Prospects for generating polarized electron beams for a linear collider using an rf gun

Abstract

The next generation of linear colliders — represented by the Japanese Linear Collider (JLC) and the Next Linear Collider (NLC) — will probably utilize polarized electrons generated by a photocathode gun. A photocathode gun with high polarization ( P e) photocathodes (up to P e ∼ 80% achieved to date) is currently providing polarized electrons for the SLC. The SLC source requires subharmonic bunching at low energy to reduce the bunch length prior to S-band bunching and a damping 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 vacuum 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 bandgap threshold. These problems have so far precluded any serious attempt to design an rf gun for polarized electrons. Several technical advances that now improve the prospects for a practical polarized electron rf gun are described. Finally, new ideas for high polarization photocathodes that permit operation in a relatively poor vacuum and techniques being explored to mitigate the low QE “charge limit” are discussed.