K. F. Pao

Stable, high efficiency gyrotron backward-wave oscillator

Stability issues have been a major concern for the realization of broadband tunability of the gyrotron backward-wave oscillator (gyro-BWO). Multimode, time-dependent simulations are employed to examine the stability properties of the gyro-BWO. It is shown that the gyro-BWO is susceptible to both nonstationary oscillations and axial mode competition in the course of frequency tuning. Regions of nonstationary oscillations and axial mode competition are displayed in the form of stability maps over wide-ranging parameter spaces. These maps serve as a guide for the identification and optimization of stable windows for broadband tuning. Results indicate that a shorter interaction length provides greater stability without efficiency degradation. These theoretical predictions have been verified in a Ka-band gyro-BWO experiment using both short and long interaction lengths. In the case of a short interaction length, continuous and smooth tunability, in magnetic field and in beam voltage, was demonstrated with the high interaction efficiency reported so far. A maximum 3-dB tuning range of 1.3 GHz with a peak power of 149 kW at 29.8% efficiency was achieved. In a comparative experiment with a longer interaction length, the experimental data are characterized by piecewise-stable tuning curves separated by region(s) of nonstationary oscillations, as predicted by theory.

Selective suppression of high order axial modes of the gyrotron backward-wave oscillator

Selective suppression of high order axial modes of the gyrotron backward-wave oscillator (gyro-BWO) is investigated in theory and in experiment. The gyro-BWO interaction is much more efficient in a down-tapered interaction structure, while it is also more susceptible to the problem of axial mode competition in such a structure. Because higher order axial modes (at a higher oscillation frequency) penetrate deeper into the interaction structure, application of distributed wall loss at the downstream end of the interaction structure is shown in theory to be effective for selective suppression of these modes without degrading the efficiency of the desired fundamental axial mode. A stable gyro-BWO operating in a single mode throughout the entire beam pulse is demonstrated on the basis of this principle. Theoretical and experimental results are found to be in good agreement.

Self-Consistent Effects on the Starting Current of Gyrotron Oscillators

Self-consistent effects on the starting current of gyrotron oscillators are examined. Field profiles in the open cavity are shown to be sensitive to the interaction dynamics. This can either significantly raise or lower the oscillation threshold, particularly for the low-Q modes. The transition from resonant-mode oscillations at the low magnetic field to backward-wave oscillations at the high magnetic field is demonstrated.

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, 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.

Stability and tunability of the gyrotron backward-wave oscillator

Summary form only given. The stability issue in the gyrotron backward wave-oscillator (gyro-BWO) is the major limitation of demonstrating its capability of broadband and continuous tunability. A time-dependent code is employed to examine the stability of gyro-BWO for either magnetic-field or beam-voltage tuning. Numerical results indicate that shortening the interaction length stabilizes the operation of gyro-BWO without sacrificing the efficiency for a tapered waveguide structure. Besides, the up-tapered magnetic field can also enhance the efficiency and, more importantly, suppress the unwanted oscillation at the upstream section. A Ka-band gyro-BWO experiment is conducted. The interaction structure consists of a 3 cm uniform section connected at each end to a slightly tapered section of 4 cm in length. The circuit is immersed in a uniform magnetic-field with the exception of the upstream waveguide tapered section. Appreciable efficiency with continuous frequency tunability is experimentally demonstrated for both magnetic-field and beam-voltage tunings. A peak power of 145 kW at 29% efficiency with a smooth tuning range of 1.8 GHz has been achieved

Nonlinearly driven oscillations in the gyrotron traveling-wave amplifier

By delivering unprecedented power and gain, the gyrotron traveling-wave amplifier (gyro-TWT) offers great promise for advanced millimeter wave radars. However, the underlying physics of this complex nonlinear system is yet to be fully elucidated. Here, we report a new phenomenon in the form of nonlinearly driven oscillations. A zero-drive stable gyro-TWT is shown to be susceptible to a considerably reduced dynamic range at the band edge, followed by a sudden transition into driven oscillations and then a hysteresis effect. An analysis of this unexpected behavior and its physical interpretation are presented.

Stability and Tunability of a Gyrotron Backward-wave Oscillator

Stability issues have been a major concern for the realization of broadband tunability of the gyrotron backward wave-oscillator (gyro-BWO). A time-dependent code is employed to examine the stability properties of the gyro-BWO over a wide range in magnetic field and electron beam voltage. On the basis of the results, an optimized Ka-band gyro-BWO experiment has been designed and conducted. Continuous and smooth tunability, in magnetic field and in beam voltage, was demonstrated with high interaction efficiencies. A 3-dB tuning range of 1.4 GHz with a peak power of 130 kW at 27% efficiency was achieved.

W-band TE 01 gyrotron backward-wave oscillator with distributed losses

Distributed wall losses are adopted to enhance the stability and tunability of a W-band TE01 gyrotron backward-wave oscillator (gyro-BWO). Simulation results reveal that distributed wall losses can effectively suppress the competing transverse modes and high-order axial modes, but do not significantly degrade the performance of a gyro-BWO operating at the fundamental axial mode. Extensive numerical calculations are conducted. The effects of guiding center radius and waveguide tapering on the start-oscillation currents are examined. Preliminary tuning properties of the gyro-BWO are presented. The gyro-BWO is predicted to yield a peak output power of 100 kW centered at 96 GHz with an efficiency of 20 % and a 3-dB tuning bandwidth of 1.8 GHz at a beam of 100 kV and 5 A.

Study of axial modes in the gyrotron backward-wave oscillator

In a resonator structure, the axial field profile of a cold mode results from constructive superposition of reflected waves at both ends. For the gyromonotron oscillator, end reflections also constitute the feedback loop during the field build-up. The hot mode has essentially the same field profile and oscillation frequency as those of the cold mode. The gyrotron backward-wave oscillator (gyro-BWO), on the other hand, employs a waveguide structure in which no cold resonant modes exist. In addition, the feedback loop consists of the forward moving beam and the backward propagating wave. End reflections in principle play no role in forming the gyro-BWO field. The field pattern of the oscillating mode must then depend entirely on the beam-wave interaction. Field forming processes as well as the resultant axial field profile in the gyro-BWO are thus expected to be fundamentally different from those of the gyromonotron. Indeed, a recent study has shown that the field in the gyro-BWO contracts nonlinearly as the beam current builds up. Here, we examine the mechanisms for the formation of the axial modes in the gyro-BWO in the linear regime.Formation of different orders of axial modes in the gyrotron backward-wave oscillator are shown to be governed by the optimum conditions for the beam-wave interaction. Distinctive linear properties of oscillating modes in the nonresonant structure are revealed and interpreted physically in such a perspective. Implications of these properties to the nonlinear behavior are examined with time-dependent simulations. Self-modulation, rather than axial modes competition, is found to be the cause of nonstationary behavior and regimes of stable frequency tuning are identified as a remedy.

Fundamental and harmonic mode competition in the gyrotron oscillator

We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotoron oscillator. It is shown through the particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. A physical interpretation is given.