.e. voids),
possibility of making pure metal films and compounds
possibility of the deposition upon components of sophisticated shapes.
Ultra - high vacuum conditions
- Pressure below Torr makes possible the practical elimination of
impurities, like water vapors, nitrogen and CxHY

8. Shielding tube enables the deposition upon the RF Gun endplate,
Planar 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.

9. Arc Plasma Transportation
Values of the measured ion-current density along the axis of the investigated shielding tube.

10. Formation 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

11. Pb thin films characterization: SEM
SEM pictures of Pb films deposited by means of cathodic vacuum arc
(courtesy of BNL)
magn. 250
magn. 3000

12. Pb 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)

13. Pb thin films characterization: GD-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

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

15. First result of Q measurement of Nb-Pb RF Gun (at JLab)
Q of Nb-PB RF Gun measured at JLab (October 2006, Courtesy of P. Kneisel)

16. Conclusions
Niobium, although it is a good superconductor, is a relatively poor photocathode.
For moderate average currents, SC lead plating the cathode may be an alternative to niobium.
UHV arc deposited lead film was chosen for further investigations.
QE266nm= 0.035%, QE213nm= 0.27%, QE190nm= 0.55%
Very high density and purity of arc deposited Pb films were confirmed.
Pb films deposition upon the RF Gun end-plate is a challenge.
The first results of Qo measurements are quite satisfactory ones.

17. Future plans
Within the collaboration with BNL, QE measurements will be carried out on samples without and with a SRF cavity at cryogenic temperatures, in order to investigate the behavior of the SC lead under laser irradiation.
Possible approaches for improving the UHV cathodic arc deposition of Pb photocathodes:
- a new T-type magnetic filter for better plasma transport
efficiency,
- a kHz pulse bias and arc discharge current (instead of DC
ones) for higher film density → higher QE.
The deposition of Mg thin films??? (see L. Cultrera et al., Metal Film Photo-Cathodes For High Brightness Electron Injectors, Proc. EPAC2006, Edinburg, UK; MOPCH02.