Discrete frequency hopping in long-pulse high-power gyrotrons

Careful investigation of the RF oscillation frequency generated by MW class long-pulse 140 GHz gyrotrons for the Wendelstein 7-X (W7-X) stellarator found discrete frequency hopping in steps of 5-10 MHz during the initial frequency down-tuning as well as in long-pulse “quasi” steady-state operation. The classical explanation for this behavior, such as electron beam acceleration voltage ripple and the classical long-line effect of the load, are reviewed, and are found not to match to the experimental observations. As alternative hypotheses we propose a long-line effect involving multiple internal reflections, or small discrete changes in the effective cavity radius, caused by discontinuous thermo-mechanical expansion.