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 fusion applications. The undulator of the FEM consists of two sections with different strengths and different lengths separated by a gap without undulator field. The design provides arbitrary focussing. Single frequency codes predict a much higher output power for the two-section undulator than for a one-section undulator. However, the different undulator 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 investigation. First results are presented in this paper. The longitudinal mode structure of the FEM is simulated in a multi-pass, multi-frequency code. In this code the electrons are described fully 3D, non-wiggler averaged and in the long pulse limit. AC longitudinal space-charge forces are included. The radiation field is considered to have the known transverse radial dependence of a HE11-mode, due to the rectangular corrugated waveguide of the FEM. A multi-pass calculation in the single-frequency limit is presented, where the field grows into saturation.