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Polarized Photocathode R&D for Future Linear Colliders

It is a challenge to generate full charge electrons from the electron sources without compromising polarization for the proposed ILC and CLIC. It is essential to advance polarized photocathodes to meet the requirements. SLAC has worldwide unique dedicated test facilities, Cathode Test System and dc‐Gun Test Laboratory, to fully characterize polarized photocathodes. Recent systematic measurements on a strained‐well InAlGaAs/AlGaAs cathode at the facilities show that 87% polarization and 0.3% QE are achieved. The QE can be increased to ∼1.0% with atomic hydrogen cleaning. The surface charge limit at a very low current intensity and the clear dependence of the polarization on the surface charge limit are observed for the first time. Ongoing programs to develop photocathodes for the ILC and CLIC are briefly introduced.