Initial measurements of the UCLA rf photoinjector

Abstract

An rf photocathode gun which, along with a compact linac, forms the injection system for a planned 10 mm free-electron laser amplifier experiment, has been commissioned in the Particle Beam Physics Laboratory at UCLA. This high-gradient gun, based on the Brookhaven design, has emitted several picosecond, 100 A electron beams of up to 4 MeV in energy. These beams have been characterized by a variety of diagnostics. The quantum efficiency of the copper cathode used has been measured at normal incidence, and at 70° incidence, where the polarization dependence was also examined. Limits on laser intensity due to surface damage, and to longitudinal space charge suppression of photoemission have been explored. The energy and energy spread of the beam were characterized using a dipole spectrometer, while the time structure was examined using a picosecond resolution streak camera. Both energy spread and pulse length were found to be adversely affected by longitudinal space charge forces. The emittance of the beam was measured using the pepper pot technique, and its dependence on space charge and rf phase were found. The impact of these results on improving the design and operation of high brightness photoinjectors is discussed, in particular with respect to SASE FEL amplifiers such as the UCLA 10 mm FEL, and the proposed SLAC X-ray FEL.