A GaAs Type Source of Polarized Electrons at the Mainz Race Track Microtron MAMI

The source of polarized electrons attached to the 855 MeV race track microtron MAMI in Mainz is based on photoemission from III-V-semiconductors. Strained layers of GaAsP are used at present. Typical degree of spin-polarization of emitted electrons is 75% with 830 nm light irradiating the cathode of the source gun. The light is produced by a gain switched laser diode followed by a laser diode amplifier. The diode is driven by the 2.45 GHz radiofrequency of the accelerator. So a 2.45 GHz pulsed beam in synchronism with the accelerator-rf is injected into MAMI. Thereby a capture efficiency of injection better than 93% is achieved. Beam currents up to 20 µA that are spin-polarized to a degree of 75% are available at MAMI for long term experiments. Total beam time scheduled for experiments is above 2000 h/a.