D-band frequency step-tuning of a 1 mw gyrotron using a Brewster output window

In order to demonstrate the usability of gyrotron oscillators as frequency step tunable high power millimeter-wave sources, experiments on a 1 MW, 140 GHz TE22,6 gyrotron with a built-in quasi-optical (q. o.) mode converter have been performed. By varying the operating parameters of the tube, a series of oscillations in the frequency range from 114 GHz to 166 GHz were excited. To avoid reflections, caused by the required vacuum barrier window, the gyrotron was equipped with a Brewster window. The achieved output power levels between 0.85 and 1.05 MW are compared to measurements carried out with the same tube using a conventional single-disk window. These experiments showed that even by using a q. o. mode converter, the influence of window reflections on the gyrotron oscillatory behavior cannot be removed completely.