Spectral Dynamics of the Fem

The FOM Fusion FEM project involves the construction and operation of a 1-MW, 100 ms pulse, rapidly tunable FEM in the 130-250 GHz range for fusion applications. The undulator is a novel step-tapered undulator, consisting of two sections with different strengths and lengths and equal periodicities, and separated by a fieldfree gap. The purpose of this novel proposal is to enhance the efficiency at high output power. The associated high gain in the linear and in the non-linear regime provide a unique oscillator. The spectral dynamics of the high-current FEM with a low-quality cavity is calculated with a multi-pass, multi-frequency code. In this code the electrons are described 3D. The equations in the model are not averaged over a wiggler period. The continuous beam limit is considered. The radiation field is described as a sum over discrete frequencies. The millimeter wave field has the transverse radial dependence of the HE(11)-mode in the rectangular corrugated waveguide. The linear gain curve of the step-tapered undulator has a completely different spectrum than the single undulator. Furthermore the gain of the FEM is so high that non-linear interaction already occurs within a few passes. In the fully non-linear regime the gain is still relatively high and the output power reaches the required high level. Already in an early phase the spectral dynamics is strongly influenced by non-linear competition between the various maser modes. This non-linear mode competition is investigated, in particular the evolution of the sidebands is analized. It is observed that the spectral signal at the resonant frequency of the second undulator is suppressed. This suppression is observed for several gap lengths, Furthermore the spectrum can change with the variation of the gap length from quasi-stable to chaotic.