Abstract
${\mathrm{Cs}}_{x}{\mathrm{K}}_{y}\mathrm{Sb}$ 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 $<20\text{ }\text{ }\mathrm{nm}$ to $>1\text{ }\text{ }\mathrm{um}$), 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.
Highlights
Free electron lasers [1], energy recovery linacs [2] and electron-cooling [3] applications require unpolarized electron beams with high bunch charge and high average current
This work shows that photocathodes fabricated with different Sb thicknesses over an estimated range 1 μm provided similar thermal emittance per rms laser spot size, and similar to values reported in literature of nominally “thin” photocathodes, namely 0.4 mm mrad=mm [4] to 0.5 mm mrad=mm [18,19,20,21]
These measurements are consistent with recent measurements at x-ray light sources that indicate photocathode formation to be an exothermic reaction, with the Sb foundational layer completely transformed in the process [22]
Summary
Free electron lasers [1], energy recovery linacs [2] and electron-cooling [3] applications require unpolarized electron beams with high bunch charge (many nano-Coulombs) and high average current (tens to hundreds of milliamperes). This work shows that photocathodes fabricated with different Sb thicknesses over an estimated range 1 μm provided similar thermal emittance per rms laser spot size, and similar to values reported in literature of nominally “thin” photocathodes, namely 0.4 mm mrad=mm [4] to 0.5 mm mrad=mm [18,19,20,21]. These measurements are consistent with recent measurements at x-ray light sources that indicate photocathode formation to be an exothermic reaction, with the Sb foundational layer completely transformed in the process [22]
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