Ilc @ slac r d program for a polarized rf gun
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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.

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Ilc @ slac r d program for a polarized rf gun

ILC @ SLAC R&D Program for a Polarized RF Gun

J. E. Clendenin

Stanford Linear Accelerator Center

PST 2005, 15 Nov 2005

Tokyo, Japan


Co authors
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


Outline
Outline

  • Promise of polarized rf guns

  • Potential problems

  • Elements of R&D program

  • Conclusions

PST 2005, 15 Nov 2005

Tokyo, Japan


Present situation
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


The route to improvement
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


The rf gun solution
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


New potential problems
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


First attempted operation a failure
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


R d choice of rf structure
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


Superfish output for hom gun
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


Pwt design
PWT design

D. Yu et al., PAC 2003

PST 2005, 15 Nov 2005

Tokyo, Japan


R d improve pumping scheme
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


R d compare conductances
R&D: Compare conductances

PST 2005, 15 Nov 2005

Tokyo, Japan


R d cathode plug
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


R d simulations
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


S band pwt gun simulations
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


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

R&D: Quantify expected cathode damage

SLAC small spot system

PST 2005, 15 Nov 2005

Tokyo, Japan


R d choice of materials fabrication assembly cleaning
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


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]

Proof of principle experiment

PST 2005, 15 Nov 2005

Tokyo, Japan


Result
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


R d overall
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


Conclusions
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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