Increasing the diffraction losses in gyrotron beam tunnels for improved suppression of parasitic oscillations

Towards a 1.5 MW, 140 GHz gyrotron for the upgraded ECRH system at W7-X

“…In order to mitigate the risk of parasitic mode excitation by beam currents of that order, a new baseline beam tunnel has been designed, using the code NESTOR [22]. Although the BeO/SiC ceramic material for the absorbing rings has been kept the same as in the beam tunnel of the 1 MW gyrotron, the geometry of the dielectric and copper rings has been optimized according to the recent findings of [23] and a considerable increase of the diffraction losses of the possible parasitic modes has been achieved. According to cold-cavity calculations, the performance of the new baseline beam tunnel is improved by approximately 25% (in terms of number and quality factor of potential parasitic modes in the frequency range of 115-135 GHz), compared to the beam tunnel of the 1 MW gyrotron.…”