Use of non evaporable getter pumps to ensure long term performances of high quantum efficiency photocathodes

Abstract

High quantum efficiency photocathodes are routinely used as laser triggered emitters in the advanced high brightness electron sources based on radio frequency guns. The sensitivity of “semiconductor” type photocathodes to vacuum levels and gas composition requires special care during preparation and handling. This paper will discuss the results obtained using a novel pumping approach based on coupling a 20 l s−1 sputter ion getter pump with a CapaciTorr® D100 non evaporable getter (NEG) pump. A pressure of 8⋅10−8 Pa was achieved using only a sputter ion pump after a 6 day bake-out. With the addition of a NEG pump, a pressure of 2⋅10−9 Pa was achieved after a 2 day bake-out. These pressure values were maintained without power due to the ability of the NEG to pump gases by chemical reaction. Long term monitoring of cathodes quantum efficiencies was also carried out at different photon wavelengths for more than two years, showing no degradation of the photoemissive film properties.