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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.

This paper reports the results of the high-power test of a remote steering launcher mockup at 140 GHz, which were performed at the ECRH installation for the future stellarator W7-X at IPP Greifswald. The mock-up test system consists of a 6.62 m long square corrugated waveguide with a steerable optic at the entrance and various diagnostics at the exit of the waveguide. A straight launcher and version with 2 integrated mitre-bends were investigated. The high-power tests of the straight setup have been performed with powers up to P 0 = 700 kW (typically 500 kW) and pulse lengths of up to 10 seconds. In the dog-leg verison, arcing limited the power and pulse length. For both polarizations (parallel and perpendicular to the steering plane), no arcing was observed within this range of power and pulse-length for the straight setup, in spite of the fact, that the experiments were performed under ambient atmospheric conditions. After the integration of 2 mitre bends in the setup, arcing limited the usable parameter range. The ohmic loss P Ω of the waveguide was measured via the temperature increase of the waveguide wall, and was used to calibrate the calculated angular dependence of the total ohmic losses of the waveguide. Short-pulse radiation pattern measurements with thermographic recording show high beam quality and confirm the steering range of −12• < φ < 12• . The version with two mitre bends produces similar results but with an increased level of sidelobes and arcing problems.

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