Performance of magnesium cathode in the S-band RF gun

In this paper, we present the preliminary results of the performance of the magnesium cathode in a high frequency RF gun. The quantum efficiency of Mg showed a dramatic improvement upon laser cleaning, increasing from 10/sup -5/ to 4/spl times/10/sup -4/ after two hours of cleaning, and to 2/spl times/10/sup -3/ after systematic cleaning. The cleaning procedure for this increase is described in detail. Charge measured as a function of the laser injection phase relative to the RF phase indicates that the temporal variation of the field on the cathode both due to the RF and the shielding effect of the emitted electrons play a critical role in the emission and extraction of electrons. A model that includes this variation is numerically fitted to the measured charge and the results are presented. The unexpected outcome of the fit was the low field enhancement factor (0.1) predicted by the model for the photoemission. The physical origin of this is still under investigation.