COLLECTOR MATERIAL DESORPTION TESTS’ A

To obtain long cathode lifetimea, the surface of the spent beam collector for the photocathode RF power source (called the Lasertron) must have a low coefficient of darorption of gas moleculea by electrons. We uuxmbled a low voltage (5 10 kV) ultrahigh vacuum electron gun md target system to measure the desorption rates of CO and COz under conditions where the power density, charge density and pressure were the mme order as expected in the lasertron. Coppu coated with a thii layer of a nonevaporable getter (NEC) was found to have a coefficient of desorption of COz (CO) at least 30 (5) t imes less than vacuum degazsed high grade OFHC copper. The NEG material may be coated on the usual collector geometries and therefore can substantially reduce operating tube pressure in conventional klystron and other high current tuben. 2