Experimental study of an injection-locked gyrotron backward-wave oscillator

Parametric phase locking in an electron rf oscillator

We propose a way to achieve phase locking of output rf radiation produced by an oscillator driven by a cw electron beam. The locking mechanism is provided by fast periodic modulation of electromagnetic properties of the operating cavity. Ohmic loss and/or the eigenmode frequency are modulated using induced photoconductivity of a semiconductor insert, as affected by a laser with a pulse repetition rate equal to twice the rf frequency.

High power wideband gyrotron backward wave oscillator operating towards the terahertz region

Experimental results are presented of the first successful gyrotron backward wave oscillator (gyro-BWO) with continuous frequency tuning near the low-terahertz region. A helically corrugated interaction region was used to allow efficient interaction over a wide frequency band at the second harmonic of the electron cyclotron frequency without parasitic output. The gyro-BWO generated a maximum output power of 12 kW when driven by a 40 kV, 1.5 A, annular-shaped large-orbit electron beam and achieved a frequency tuning band of 88-102.5 GHz by adjusting the cavity magnetic field. The performance of the gyro-BWO is consistent with 3D particle-in-cell numerical simulations.

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.

Observation of chaotic dynamics in a powerful backward-wave oscillator

Self-modulation regimes of generation in a powerful 10-micros X-band backward-wave oscillator were studied theoretically and experimentally. The sequence of the self-modulation patterns and corresponding bifurcation values observed as the current was increased were in good agreement with the results of simulations. It was found that at a current of 120 A chaotic self-modulation set in at a power of 2 MW and a relative spectral width of 4%.

Nonstationary phenomena in tapered gyro-backward-wave oscillators

Recently, it was found that a gyro-backward-wave oscillator (gyro-BWO) with a tapered waveguide can exhibit stationary oscillations at currents which exceed the starting current by more than 2 orders of magnitude. This result, at first glance, contradicts the theory of transients in untapered linear-beam BWO's, which predicts nonstationary oscillations at currents exceeding the start current by a factor of 3. To explain this behavior of the tapered gyro-BWO, its nonstationary theory is developed. The results obtained demonstrate the consistency of the tapered gyro-BWO operation with the known theory of transients and also show that the tapering may provide stationary oscillations at high currents.

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.