PHOTOEMISSION USING FEMTOSECOND LASER-PULSES

Successful operation of a short wavelength FEL requires an electron bunch of current > 100 A and normalized emittance < 1 mm mrad. Recent experiments show that rf guns with photocathodes as the electron source may be the ideal candidate for achieving these parameters. To reduce the emittance growth due to space charge and rf dynamics effects, the gun may have to operate at high field gradient (hence at high rf frequency) and a spot size small compared to the aperture. This may necessitate the laser pulse duration to be in the subpicosecond regime to reduce the energy spread. We will present the behavior of metal photocathodes upon irradiation with femtosecond laser beams, comparison of linear and nonlinear photoemission, and scalability to high currents. Theoretical estimate of the intrinsic emittance at the photocathode in the presence of the anomalous heating of the electrons, and the tolerance on the surface roughness of the cathode material, will be discussed.