Emittance and Quantum Efficiency Measuremen 1.6 Cell S-band Photocathode Rf Gun with Mg C

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 at 110 MV/m and 30° laser phase for the Cu and 8 10 at 90 MV/m and 30° laser phase for the Mg cathode. INTRODUCTION The cathode is one of the most important components of a photocathode rf gun as it defines the quantum efficiency (QE) and the minimum achievable emittance or so called thermal emittance. In addition the cathode also affects the maximum attainable field in the gun due to rf breakdown at the cathode to back plate joint. The ideal cathode would exhibit high QE, low thermal emittance and would not limit the maximum attainable field. A peak on axis field of 120 MV/m is required in order to achieve an emittance of 1 μm with 1 nC of charge as desired for the Linac Coherent Light Source (LCLS) [1]. The maximum required field limits the choice of cathode materials to metals. This paper reports the results of measurements made at the SLAC Gun Test Facility with a Mg cathode. The results are also compared with theoretical values and previously reported Cu cathode results [2]. The Cu cathode used in the study is a 1 cm diameter, single crystal (100 orientation) brazed into the polycrystalline Cu back plate. After brazing, the cathode was polished with 0.25 μm diamond paste and installed on the gun in a N2 environment. The 2 cm diameter Mg insert was friction welded into the Cu back plate. After welding, the Mg cathode and back plate surface were machined using single point diamond tools. The cathode was offset from the lathe center to eliminate a machining defect at the center of the cathode. The Mg cathode was installed on the gun in air so that the electric field on axis could be measured with a bead drop measurement. The maximum field attained with the Cu cathode was 127 MV/m with reliable operation at 110 MV/m and typically 2 10 Torr vacuum pres beam operation. The Mg cathode fields due to rf breakdown. The max was 107 MV/m with reliable operatio 10 Torr vacuum pressure with e comparison, a polycrystalline Cu cath weld joint was operated up to 140 operation at 125 MV/m. THEORY The definition of thermal emit equation 1 where x is the beam po transverse momentum [3]. Assum laser pulse shape and averaging ove distribution leads to equation 2 whe beam radius, Ek is the electron kineti given by the sum of the Fermi energ and Eb is the metal barrier energy wh Fermi energy and work function less reduction. Using the definition of Q emitted electrons per incident phot computed as shown in equation 3 w reflectivity. After integration th equation 4 where EF is the Fermi ene