The axial modes of the gyrotron backward-wave oscillator (gyro-BWO) each exhibit a distinctive asymmetry in axial field profile. 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 different 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.
The gyrotron backward-wave oscillator (gyro-BWO) features broadband tunability, but ragged tuning curves are frequently observed experimentally. Accordingly, a Ka-band gyro-BWO experiment with external circuit mismatch was conducted to examine its tuning properties at two reflected strengths: one is slightly mismatched (15 dB reflection) and the other can be categorized as matched (30 dB reflection). Stepwise frequency tunings by varying the magnetic field, the beam voltage, and the beam current were observed under mismatched conditions. A self-locking model was introduced using the concept of injection-locking, where the output and reinjected signals tend to form a stable phase relation, favoring certain discrete oscillation frequencies. The observed frequencies agree closely with the calculated frequencies. Smooth tuning curves were also obtained, revealing a remedy for the stepwise tuning of a gyro-BWO.
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