PROSPECTS FOR GENERATING POLARIZED ELECTRON-BEAMS FOR A LINEAR COLLIDER USING AN RF GUN

The next generation of linear colliders - represented by the Japanese Linear Collider (JLC) and the Next Linear Collider (NLC) - will probab ly utilize polarized electrons generated by a photocathode gun. A phot ocathode gun with high polarization (P(e)) photocathodes (up to P(e) a pproximately 80% achieved ro date) is currently providing polarized el ectrons for the SLC. The SLC source requires subharmonic bunching at l ow energy to reduce the bunch length prior to S-band bunching and a da mping ring at high energy to reduce the transverse emittance. The use of an rf gun can eliminate the former and possibly simplify the latter . However, rf guns as presently developed have serious problems with v acuum contamination, which would quickly lower the quantum efficiency (QE) of a semiconductor photocathode. In addition, the ''charge limit' ' previously reported for high peak current pulses puts a limit on the laser power usable for photoexciting a low QE cathode near the bandga p threshold. These problems have so far precluded any serious attempt to design an rf gun for polarized electrons. Several technical advance s that now improve the prospects for a practical polarized electron rf gun are described. Finally, new ideas for high polarization photocath odes that permit operation in a relatively poor vacuum and techniques being explored to mitigate the low QE ''charge limit'' are discussed.