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.