Thermal emittance and response time measurements of negative electron affinity photocathodes

The thermal emittance and temporal response of a photocathode set an upper limit on the maximum achievable electron beam brightness from a photoemission electron source, or photoinjector. We present measurements of these parameters over a broad range of laser wavelength for two different negative electron affinity (NEA) photocathodes. The thermal emittance of NEA GaAs and GaAsP has been measured by two techniques-a measurement of the beam size downstream from a solenoid, whose strength was varied, and a double slit transmission measurement-for different laser spot sizes and shapes. The effect of space charge on the beam spot size allows a good estimation of the photoemission response time from these cathodes. Both cathodes show a subpicosecond response for laser wavelengths shorter than 520 nm. (C) 2008 American Institute of Physics.