Mechanisms of efficiency enhancement in gyrotron backward-wave oscillators with tapered magnetic fields

Dynamics of mode competition in the gyrotron backward-wave oscillator

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.

Numerical analysis of an injection-locked gyrotron backward-wave oscillator with tapered sections

Injection-locked operation of a gyrotron backward-wave oscillator (gyro-BWO) is investigated by means of our time-dependent self-consistent code. Numerical results for a 100 kW, Ka-band, TE11-mode gyro-BWO developed at NTHU, Hsinchu, Taiwan, are compared to the experimental ones. The results are in good agreement showing, in both cases, the asymmetric form of the locking bandwidth curve. Comparison of these results obtained for the case of injection of the external signal into the upstream port, as has been done in the experiments, to the results for injection into the downstream port is made. The results of the comparison demonstrate that the asymmetry of the locking bandwidth curves must be attributed to the influence of the injection signal on the electron bunching that takes place in the input tapered section. Moreover, in some cases, this influence results also in a significant increase of the locking bandwidth (up to a factor of 5). An increase of the output power of the locked gyro-BWO over the output power of the free running one is also investigated and compared with the experimental data.

Linear and time-dependent behavior of the gyrotron backward-wave oscillator

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.

Effective Co-generation of opposite and forward waves in cyclotron-resonance masers

A novel type of the frequency-tunable oscillator based on simultaneous generation of the forward and opposite waves at the same frequency, but at different cyclotron harmonics, is proposed. A spatially periodic helical electron beam allows for strong coupling of the waves. The opposite wave provides the broadband feedback and electron bunching, whereas the forward wave amplifies the arising signal and withdraws the rf power from the interaction region. A 15% efficiency and 5% frequency bandwidth have been achieved in the first experiment.

Saturated behavior of the gyrotron backward-wave oscillator

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.