27-10-2012, 12:12 PM
Recent Progress at SLAC Extracting High Charge from Highly-Polarized Photocathodes for Future-Collider Applications
ABSTRACT
Future colliders such as NLC and JLC will require a highly-polarized macropulse with charge that is more than an order
of magnitude beyond that which could be produced for the SLC. The maximum charge from the SLC uniformly-doped GaAs
photocathode was limited by the surface charge limit (SCL). The SCL effect can be overcome by using an extremely high
((ge)10(sup 19) cm(sup -3)) surface dopant concentration. When combined with a medium dopant concentration in the
majority of the active layer (to avoid depolarization), the surface concentration has been found to degrade during normal heat
cleaning (1 hour at 600 C). The Be dopant as typically used in an MBE-grown superlattice cathode is especially susceptible
to this effect compared to Zn or C dopant. Some relief can be found by lowering the cleaning temperature, but the long-term
general solution appears to be atomic hydrogen cleaning.
ABSTRACT
Future colliders such as NLC and JLC will require a highly-polarized macropulse with charge that is more than an order
of magnitude beyond that which could be produced for the SLC. The maximum charge from the SLC uniformly-doped GaAs
photocathode was limited by the surface charge limit (SCL). The SCL effect can be overcome by using an extremely high
((ge)10(sup 19) cm(sup -3)) surface dopant concentration. When combined with a medium dopant concentration in the
majority of the active layer (to avoid depolarization), the surface concentration has been found to degrade during normal heat
cleaning (1 hour at 600 C). The Be dopant as typically used in an MBE-grown superlattice cathode is especially susceptible
to this effect compared to Zn or C dopant. Some relief can be found by lowering the cleaning temperature, but the long-term
general solution appears to be atomic hydrogen cleaning.