Advances in Polarized Electron Sources for High-

Energy Accelerators

Jym Clendenin

CharlieFestSLAC, 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’s3-step model

for GaAs a few eV, reducedto ~1 eV with Cs,O

Bands bend downwith p doping,~0.75 eV for GaAs

Net result:Vacuum level below CBM in bulk(negative electron affinity)

Polarization for bulk GaAsEnergy

vsMomentum

Spin-orbit split-off band below VBM by SO=0.35 eVPmax = (3-1)/(3+1)=0.5

Symmetryat

Polarizationvs excitation photonenergy

Surface Charge Limit

Cannot increase charge in asingle 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 requiredcharge(~81010 e-)

P~25%, source availability ~90%

Re-activated (A)when re-cesiationcycles became tooshort

Gun improvementsbegun in ’92

Load lock Channel cesiators Nanoammeters Low field electrodes Larger diameter GaAs cathodes Lower cathode temperature

Load lock attached to rear of gunwith top of corona shield removed

Bi-axial compressive strainlifts the degeneracy of thehh and lh bands at

a~1% yields of 50-80 meV

Single-layerStrainedGaAsP/GaAs

SLC YAG-pumped Ti:sapphire laser system

The Ti:sapphire laser cavity

QE lifetime extended by cooling cathode

SLC 1993-1998 P~80% using GaAsP/GaAs cathode I at source ~8x1010 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

Parameter SLC NLC ILCat Source Design NC-SB SC-

LB ne nC 20 2.4

6.4

z ns 3 0.5 2

Ipulse, avg A 6.7 4.8 3.2

Ipulse, peak A 11 (SCL)

Toward the next collider

Charge requirements at source

NLC/ILC peak current < SLC, but total charge per macropulse muchhigher NLC: 2.9x1012 e- in 270 ns ILC: 1.1x1014 e- in 0.94 ms

SCL not visible for dopant concentration ≥21019 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/cm2.

But higher doping depolarizes spin.

GaAs0.64P0.36/GaAs SL with 5-nm GaAs final layer doped to 51019 cm-3

6

5

4

3

2

1

0

Cur

rent

(A

)

43210

Laser Power (kW)

SVT-4353780nm,14mmØ

With Q-Switching Without Q-Switching

Peak current exceedsthat required for theNLC micropulse

(75 ns)

(250 ns)

Same flashlamp-pumpedTi:sapphire laser as forE-158

QE performance of SVT-4249 (E158-III cathode)after ~1 year

QE profile for SVT-4249

August 21, 2003 June 28, 2005

GaAs0.64P0.36/GaAs SL (4+4 nm x 12) grown by SVT using MBE

GaAs0.66P0.34/GaAs0.95P0.05 single strained-layer 90-nm grown by SVT using MBE

QE at Pe max:

1.2%0.3%

Pe maxCTS/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