Development of Polarized Electron Source for Linear Colliders by Using Superlattice Photocathhode

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We achieved 715 polarization of photoemission from the ALGA-As-GaAs superlattice in early 1991. The study described below has been performed to proceed the fruits further aimed to apply the superlattice in future linear colliders (JLC for example).We have studied the spin depolarization during the diffusion of electrons in the conduction band. The superlattice is p-doped to achieve negative-electron-affinity, which is necessary to use the superlattice for a photocathode. We applied lower acceptor concentration in most part of a superlattice, on the other hand normal(5x10^<18>/cm^3), or even higher, concentration near the surface. We have found this type of modulation doping improves both polarization and quantum efficiency of a photocathode.We have studied that how polarization depends on the total thickness of the superlattice. The extrapolation to zero thickness gave a polarization of -75%. We tested sample which has various surface conditions. Non of them showed significant improvement of the polarization. We have concluded there was no source of large depolarization near the surface. Therefore the polarization of electrons exited in the conduction band was -75%.We have tested large charge-extraction from a superlattice. This test was performed at SLAC as a part of the Japan-SLAC collaboration. The result was successful ; 2.3x10^<11>electrons were extracted in 2.5nsec. No charge Limit of the photochatode material was observed. The resultant number of electrons was limited by Space Charge Limit of the gun. This result is very encouraging becaouse the charge Limit phenomenon is supposed to be the most serious problem of a photocathode gun in a future linear collider.