Simulation of Parasitic Backward-Wave Excitation in High-Power Gyrotron Cavities

The possibility of parasitic excitation of backward waves directly in the gyrotron cavity is demonstrated, by simulation, for two existing high-power gyrotrons. These are the 140-GHz 1-MW TE $_{{28},{8}}$ -mode gyrotron for the stellarator W7-X and the 140-GHz 1.5-MW TE $_{{28},{10}}$ -mode gyrotron, also for W7-X. The parasitic backward waves, namely, the TE $_{{23},{7}}$ mode in the 1-MW gyrotron and the TE $_{{-{24},{10}}}$ mode in the 1.5-MW gyrotron, are excited at high frequencies (RF), which are of the order of $\sim $ 10% lower than the nominal operating frequency and which can lead to significant performance degradation, with respect to the output power, efficiency, and stability of the tube. This finding offers an additional possibility, besides parasitic mode excitation in the gyrotron beam tunnel or after the gyrotron cavity, for the origin of experimentally observed RF parasitic oscillations in high-power, high-frequency gyrotrons, operating in high-order modes. To strengthen the confidence in the simulation, the results of two codes, each using different modeling of the interaction between the electron beam and the RF wave, are compared and the appropriateness of the modeling with respect to the accurate simulation of backward waves is discussed in detail.