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					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
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              Spicer’s
(negative electron affinity)   3-step model
Polarization for bulk GaAs
  Energy          Symmetry          Polarization
    vs              at G            vs excitation photon
 Momentum                           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 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1010 e-)

Re-activated (A)
when re-cesiation
cycles became too
short


 P~25%, source availability ~90%
      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
    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
   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                   SLC         NLC     ILC
at Source                   Design     NC-SB SC-LB

ne                  nC      20           2.4        6.4
Dz                  ns      3            0.5        2
Impulse, avg        A       6.7          4.8        3.2
Impulse, peak       A       11 (SCL)

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
                       SVT-4353
                                                                               Peak current exceeds
                       780nm,14mmØ                                             that required for the
              5                                                                NLC micropulse

              4
                                                                               Same flashlamp-pumped
Current (A)




                                                                               Ti:sapphire laser as for
              3
                                                                               E-158

              2


              1                                  With Q-Switching (75 ns)
                                                 Without Q-Switching
                                                                (250 ns)
              0
                  0           1             2             3                4
                                     Laser Power (kW)
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                                                          QE at Pe max:
CTS/Møller):
                                                                1.2%
86(90)%                                                         0.3%
81(85)%




      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
   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

				
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