Abstract
Linear behavior of the gyrotron backward-wave oscillator (gyro-BWO) is elucidated in the perspective of optimum beam-wave interactions. Knowledge of the formation processes of individual axial modes provides the requisite groundwork for an examination of the nonstationary behavior, as well as the accessibility and identity of high-order axial modes (HOAMs). Nonstationary oscillations at the onset stage exhibit self-modulation behavior, which might cause the erratic frequency tuning commonly observed in experiments. Thus, regimes of stable magnetic field tuning are identified as a result.