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R&D Program for a Polarized RF Gun

This presentation discusses the promise, potential problems, and elements of an R&D program for a polarized RF gun, and presents potential solutions and improvements for the current situation.

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R&D Program for a Polarized RF Gun

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  1. ILC @ SLAC R&D Program for a Polarized RF Gun J. E. Clendenin Stanford Linear Accelerator Center PST 2005, 15 Nov 2005 Tokyo, Japan

  2. Co-authors: • A. Brachmann, D. H. Dowell, E. L. Garwin, K. Ioakeimidi, R. E. Kirby, T. Maruyama, C. Y. Prescott (SLAC) • R. Prepost (U. Wisconsin) PST 2005, 15 Nov 2005 Tokyo, Japan

  3. Outline • Promise of polarized rf guns • Potential problems • Elements of R&D program • Conclusions PST 2005, 15 Nov 2005 Tokyo, Japan

  4. Present situation • Accelerator based sources for polarized electron beams utilizing GaAs photocathodes have proven successful using a dc-bias of a few 100s kV and fields of a few MV/m at the photocathode. Success has been dependent on eliminating HV breakdown, achieving vacuum <10-11 Torr and average dark current <10-20 nA • Due to relatively low energy of extracted bunch, space charge density must be kept low by using long bunch length and/or large bunch radius • Thus these sources require rf bunching systems. Resulting emittance, both transverse and longitudinal, significantly compromised PST 2005, 15 Nov 2005 Tokyo, Japan

  5. The route to improvement • If the extraction field and beam energy are increased, higher current densities can be supported at the cathode • The source laser system can then be used to generate the high peak current, relatively low duty-factor micropulses required by the ILC without the need for post-extraction rf bunching • Electron capture and transport efficiency will be improved • Damping ring probably can not be eliminated, but operational reliability and efficiency would be improved PST 2005, 15 Nov 2005 Tokyo, Japan

  6. The RF gun solution • A polarized rf gun incorporating GaAs photocathode in the first cell increases both field and energy, enabling ILC microbunch to be generated in gun and directly inserted into injector accelerator. • Net result: injection system for a polarized rf gun can be identical to that for an unpolarized rf gun • Also: • Increases the cathode quantum yield due to Schottky effect • Decreases the surface charge limitation, while at the same time the beam will exit the gun with sufficient energy to significantly reduce space charge effects during transport to the injector accelerator section PST 2005, 15 Nov 2005 Tokyo, Japan

  7. New potential problems • Vacuum poor: mid-10-10 Torr when rf on • Peak dark current high: 40, 170 mA at 35, 40 MV/m [I. Bohnet et al., DIPAC 2003, p. PT29 (1.5-cell L-band rf gun with Cs2Te photocathode at DESY/Zeuthen)] • Back bombardment of cathode by e- and ions limits QE lifetime PST 2005, 15 Nov 2005 Tokyo, Japan

  8. First attempted operation a failure 1/2 –cell S-band gun at BINP operated at up to 100 MV/m peak field at cathode, rf pulse=2 ms, PRR=0.5 Hz [A. Aleksandrov et al., EPAC 1998] PST 2005, 15 Nov 2005 Tokyo, Japan

  9. R&D: Choice of rf structure • Criteria: best vacuum, low FE • Choices: • 1.5(6)-cell “pill box” • 7-10 cell PWT integrated • HOM (TM012,p) Cross section of the HOM TM012 rf gun (solid line) superimposed on standard 1.6 cell TM010 gun (dotted line), where the units for r and z are the same [J.W. Lwellen, PRLST-AB 4 (2001) 040101] PST 2005, 15 Nov 2005 Tokyo, Japan

  10. Superfish output for HOM gun Outer wall truncated Ez(z,r=0) virtually same as for 1.6-cell TM010,p gun, but shunt impedance about 1/2 [J.W. Lewellen, private communciation] PST 2005, 15 Nov 2005 Tokyo, Japan

