Photoemission cathodes are important sources for electron beams, from Free Electron Lasers (FELs) to accelerator applications, due to the high quality electron beams that can be achieved. Metallic photocathodes are rugged and have fast response times, but low QE and require UV illumination. Direct band-gap p-type semiconductors have the highest QE and operate at longer wavelengths, but are chemically reactive and easily poisoned by H20 and CO2, damaged by back ion bombardment, and (for NEA III-V photocathodes) insufficiently responsive for pulse shaping in an rf injector due to their long emission time. The ideal photocathode will have a high QE at the longest possible wavelength, be capable of in situ repair or rehabilitation, and demonstrate good lifetime. To meet the particular needs of a megawatt (MW) class FEL, a photocathode must produce 1 nC of charge in a 10-50 ps pulse every nanosecond (ns) (100 A peak and 1 A average current) in applied fields of 10-50 MV/m and background pressures of 0.01 mTorr – and to do so for several seconds.
Even if such a photocathode were available, making predictions of its performance is a complex challenge. Useful models of photoemission must account for cathode surface conditions and material properties, as well as drive laser parameters. We shall report on our efforts to develop dispenser photocathodes and to model photoemission from them: they are rich in physics as a combination of field, thermal, and photoemission effects contribute to the electron emission process. We shall relate the models to our measurements as well as studies in the literature. The model accounts for surface conditions (coating, field enhancement, reflectivity), laser parameters (duration, intensity, wavelength), and material characteristics (reflectivity, laser penetration depth, scattering rates) to predict current distribution and quantum efficiency. While we focus on dispenser photocathodes in particular, as they introduce complications such as coverage non-uniformity, field enhancement, and work function variation with degree of coverage by coatings such as barium and cesium, we shall also discuss other photocathode configurations.


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