Simulations of the Performance of the Fem Oscillator for Fusion at 130-250 Ghz

The performance of a 1 MW, long pulse, 130-250 GHz free-electron maser being designed in the FOM-Institute of Plasmaphysics for fusion applications has been simulated by a fully 3-D, AC and DC space charge included, non-wiggle averaged, particle-pusher code. In comparison with previous designs a new shorter construction of a step-tapered undulator providing suppression of the double frequency amplification on the linear stage and a perfect matching of the power grow on the nonlinear stage is considered. The dependences of the linear gain and output power on the operating frequency, reflection coefficient, gap length between two undulator sections and electron beam focusing efficiency are examined. Particular attention has been devoted to the influence of space charge on the operation of the FEM oscillator. Longitudinal space charge reduces the linear gain but increases the level of power which can be reached inside the resonator. It has been shown that for a fixed undulator length there is an optimum value of the electron beam current density providing sufficiently high linear gain and output power.