Predicted operating conditions for maintaining mode purity in the 1 MW 200 GHz FOM free electron maser

A Free Electron Maser (FEM) oscillator is now under construction at the FOM Institute (Rijnhuizen), Netherlands with the goal of producing 1 MW long pulse to CW microwave output in the range 130 GHz to 250 GHz with application to plasma heating. A newly developed multi-frequency FEM simulation code (MALT1D) using the windows approximation in the high gain regime and including AC space charge is used to model possibilities for longitudinal mode competition and resulting non-stationary chaotic output. The code predicts that stationary single frequency output can be obtained by taking advantage of the frequency dependence of the feedback reflection coefficient and by optimizing the gap spacing between the two wiggler sections.