Investigations on an experimental 170 GHz coaxial cavity gyrotron

As part of the European program for development of a 2 MW, CW, 170 GHz coaxial cavity gyrotron, the experimental studies on a short pulse (~ few ms) gyrotron has been continued. A hypothesis concerning the excitation of observed parasitic low frequency (LF) oscillations around 260 MHz has been proven experimentally. Small modifications of the shape of the coaxial insert resulted in an increase of the starting current for the LF oscillations by about a factor of 3 (to above 40 A) causing a strong reduction of the LF amplitude. The influence of the level of microwave stray radiation on gyrotron performance (range of single mode oscillation and efficiency) has been investigated. To improve the performance of the quasi optical (q.o.) RF output system a new launcher with a different wall corrugation and a new adapted phase correcting mirror has been tested both at low power and in the gyrotron. The results are going to be analyzed and compared with simulations.