[SLAC Pubs and Reports]
RF Guns for Generation of Polarized Electron Beams
Several accelerators, including the SLC, JLAB, Mainz, Bates/MIT, and Bonn
have successfully operated for medium and high energy physics experiments using
polarized electron beams generated by dc-biased guns employing GaAs
photocathodes. Since these guns have all used a bias on the order of 100 kV,
the longitudinal emittance of the extracted bunch is rather poor. Downstream rf
bunching systems increase the transverse emittance. An rf gun with a GaAs
photocathode would eliminate the need for separate rf bunchers, resulting in a
simpler injection system. In addition, the thermal emittance of GaAs-type
cathodes is significantly lower than for other photocathode materials. The
environmental requirements for operating activated GaAs photocathodes cannot be
met by rf guns as currently designed and operated. These requirements,
including limits on vacuum and electron back bombardment, are discussed in some
detail. Modifications to actual and proposed rf gun designs that would allo!
w these requirements to be met are presented.
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