First mm-wave generation in the FOM free electron maser

A free electron maser (FEM) has been built as a pilot experiment for a mm-wave source for applications on future fusion research devices such as ITER, the International Tokamak Experimental Reactor. A unique feature of the Dutch Fusion-FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the first phase of the project, the so-called inverse set-up is used. The electron gun is mounted inside the high-voltage terminal. The entire beam line was tested successfully with extremely low loss current, lower than 0.05%. First generation of mm-waves was achieved in October 1997. Up to now the highest peak power measured is 700 kW at 200 GHz. This was achieved with a beam current of 8 A and an accelerator voltage of 1.77 MV. Output power, start-up time and frequency correspond well with simulation results. Parameter scans for the longitudinal undulator gap, accelerator voltage and reflection coefficient have given a wide range of interesting data of which a few highlights are given