A high power, tunable free electron maser for fusion

The Fusion-FEM experiment, a high-power, electrostatic free-electron maser being built at the FOM-Institute for Plasma Physics 'Rijnhuizen', is operat ed at various frequencies. So far, experiments were done without a depresse d collector, and the pulse length was limited to 12 mus. Nevertheless, many aspects of generation of mm-wave power have been explored, such as the dep endency on the electron beam energy and beam current, and cavity settings s uch as the feedback coefficient. An output power of 730 kW at 206 GHz is ge nerated with a 7.2 A, 1.77 MeV electron beam, and 360 kW at 167 GHz is gene rated with a 7.4 A, 1.61 MeV electron beam. It is shown experimentally and by simulations that, depending on the electron beam energy, the FEM can ope rate in single-frequency regime. The next step of the FEM experiment is to reach a pulse length of 100 ms. The major part of the beam line, the high v oltage systems, and the collector have been completed. The undulator and mm -wave cavity are now at high voltage (2 MV). The new mm-wave transmission l ine, which transports the mm-wave output power from the high-voltage termin al to ground and outside the pressure tank, has been tested at low power. ( C) 2001 Elsevier Science B.V. All rights reserved.