Advances in Polarized Electron Sources for High-Energy Accelerators Jym Clendenin CharlieFest SLAC, January 27, 2006

Contents of talk SLC era Progress since SLC R&D plans for ILC

Polarized electron sources for high-energy accelerators must provide: High polarization High peak current Operational simplicity and stability Nearly zero downtime

3 elements to a GaAs-type source Vacuum structure (i.e., electron gun) Photocathode Laser system

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!

Surface charge limit depends on QE

The first SLC run with polarized e - Re-cesiated (C) when QE not sufficient to maintain required charge (~8  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

Load lock attached to rear of gun with top of corona shield removed

Bi-axial compressive strain lifts the degeneracy of the hh and lh bands at   a~1% yields  of meV Single-layer Strained GaAsP/GaAs

SLC YAG-pumped Ti:sapphire laser system

The Ti:sapphire laser cavity

QE lifetime extended by cooling cathode

SLC 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 nC  zns I  pulse, avg A 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  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.

But higher doping depolarizes spin.

GaAs 0.64 P 0.36 /GaAs SL with 5-nm GaAs final layer doped to 5  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