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Polarized RF Guns


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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