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Physical processes in the gyrotron backward-wave oscillator (gyro-BWO) are investigated theoretically. Results indicate highly current-sensitive field profiles and hence sharply contrasting linear and saturated behaviors. The linear field extends over the entire structure length, whereas the saturated profile depends strongly on the energetics in the internal feedback loop. It is shown that this distinctive feature substantially influences the basic properties of the gyro-BWO including the start-oscillation current, efficiency, power scaling, and stability of tuning.

Mode competition in the gyrotron traveling wave amplifier is shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Physical origins of spurious oscillations are analyzed and characterized. Fundamental understanding of these processes leads to a device concept which provides zero-drive stability at ultrahigh gain. The scheme was verified in a proof-of-principle experiment in the $\mathrm{Ka}$ band, producing 93 kW saturated peak power at $26.5%$ efficiency, 70 dB gain, and a 3 dB bandwidth of 3 GHz.

The dynamics of axial mode competition in the gyrotron backward‐wave oscillator (gyro‐BWO) has been investigated in theory and experiment. Because of the difference in field‐shaping processes, the axial modes of the gyro‐BWO each exhibit a distinctive asymmetry in field distribution along the axis. Multi‐mode particle simulations and time‐frequency analyses reveal a consistent pattern of axial mode competition in which a fast‐growing and well‐established mode is subsequently suppressed by a slowly‐growing mode with a favorable field profile. This is verified in a Ka‐band gyro‐BWO experiment.

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.

The transition from the stationary state to a sequence of nonstationary states in the gyromonotron oscillator is experimentally characterized for the first time. We have also demonstrated the stationary operation of a gyrotron backward-wave oscillator at a beam current far in excess of the generally predicted nonstationary threshold. This difference in nonlinear behavior has been investigated and shown to be fundamental with a comparative analysis of the feedback mechanisms, energy deposition profiles, and field shaping processes involved in these two types of oscillations.