Photoemission from p-type semiconductors Spicer’s 3-step model for GaAs a few eV, reduced to ~1 eV with Cs,O Bands bend down with p doping, ~0.75 eV for GaAs Net result: Vacuum level below CBM in bulk (negative electron affinity)
Polarization for bulk GaAs Energy vs Momentum Spin-orbit split-off band below VBM by SO =0.35 eV P max = (3-1)/(3+1)=0.5 Symmetry at Polarization vs excitation photon energy
Surface Charge Limit Cannot increase charge in a single pulse by simply increas- ing the laser energy!
The first SLC run with polarized e - Re-cesiated (C) when QE not sufficient to maintain required charge (~8 10 10 e - ) P~25%, source availability ~90% Re-activated (A) when re-cesiation cycles became too short
Gun improvements begun in ’92 Load lock Channel cesiators Nanoammeters Low field electrodes Larger diameter GaAs cathodes Lower cathode temperature
SLC 1993-1998 P~80% using GaAsP/GaAs cathode I at source ~8x10 10 e - for each of the 2 micropulses With LL, no need to re-activate Availability >97% Operated entirely by MCC staff except for YAG flashlamp changes every few weeks
ParameterSLC NLCILC at SourceDesign NC-SB SC-LB n e nC20 2.4 6.4 zns3 0.5 2 I pulse, avg A6.7 4.8 3.2 I pulse, peak A11 (SCL) Toward the next collider Charge requirements at source NLC/ILC peak current < SLC, but total charge per macropulse much higher NLC: 2.9x10 12 e - in 270 ns ILC: 1.1x10 14 e - in 0.94 ms
SCL not visible for dopant concentration ≥2 10 19 cm -3 Uniformly doped, unstrained, 100-nm GaAs cathodes. QE=0.45, 0.9, 0.4, 0.4% in order of increasing dopant density. Laser energy increases in equal steps to 150 W/cm 2.
GaAs 0.64 P 0.36 /GaAs SL with 5-nm GaAs final layer doped to 5 10 19 cm -3 Peak current exceeds that required for the NLC micropulse (75 ns) (250 ns) Same flashlamp-pumped Ti:sapphire laser as for E-158
QE performance of SVT-4249 (E158-III cathode) after ~1 year
QE profile for SVT-4249 August 21, 2003 June 28, 2005
GaAs 0.64 P 0.36 /GaAs SL (4+4 nm x 12) grown by SVT using MBE GaAs 0.66 P 0.34 /GaAs 0.95 P 0.05 single strained-layer 90-nm grown by SVT using MBE QE at Pe max: 1.2% 0.3% Pe max CTS/Møller): 86(90)% 81(85)% CTS Measurements
ILC R&D Plans Photocathodes for higher polarization and/or QE: AlInGaAs/AlGaAs SL high- strain or low CB offset; AlInGaAs/GaAsP SL strain-compensated; grided cathodes; GaN based cathodes for robustness Higher voltage gun: new materials for DC gun; prototype RF gun Lasers: generate ILC macropulse in visible
Workshop on Polarized Electron Sources, Mainz, Germany, Oct., 2004