AbstractExperiments have been performed with the free-electron maser (FEM) at Rijnhuizen, a high-power mm-wave source. A unique feature of the 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 so-called inverse set-up, where the electron gun is mounted inside the high-voltage terminal, a peak power of 730 kW was measured at 200 GHz and of 350 kW at 167 GHz [1], [2]. Furthermore, we made the design work to extend the pulse-length to 1 s. Detailed thermal behavior of the critical components is studied. Both the cavity mirrors and the depressed-collector electrodes seem to have adequate cooling.