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

Advances in Polarized Electron
Sources for High-Energy
Accelerators
Jym Clendenin
CharlieFest
SLAC, January 27, 2006
Contents of talk
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SLC era
Progress since SLC
R&D plans for ILC
Polarized electron sources for
high-energy accelerators must
provide:
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High polarization
High peak current
Operational simplicity and
stability
Nearly zero downtime
3 elements to a GaAs-type
source
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Vacuum structure (i.e., electron
gun)
Photocathode
Laser system
Photoemission from p-type
semiconductors
F 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)
Spicer’s
3-step model
Polarization for bulk GaAs
Energy
vs
Momentum
Symmetry
at G
Polarization
vs excitation photon
energy
Spin-orbit split-off band below VBM by DSO=0.35 eV
Pmax = (3-1)/(3+1)=0.5
Surface Charge Limit
Cannot increase charge in a
single pulse by simply increasing 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1010 e-)
Re-activated (A)
when re-cesiation
cycles became too
short
P~25%, source availability ~90%
Gun improvements
begun in ’92
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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
Single-layer
Strained
GaAsP/GaAs
Bi-axial compressive strain
lifts the degeneracy of the
hh and lh bands at G
da~1% yields d of 50-80 meV
SLC YAG-pumped Ti:sapphire
laser system
The Ti:sapphire laser cavity
QE lifetime extended by cooling cathode
SLC 1993-1998
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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
Toward the next collider
Charge requirements at source
Parameter
at Source
ne
Dz
Impulse, avg
Impulse, peak
SLC
Design
nC
ns
A
A
20
3
6.7
11 (SCL)
NLC
ILC
NC-SB SC-LB
2.4
0.5
4.8
6.4
2
3.2
NLC/ILC peak current < SLC, but total charge per macropulse much
higher
NLC: 2.9x1012 e- in 270 ns
ILC: 1.1x1014 e- in 0.94 ms
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.
SCL not visible for dopant concentration ≥21019 cm-3
But higher doping depolarizes spin.
GaAs0.64P0.36/GaAs SL with 5-nm GaAs final
layer doped to 51019 cm-3
6
Peak current exceeds
that required for the
NLC micropulse
SVT-4353
780nm,14mmØ
5
Current (A)
4
Same flashlamp-pumped
Ti:sapphire laser as for
E-158
3
2
With Q-Switching (75 ns)
Without Q-Switching
1
(250 ns)
0
0
1
2
Laser Power (kW)
3
4
QE performance of SVT-4249 (E158-III cathode)
after ~1 year
QE profile for SVT-4249
August 21, 2003
June 28, 2005
CTS Measurements
Pe max
CTS/Møller):
86(90)%
81(85)%
QE at Pe max:
1.2%
0.3%
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
ILC R&D Plans
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Photocathodes for higher polarization
and/or QE: AlInGaAs/AlGaAs SL highstrain 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