[SLAC] [SLAC Pubs and Reports]
SLAC-PUB-7854
Polarized RF Guns
Abstract
RF guns employing photocathodes are now well established as viable
electron sources for accelerator applications. For high-energy
accelerators, the desirable properties of such sources include the
relative ease of pulse formation (using the source laser system to
establish the pulse shape and number) and a low transverse beam
emittance, eliminating the need for rf chopping and bunching systems
and reducing the demands on emittance-reducing damping rings. However,
most high-energy accelerators now require polarized
electrons. Polarized electron beams can in principle be generated by
substituting a III-V semiconductor, such as GaAs, for the traditional
photocathode in a conventional rf gun structure. In the past, the
principal criterion for selection of photocathodes for rf guns has
been the ability to maintain a reasonable quantum yield from the
cathode over a relatively long operating period. It is well known that
GaAs is significantly more sensitive to the vacuum environment than
any of the cathode types that have been used to date. However,
advances in our understanding of how to operate GaAs cathodes in
hostile environments combined with similar advances in our ability to
prepare gun structures that will significantly reduce dark current and
rf breakdown lend credibility to the prospect of successfully
operating an rf gun with an activated GaAs cathode. The status of
research related to polarized rf guns is reviewed and the outline of a
future polarized rf source combining the best of what is known today
is presented.
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