Investigation on Mode Competition Between the Fundamental and Second-Harmonic Modes in the Complex Gyrotron

Stable operation in a single mode is an important goal of high-power gyrotron. Both multimoding and switching into unwanted modes can lead to lower efficiency. Based on the multimode, multifrequency, and time-dependent fixed field theory, a numerical code has been written for simulating the mode competition in the complex cavity gyrotron operating at the second harmonic, which is confirmed to be correct by comparison with the self-consistent simulation. With the help of the code, the mode competition in the designed complex gyrotron with the frequency of 0.42 THz is simulated by tracking several competition modes without considering ohmic losses, whose influence is roughly estimated. Through mode competition simulations, it is verified that the complex gyrotron with gradually tapered cavity has a good capability to suppress the mode competition, especially the competition from the fundamental modes. Meanwhile, the simulations show that the single mode oscillation of the desired mode TE17.4 at 150-kW level can be expected when the optimal operating condition is that the beam voltage is 50 kV, the beam current is 6 A, and the applied magnetic field is 7.98 T.