Observation of Discrete Frequency Hopping in MW-Class Gyrotrons During Long-Pulse Operation

A detailed investigation of the RF oscillation frequency produced by megawatt-class continuous wave 140 GHz gyrotrons for the stellarator W7-X found discrete jumps of a few megahertz during the initial frequency downtuning phase, as well as in quasi-steady long-pulse operation. The common explanations for such behavior, such as voltage ripple and the classical long-line effect, are reviewed, and are found not to match the experimental observations properly. As alternative hypotheses, we suggest a long-line effect involving higher-order reflections inside the gyrotron, or discrete changes in the cavity radius, caused by discontinuous thermomechanical expansion.