advances in polarized electron sources for high-energy accelerators jym clendenin charliefest slac,...
TRANSCRIPT
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Advances in Polarized Electron Sources for High-
Energy Accelerators
Jym Clendenin
CharlieFestSLAC, January 27, 2006
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Contents of talk
SLC era Progress since SLC R&D plans for ILC
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Polarized electron sources for high-energy accelerators must provide:
High polarization High peak current Operational simplicity and
stability Nearly zero downtime
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3 elements to a GaAs-type source
Vacuum structure (i.e., electron gun)
Photocathode Laser system
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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)
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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
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Surface Charge Limit
Cannot increase charge in asingle pulse by simply increas-ing the laser energy!
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Surface charge limit depends on QE
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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
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Gun improvementsbegun in ’92
Load lock Channel cesiators Nanoammeters Low field electrodes Larger diameter GaAs cathodes Lower cathode temperature
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Load lock attached to rear of gunwith top of corona shield removed
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Bi-axial compressive strainlifts the degeneracy of thehh and lh bands at
a~1% yields of 50-80 meV
Single-layerStrainedGaAsP/GaAs
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SLC YAG-pumped Ti:sapphire laser system
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The Ti:sapphire laser cavity
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QE lifetime extended by cooling cathode
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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
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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
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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.
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But higher doping depolarizes spin.
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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
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QE performance of SVT-4249 (E158-III cathode)after ~1 year
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QE profile for SVT-4249
August 21, 2003 June 28, 2005
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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
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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
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Workshop on Polarized Electron Sources, Mainz, Germany, Oct., 2004