170 GHz, 2 MW Coaxial Cavity Gyrotron - investigation of the parasitic oscillations and efficiency of the RF-output system -*

The mechanism of excitation of parasitic LF oscillations inside a coaxial gyrotron has been studied. At present a new coaxial insert with a modified profile on the cathode side of the resonator is under test. A suppression or at least increase of the starting current for the excitation of the LF oscillations is expected. As a first step to increase the Gaussian content of the RF output beam a launcher with only a 3rd order azimuthal perturbation was fabricated. Tests at low power have already been performed and tests with the experimental pre-prototype gyrotron are in progress. The results are presented and discussed.