INVESTIGATION OF MULTIFREQUENCY GENERATION IN THE FEM

The FOM Fusion FEM project involves the construction and operation of a 1-MW, 100 ms pulse, rapid tunable FEM in the 130-250 GHz range for f usion applications. The undulator of the FEM consists of two sections with different strengths and different lengths separated by a gap with out undulator field. The design provides arbitrary focussing. Single f requency codes predict a much higher output power for the two-section undulator than for a one-section undulator. However, the different und ulator strengths lead to different resonance conditions and therefore, in principle, multiple frequencies can be generated. This problem of multi-frequency generation due to the two-section undulator is under i nvestigation. First results are presented in this paper. The longitudi nal mode structure of the FEM is simulated in a multi-pass, multi-freq uency code. In this code the electrons are described fully 3D, non-wig gler averaged and in the long pulse limit. AC longitudinal space-charg e forces are included. The radiation field is considered to have the k nown transverse radial dependence of a HE11-mode, due to the rectangul ar corrugated waveguide of the FEM. A multi-pass calculation in the si ngle-frequency limit is presented, where the field grows into saturati on.