Nonlinear RF and space-charge induced emittance growth in a thermionic injector accelerating cavity

The performance of a high-power free-electron laser depends crucially on the attainment of low beam emittances and short pulse lengths from the injector. One potential injector design involves generating a beam from an rf-gated thermionic cathode that is located within a 700 MHz accelerating cavity. The proposed RF cavity design is modeled using the field-solver Poisson Superfish. Particle energies in the cavity range from a few hundred eV at the cathode up to 1 MeV at the cavity exit. At these low energies, space-charge effects play a crucial role in determining the phase-space evolution and emittance growth of the beam. To examine these effects, the accelerating cavity region is modeled using the 3-d particle-in-cell code PARMELA. We explore the use of an additional 3^rd-harmonic 2.1 GHz accelerating structure to minimize RF-induced curvature in the longitudinal phase space and to improve the longitudinal bunching of the beam.