PST 2005, 15 Nov 2005

1
ILC _at_ SLAC RD Program for a Polarized RF Gun

• J. E. Clendenin
• Stanford Linear Accelerator Center

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)

3
Outline

• Promise of polarized rf guns
• Potential problems
• Elements of RD program
• Conclusions

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 lt10-11 Torr and average dark
  current lt10-20 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

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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

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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

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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

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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 pulse2 ms,
• PRR0.5 Hz
• A. Aleksandrov et al., EPAC 1998

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RD 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
10
Superfish output for HOM gun
Outer wall truncated Ez(z,r0) virtually same as
for 1.6-cell TM010,p gun, but shunt
impedance about 1/2
J.W. Lewellen, private communciation
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PWT design
D. Yu et al., PAC 2003
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RD 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

13
RD Compare conductances
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RD 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

15
RD 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

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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 lt55 MV/m a great advantage for
this design
90?
0?
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RD 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
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RD 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

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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

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Result

• Peak dark current lt25 pA _at_ 140 MV/m peak surface
  field
• b50
• RGA peak heights unchanged between RF on/off!
• Prediction IAvg ltlt0.1 pA for ILC DF5x10-3

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RD 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

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 RD activities at other labs welcomed!