Thermal emittance measurements from negative electron affinity photocathodes

Recent computational optimizations have demonstrated that it should be possible to construct electron injectors based on photoemission cathodes in very high voltage DC electron guns in which the beam emittance is dominated by the thermal emittance from the cathode. Negative electron affinity (NEA) photocathodes are known to have a naturally low thermal emittance. However, the thermal emittance depends on the illuminating wavelength, the degree of negative affinity, and the band structure of the photocathode material. As part of the development of a high brightness, high average current photoemission electron gun for the injector of an ERL light source, we have measured the thermal emittance from negative affinity GaAs and GaAsP photo- cathodes. The measurements were made by obtaining the electron beam spot size downstream of a counter-wound solenoid lens as a function of the lens strength. Electron beam spot sizes were measured by two techniques: a 20 mum wire scanner, and a BeO screen. Excellent agreement in measured emittance has been obtained between these two methods and direct transverse phase space mapping using a 20 mum slit and the screen. Results are presented for both cathode types. The Cornell ERL injector performance is evaluated based on the results of these measurements.