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

Polarized Electron Sources for Future Colliders: Present Status and Prospects for Improvement. J. Clendenin Stanford Linear Accelerator Center ALCPG Workshop SLAC, January 9, 2004. Outline. 1. Photocathode development a. Higher P e b. Surface charge limit c. Other concerns 1) Lifetime

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

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  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 < 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 splitting—now ~80 meV • Drift toward surface—lower dopant density or provide electric field • BBR confinement—reject low-energy e-? • The BBR is most likely area for improvement—to 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 surface—recombination with trapped electrons—precluding rise in surface affinity

  9. Quantum Efficiency (QE) Lifetime (t) • QE decays at rate 1/t. QE restored by applying additional Cs (total time 15-30 min.) Want t>>100 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

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

  11. 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)

  12. 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”

  13. 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

  14. 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

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