SLC polarized beam source ultra-high-vacuum design

The authors describe the design of the ultra-high vacuum system for the beam-line from the 160 kV polarized electron gun to the linac injector in the Stanford Linear Collider (SLC). The polarized electron source is a GaAS photocathode, requiring 10/sup -11/ Torr-range pressure for adequate quantum efficiency and longevity. The photocathode is illuminated by 3 ns-long laser pulses. Photocathode maintenance and improvements require occasional substitution of guns with rapid restoration of UHV conditions. Differential pumping is crucial since the pressure in the injector is more than 10 times greater than the photocathode can tolerate and since electron-stimulated gas desorption from beam loss in excess of 0.1% of the 20 nC pulses may poison the photocathode. The design for the transport line contains a differential pumping region isolated by a pair of valves. Exchange of guns requires venting only this isolated region, which can be restored to UHV rapidly by baking. The differential pumping is performed by nonevaporable getters and an ion pump.<>