Approaches in modelling a waveguide RF-linac THZ-FEL

A widely used approach in describing radiation fields generated by short pulse rf linac FEL oscillators is based on modal expansion of the intracavity fields and solving the wave equation for the amplitudes of the excited transverse cavity eigenmodes, in time and axial dimension. In this time domain approach, mode amplitudes are calculated by adopting the slowly varying envelope approximation (SVEA). The longitudinal propagation number k_z of the carrier wave is usually assumed to represent a group velocity and a wave impedance of the frequencies encompassed within the spectral bandwidth of the optical pulse envelope as it evolves within the interaction region. However, in cases where dispersive effects arising from a waveguided cavity become influential, the space-frequency approach is a straightforward method to tackle the problem without introducing further approximations. One such case is being studied based on a highly slippage dominated short pulse rf-linac FEL operating at long wavelengths.