Experiments have been performed with the free-electron maser (FEM) at Rijnh
uizen, a high-power mm-wave source. A unique feature of the FEM is the poss
ibility 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 7
30 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 an
d the depressed-collector electrodes seem to have adequate cooling. (C) 200
1 Elsevier Science B.V. All rights reserved.