ILC @ SLAC R&D Program for a Polarized RF Gun
J. E. Clendenin
Stanford Linear Accelerator Center
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)
Outline
Promise of polarized rf guns Potential problems Elements of R&D program Conclusions
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
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
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
New potential problems
Vacuum poor: mid-10-10 Torr when rf on Peak dark current high: 40, 170 A 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
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 s,
PRR=0.5 Hz [A. Aleksandrov et
al., EPAC 1998]
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,)
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]
Superfish output for HOM gun
[J.W. Lewellen,private communciation]
Outer wall truncated
Ez(z,r=0) virtuallysame as for 1.6-cellTM010, gun, butshunt impedanceabout 1/2
PWT design
D. Yu et al., PAC 2003
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
R&D: Compare conductances
Gun Design Conductance P (Torr) for:
(l/s) 10-11 Torr-l/s cm-2 10-12 Torr-l/s cm-2
BNL 1.6-cell SB with conventional pumping
3.7 1.610-9 1.610-10
with sieve 12 510-10 510-11
PWT (2/2+7 to 10 cells) 28 210-10 210-11
PWT (1.5 cells) 50 1.210-10 1.210-11
HOM 75 810-11 810-12
R&D: Cathode plug
GaAs crystal ~600 m 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
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
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
900
R&D: Quantify expected cathode damage
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
SLAC small spot system
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
Proof of principle experiment
Single full-cell S-band at KEK
– HIP Cu– Class 1 clean room– Ultra-high purity water
rinsing
[H. Matsumoto, Linac 1996, p. 62]
Result:
Peak dark current <25 pA @ 140 MV/m peak surface field
~50 RGA peak heights unchanged between RF
on/off! Prediction: IAvg <<0.1 pA for ILC DF=5x10-3
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
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!