Study of partial-waveguide rf-linac FELs for intense THz-pulse generation

In this paper, we present a time-domain analysis of a short pulse partial-waveguide FEL oscillator employing toroidal mirrors and a hole outcoupling. The use of toroidal mirrors with optimized radius of curvatures helps to reduce cavity losses arising from the mismatch of the free space propagating optical field into a waveguided one. We introduce semi-analytical expressions for the calculation of the scattering matrix elements describing the loss and mode coupling mechanism as well as the amount of the extracted power from the cavity. The formulation is implemented in a time-domain FEL code based on modal expansion approach. The described model is applied to a partial-waveguide FEL system and simulation results are compared with measurements.