INITIAL MEASUREMENTS OF THE UCLA RF PHOTOINJECTOR

An rf photocathode gun which, along with a compact linac, forms the in jection system for a planned 10 mm free-electron laser amplifier exper iment, has been commissioned in the Particle Beam Physics Laboratory a t UCLA. This high-gradient gun, based on the Brookhaven design, has em itted several picosecond, > 100 A electron beams of up to 4 MeV in ene rgy. 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-degrees incidence, where the polarization dependence was also examined. Limits on laser intensity due to surfac e damage, and to longitudinal space charge suppression of photoemissio n have been explored. The energy and energy spread of the beam were ch aracterized using a dipole spectrometer, while the time structure was examined using a picosecond resolution streak camera. Both energy spre ad and pulse length were found to be adversely affected by longitudina l space charge forces. The emittance of the beam was measured using th e pepper pot technique, and its dependence on space charge and rf phas e were found. The impact of these results on improving the design and operation of high brightness photoinjectors is discussed, in particula r with respect to SASE FEL amplifiers such as the UCLA 10 mm FEL, and the proposed SLAC X-ray FEL.