  11. PWT design D. Yu et al., PAC 2003 PST 2005, 15 Nov 2005 Tokyo, Japan

  12. R&D: Improve pumping scheme • Typically conductance limited • Increase conductance by using: • Z-slots a là AFEL • Multiple small holes (sieve) • Surround rf cavity with UHV chamber • Use massive NEG pumping plus some ion pumping PST 2005, 15 Nov 2005 Tokyo, Japan

  13. R&D: Compare conductances PST 2005, 15 Nov 2005 Tokyo, Japan

  14. R&D: Cathode plug • GaAs crystal ~600 mm thick, maybe 1 cm dia., can be nicely mounted flush to Mo plug • Plug itself maybe 2 cm dia., must be loose enough to insert/remove remotely • RF seal for plug presents a serious potential source of FE electrons • Need find innovative RF seal technique PST 2005, 15 Nov 2005 Tokyo, Japan

  15. R&D: Simulations • Ion back bombardment • Not expected to be a problem [J.W. Lewellen, PRST-AB 5, 020101 (2002); R.P. Filler III et al, PAC05] • Electron back bombardment • Influenced by peak field and by solenoid value [J.H. Han et al, PRST-AB 8, 033501 (2005)] • Scope of analysis needs to be expanded PST 2005, 15 Nov 2005 Tokyo, Japan

  16. S-band PWT gun simulations Threshold peak axial field, for FE e- from the first iris at an annular distance r from the cell axis (d from the center plane of the disk) to reach cathode surface for indicated emission phase; solid line represents iris profile in r- r-d plane [Y. Luo et al., PAC03, p. 2126] Operating <55 MV/m a great advantage for this design 0 90 PST 2005, 15 Nov 2005 Tokyo, Japan

  17. 1. Analysis chamber 2. Loadlock chamber 3. Sample plate entry 4. Sample transfer plate 5. Rack and pinion travel 6. Sample plate stage 7. XYZµ OmniaxTM manipulator 8. Sample on XYZµ 9. Electrostatic energy analyzer 10. X-ray source 11. SEY/SEM electron gun 12. Microfocus ion gun 13. Sputter ion gun 14. To pressure gauges and RGA 15. To vacuum pumps 16. Gate valve R&D: Quantify expected cathode damage SLAC small spot system PST 2005, 15 Nov 2005 Tokyo, Japan

  18. R&D: Choice of materials, fabrication, assembly, cleaning • Materials • Class 1 OFHC Cu • HIP? • Hardened? • Fabrication • Single-point diamond? • Oil-less machining • Assembly • Clean room • Cleaning • Ultra pure water • No solvents PST 2005, 15 Nov 2005 Tokyo, Japan

  19. Single full-cell S-band at KEK HIP Cu Class 1 clean room Ultra-high purity water rinsing [H. Matsumoto, Linac 1996, p. 62] Proof of principle experiment PST 2005, 15 Nov 2005 Tokyo, Japan

  20. Result: • Peak dark current <25 pA @ 140 MV/m peak surface field • b~50 • RGA peak heights unchanged between RF on/off! • Prediction: IAvg <<0.1 pA for ILC DF=5x10-3 PST 2005, 15 Nov 2005 Tokyo, Japan

  21. R&D: Overall • Design RF gun around GaAs requirements • Construct proto-gun for testing • Test for QE and lifetime without rf • RF process with dummy cathode • SLAC L-band RF station ready in 2006 • Test activated GaAs with RF • Critical tests are QE and lifetime • Compare results with simulations PST 2005, 15 Nov 2005 Tokyo, Japan

  22. Conclusions • Polarized rf guns are desirable for ILC • New challenges not present in DC guns • The means to meet these challenges appear to exist • These means will be explored at SLAC • Related R&D activities at other labs welcomed! PST 2005, 15 Nov 2005 Tokyo, Japan

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