Thermal emittance and lifetime of alkali-antimonide photocathodes grown on GaAs and molybdenum substrates evaluated in a −300 kV dc photogun

${\mathrm{Cs}}_{x}{\mathrm{K}}_{y}\mathrm{Sb}$ photocathodes grown on GaAs and molybdenum substrates were evaluated using a $\ensuremath{-}300\text{ }\text{ }\mathrm{kV}$ dc high voltage photogun and diagnostic beam line. Photocathodes grown on GaAs substrates, with varying antimony layer thickness (estimated range from $l20\text{ }\text{ }\mathrm{nm}$ to $g1\text{ }\text{ }\mathrm{um}$), yielded similar thermal emittance per rms laser spot size values ($\ensuremath{\sim}0.4\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}/\mathrm{mm}$) but very different operating lifetime. Similar thermal emittance was obtained for a photocathode grown on a molybdenum substrate but with markedly improved lifetime. For this photocathode, no decay in quantum efficiency was measured at 4.5 mA average current and with peak current 0.55 A at the photocathode.