Mechanism of Forward Radiation in Gyrotron Oscillator With Regular Tube

Gyromonotrons are conventionally thought to rely on the convective interaction between the co-propagating beam and wave, where extended energy exchange stems from external feedback. However, numerous recent studies have demonstrated that steady forward wave radiation may be realized in a uniform, regular tube without any nonuniformities caused by structural feedback or the termination of electron beams. This study revisits the underlying mechanism for the forward wave in gyrotron oscillators. Comparative studies on the linear and nonlinear behaviors reveal an "intrinsic feedback resonance"nature in such a device due to the ultra-slow group velocity near the cutoff of the mode. Such a scenario allows the as-generated forward radiation to modulate the coming electrons through a backward-wave-like feedback loop, forming an effective absolute instability process. The quasi-degenerate nature of such bi-directional propagating waves further constructively enables the regular tube to function as a high-Q resonator, which is traditionally believed to arise from the adopted cavity structure. Briggs's theory is moreover adopted to discuss the instability in the system. Notably, these findings hold true regardless of the chosen TE or TM modes, refreshing and providing valuable insights into the underlying interaction mechanism of gyrotron devices.