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AN EXPERIMENTAL FACILITY FOR MEASURING THE ELECTRON ENERGY DISTRIBUTION FROM PHOTOCATHODES

ASTeC has spent several years developing a GaAs Photocathode Preparation Facility (PPF) which routinely produces cathodes with quantum efficiencies (Q.E. )u p to 20 % at 635 nm [1]. The goal is to use these and other cathode materials in high–average–current high–brightness injectors for particle accelerators. Electron injector brightness is limited by source beam emittance, and brightness will be increased significantly by reducing the longitudinal and transverse energy spread in the emitted electrons, thereby creating a cold beam. We are constructing an experimental system which is compatible with the PPF for measurement of the energy distribution in electrons emitted from photocathodes at room and LN2-temperature. The photocathode will be illuminated by a small, variable–wavelength light spot. The electron beam image will be projected onto a detector comprised of a 3-grid energy filter, a microchannel plate and a phosphor screen. A low–noise CCD camera will capture screen images, and the spatial and energy distribution of the emitted electrons will be extracted through analysis of these images as a function of the grid potentials. The system will include a leak valve to progressively degrade the cathode, and thus allow its properties to be measured as a function of gas exposure.