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A comparison of electron beam parameters from a 1.6 cell S-band rf gun with Cu and Mg cathode at the SLAC Gun Test Facility are reported. The lower work function of Mg compared to Cu theoretically increases the quantum efficiency for a fixed laser wavelength and also increases the thermal emittance. Slice emittance measurements at low charge (15 pC) set an upper limit on the thermal emittance of 0.6 and 1.2 microns per mm radius for the Cu and Mg cathodes respectively. The longitudinal emittance measurements with both cathodes exhibit large energy spreads emitted from the gun. The measured quantum efficiency with no laser cleaning is approximately 3 10 -5 at 110 MV/m and 30° laser phase for the Cu and 8 10 -5 at 90 MV/m and 30° laser phase for the Mg cathode.

EMITTANCE AND QUANTUM EFFICIENCY MEASUREMENTS FROM A 1.6 CELL S-BAND PHOTOCATHODE RF GUN WITH MG CATHODE