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!