Beam Physics Seminar
Photocathode operational lifetime of modern DC high voltage GaAs photoguns is limited primarily by ion backbombardment, the mechanism where residual gas at the cathode/anode gap is ionized by the extracted electron beam and back-accelerated toward the photocathode. Ions with sufficient kinetic energy damage the photocathode surface or possibly sputter away the chemicals used to create the negative electron affinity condition. Improving gun vacuum is an obvious way to prolong photocathode operating lifetime – better vacuum means there will be fewer ions to damage the photocathode surface however this is not a trivial task. In this paper, the possibility of improving photocathode lifetime by merely increasing the laser beam diameter at the photocathode is explored. For a large laser spot, ion production at the cathode/anode gap is the same, but ion damage is distributed over a larger area. Lifetime measurements were made using a bulk GaAs photocathode inside a 100 kV DC high voltage load locked gun at DC average beam currents from 1.5 to 10 mA.
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