The gyrotron backward-wave oscillator (gyro-BWO) is a coherent radiation sources featuring continuous frequency tunability. Oscillations build up via an internal feedback
loop composed of a forward moving electron beam and backward propagating waves. Recent studies indicate that stationary of non-stationary states appear alternatively as
the beam current rises. Multi-mode competition has been suggested as a possible cause due to the excitation of high-order-axial modes (HOAMs) involving tapered interaction
structures. However, unbalanced beam energy deposition is also known to result in self-modulation of a single mode. Therefore, an in-depth understanding of axial mode interactions in the gyro-BWO is of critical importance. This dissertation thus devotes to the formation of axial modes and the causes of the single mode self-modulation behavior (Chapter 3) and the multi-mode competition behavior (Chapter 4 and 5).
In the discussion of single mode self-modulation behavior, it's indicated that the occurrence of non-stationary state is caused, at first, by the very nature of dynamic energy flow from downstream end as the oscillation transitioning from transient state to saturated state. This suggests, at high Ib, the subsequent long-lasting beam/field energy modulation in an contracted feedback loop with a modulation frequency. According to this, the increase of modulation frequency as rising the Ib could be regarded as a result of the further contraction of the feedback loop (Leff) which suggests shorter modulating time and hence higher modulation frequency.
As for the discussion of the mode competition behavior, the field structures of axial modes are examined in the perspective of favorable field profile which has the advan-
tage of early interaction with the electron beam. As a result, and in sharp contrast to the behavior of the familiar resonator-based gyrotron oscillator, particle simulations of the gyro-BWO reveal a radically di®erent pattern of mode competition in which a fast-growing and well-established mode is subsequently suppressed by a later-starting mode with a more favorable field profile. This is verified in a Ka-band experiment and the interaction dynamics are elucidated with a time-frequency analysis.

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