High-brightness rf photocathode guns for single pass X-ray free-electron lasers

Single-pass X-ray FELs will require very bright, high-current, stable electron beams as drivers. A key element of the accelerator system for such a device is the electron source, which should produce 1 nC bunches with a few hundred amperes peak current and a normalized rms emittance of 1π mm mrad. The photocathode rf gun appears to be the only source capable of meeting these requirements. We discuss technological issues associated with beam dynamics, photocathodes, and drive-laser stability. The impact of drive-laser phase stability on buncher performance is discussed in detail.