The stabilizing influence of external reflections on gyrotron operation has been demonstrated in direct particle-in-cell modeling. A model with a temporal realization duration providing an opportunity to calculate frequency characteristics with a relative accuracy no worse than 2 × 10^–6 has been used for the first time. An almost twofold reduction in the spectrum width of a 28-GHz gyrotron has been observed experimentally after the introduction of reflections of the output signal.

In this work, we calculated the efficiency of second harmonic generation for sub-THz gyrotron radiation in a single-crystal InP plate embedded in a conical metal waveguide. For the values of the losses at the doubled frequency (depending on the degree of purity of the InP crystal), the dimensions of the wafer are revealed that provide the optimal effective frequency conversion. Thus, the conversion factor f –>2f can reach 5 W/kW^2 when pumped at a frequency of 263 GHz and a loss of alpha_2f = 0.1 cm^-1. The possibility of a noticeable increase in the efficiency of frequency doubling with fine tuning of the phase matching due to the selection of the size of the submicron gap between the InP plate and the plates of the metal waveguide is shown. The results obtained can be used to solve applied problems in the field of dynamic nuclear polarization spectroscopy and nuclear magnetic resonance studies.