HIGH CAPTURE EFFICIENCY FOR THE POLARIZED BEAM AT MAMI BY R.F.-SYNCHRONIZED PHOTOEMISSION

The current from a GaAs-type source of polarized electrons is limited because some photocathode detoriation processes - like the production of ions from the residual gas - tend to increase with the current itself. Therefore a high longitudinal capture efficiency of the injection into the accelerator is desirable in order to use the produced charge economically. This may be achieved by the 'synchro-laser' method: Picosecond light pulses are synchronized to the accelerator r.f. and generate a 2.45 GHz electron pulse train. Reliable operation has been achieved with a bunch length of 50 ps that resulted in a transmission from the source to the target of 90 %.