Polarized Electron Sources for Future Colliders: Present Status and Prospects for Improvement

1
Polarized Electron Sources for Future Colliders
Present Status and Prospects for Improvement

• J. Clendenin
• Stanford Linear Accelerator Center
• ALCPG Workshop
• SLAC, January 9, 2004

2
Outline

• 1. Photocathode development
• a. Higher Pe
• b. Surface charge limit
• c. Other concerns
• 1) Lifetime
• 2) Stability (primarily a laser issue)
• 3) Reliability
• 2. Gun development
• 3. Laser systems (Brachmann)

3
GaAs Direct band gap semiconductor symmetry at
G point a) unstrained b) strained
4
Polarized Electron Photoemission from GaAs
Circularly polarized light (g), near band
gap, excites electron from valence band to
conduction band Electrons drift
band-bending region (BBR) near the surface
Electron emission to vacuum from
Negative-Electron-Affinity (NEA) surface
Cathode activation p-doped GaAs, energy bands
bend down at surface Ultra-High-Vacuum lt 10-11
Torr (at RT) Heat treatment at 600 C
Application of Cesium and oxide (O2 or
NF3) Result is vacuum level at surface is lower
than conduction band minimum in the bulk, i.e.,
NEA surface
5
Polarization Achieved
SLC Pemax 78 (at source), 76 at Compton
polarimeter
6
Future Directions for Pe

• Three areas possible reduction of depolarization
• Band splittingnow 80 meV
• Drift toward surfacelower dopant density or
  provide electric field
• BBR confinementreject low-energy e-?
• The BBR is most likely area for improvementto
  study, need more reliable cathode activation
  method

7
Surface Charge Limit (SCL) Effect
Extreme case

• Some e- temporarily trapped near surface
• Result is surface affinity temporarily rises,
• reducing surface escape probability
• SLC relaxation time 10-100 ns for SLC
• photocathodes

8
Solution is to increase dopant density at surface
(gradient doping), promotes tunneling of holes
to surfacerecombination with trapped
electronsprecluding rise in surface affinity
9
(No Transcript)
10
Quantum Efficiency (QE) Lifetime (t)

• QE decays at rate 1/t. QE restored by applying
  additional Cs (total time 15-30 min.) Want tgtgt100
  h
• Key factor is vacuum both during cathode
  activation and for normal operation
• Ion back-bombardment
• Electron-stimulated gas desorption
• To date good vacuum engineering, use of
  load-lock
• New directions massive NEG arrays near cathode
  materials with low secondary-electron cofficient
  new methods of cleaning

11
Reliability

• Load-lock dramatically improves system
  reliability
• New directions more reliable cathode activation
  methods e.g., atomic hydrogen cleaning

12
Gun Development

• Limitations of present DC-biased guns
• Space-charge limits current-density at gun
• Transverse emittance limited by cathode size
• Longitudinal emittance of beam increases because
  of the low energy
• Consequently
• Cannot now use laser to generate microstructure
  (JLC/NLC)

13
Modulation of SLAC Polarized Electron Beam

• Technique pass flash-Ti laser pulse (typically
  100-300 ns) through Pockels cell modulated at 714
  MHz
• Result will be a train of mbunches spaced 1.4 ns
• Each mbunch will have 2 S-band buckets with
  some charge inbetween mbunches
• Beam-loading will limit peak current
• If Iavg in macrobunch is 0.5 A (E-158), then Ipk
  in mbunch 2 A implying 4x109 e- in single
  mpulse

14
New Directions for Gun Design

• Higher DC voltage (500 kV?)
• Goal of 20-30 MV/m at cathode
• Additional compression of microbunches probably
  required
• Pulsed voltage
• Greatly improves HV standoff
• RF gun
• Typically 10-30 MV/m extraction voltage, beam
  energy 2-5 MeV (or higher for multi-cell gun)
• Chief concern is e- back-bombardment and
  generation of secondary electrons
• Hybrid gun DC (or pulsed) gun with anode the
  input coupler to rf accelerating structure

15
Conclusions

• Electron polarization of 90 at source now
  available, prospect for 95 reasonably good
• Surface charge limit probably will not be a
  problem
• Lifetime and reliability for existing DC guns
  reasonable
• New gun designs in development that may reduce
  transverse and/or longitudinal emittance and for
  JLC/NLC permit optical formation of micropulse
  sturcture