3D design of the Fusion-FEM depressed collector using the General Particle Tracer (GPT) code

The “Rijnhuizen” Fusion Free-Electron Maser (FEM) is the pilot experiment for a high power, mm-wave source, tunable in the range 130-260 GHz. The FEM has generated 730 kW output power during 10 μs pulses. To increase the overall efficiency to over 50% and to reach a pulse length of at least 100 ms, an electron beam charge and energy recovery system is currently being designed and installed. This system consists of an electrostatic decelerator, which decels the beam from 2 MeV to an average of 200 keV, and a depressed collector. The EM-wave interaction inside the undulator can result in an energy spread of 300 keV behind the decelerator. The multi-stage collector is designed so that electrons fall on the backside of one of three electrodes, thus ensuring that secondary particles will immediately be accelerated back towards the electrodes. However, scattered primary electrons can cause back streaming, hereby reducing the efficiency and possibly damaging the machine. To reduce this back streaming to below a tolerable 0.1%, the General Particle Tracer (GPT) code is being used to calculate primary and scattered particle trajectories inside the collector. It will be shown that an off-axis bending scheme, using a rotating perpendicular magnetic field lowers the back streaming and hereby increases the pulse length of the machine. The bending scheme also improves the power dissipation in the collector