Linear and Time-Dependent Behavior of the Gyrotron Backward-Wave Oscillator

Abstract

Formation of axial modes in the gyrotron backward-wave oscillator is examined in the perspective of optimum conditions for beam-wave interactions. Distinctive linear properties are revealed and interpreted physically. Nonlinear implications of these properties (specifically, the role of high-order axial modes) are investigated with time-dependent simulations. Nonstationary oscillations exhibit self-modulation behavior while displaying no evidence of axial mode competition. Reasons for the erratic frequency tuning are investigated and stable tuning regimes are identified as a remedy.

Linear and nonlinear behaviors of gyrotron backward wave oscillators

“…3 The time-dependent simulation also showed that the onset current can be more than 100 times the start oscillation current of the gyro-BWO with a uniform waveguide, and the onset current for the nonstationary oscillations can be higher with a shorter length at no expense to the interaction efficiency. 25 Previous studies have revealed some essential properties of the gyro-BWO, and the stable operation regimes were also demonstrated by the theoretical analysis 25 and experiments. 10 However, there are some unresolved issues needed to be further studied for getting a more in-depth understanding of the intriguing electrodynamical system.…”

“…Theoretical and simulation works were then motivated by experiments for studying the essential properties of gyro-BWO, e.g., the start oscillation criteria (linear properties), [14][15][16] the saturation efficiency of the beamwave interaction, [17][18][19][20][21][22][23] and the nonstationary oscillation of the gyro-BWO (nonlinear properties). 24,25 The theoretical analysis and numerical simulation formed the basis of the experimental design and have been extensively applied on the study of the efficiency enhancement of gyro-BWO induced by the magnetic field tapering 18,19,21 or the tapering of waveguide wall radius. 22 The physical processes in the gyro-BWO are quite complicated and were scarcely discussed in previous literatures before 1990s.…”

“…Even though the start oscillation threshold of some high order axial modes is lower than the first axial mode in a tapered waveguide, the first axial mode finally dominates in the steady-state interactions due to its predominant field profile. 27 So, the nonstationary oscillation of the gyro-BWO is mainly induced by the self-modulation of the first axial mode, 3,24,25 instead of mode competition. The nonlinear field contraction can enhance the predominance of the first axial mode and makes the operations of the gyro-BWO more stable, as compared to the gyromonotron or the conventional BWO.…”

Study of a high-order-mode gyrotron traveling-wave amplifier

“…The absolute instability occurs only at a discrete set of frequencies determined by the phase coherence within the internal feedback loop. 21 Hence, it is not a common phenomenon in a cavity-based gyrodevice, which has a different set of resonant frequencies.…”

Design of an Optimized Resonant Cavity for a Compact Sub-Terahertz Gyrotron

“…For a comprehensive discussion on this topic we refer to [6][7][8][9]. It suffices only to recall here that one of the effective means for frequency tunability is based on the mechanism which involves excitation of a sequence of forward- [10] or backward-wave [10][11][12] high-order axial modes (HOAM). The results of the present study show that the uptapered resonators are beneficial in this respect too since they allow HOAM to be excited at higher efficiency levels.…”