C. T. Fan

Polarization-controllable TE21 mode converter

We report the concept and development of a Ka-band mode and polarization converter that efficiently converts a TE10 rectangular waveguide mode into either a linearly or a circularly polarized TE21 cylindrical waveguide mode. The converter is composed of a power-dividing section, a mode-converting section, and a polarization-transitioning section. The converting process in each section is displayed and the working principles are discussed. A prototype has been built and tested. The measured results agree well with the numerical calculations for both linear and circular polarizations. The measured optimum back-to-back transmission is 94% with a 1-dB bandwidth of 4.1 GHz for the linear polarization. As for the circular polarization, the measured optimum transmission is 97%, but the corresponding bandwidth is indistinct due to some resonant dips. The reasons and impact for the dips are discussed. A bandwidth of 3.9 GHz is obtained for a single circular converter; meanwhile, an approach to eliminating these un...

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.

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.

Nonlinear oscillation behavior of a driven gyrotron backward-wave oscillator

Controlling the phase and frequency of a gyrotron backward-wave oscillator (gyro-BWO) by means of injection-locking techniques is of practical importance. This study employed a nonlinear self-consistent time-independent code to analyze the nonlinear oscillation behavior of a driven gyro-BWO. There are three regimes in the driven gyro-BWO, including amplification, injection-locked oscillation, and mode competition regimes. Based on the theory of nonlinear oscillation, the amplification and injection-locked oscillation modes are the stable modes and compete with each other in the mode competition regime. An oscillator plane of the driven gyro-BWO is elucidated in the paper. This work demonstrates for the first time that the amplification mode transits to the injection-locked oscillation mode in the driven gyro-BWO. Moreover, the signification efficiency enhancement of the driven gyro-BWO over the free-running efficiency is found.

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

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.

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.

Development of frequency-tunable, terahertz gyrotron backward-wave oscillator

The gyrotron backward-wave oscillator (gyro-BWO) is a frequency tunable scheme at millimeter/terahertz regime. The continuous frequency tuning can be achieved by varying either the magnetic field or the beam voltage in a non-resonant structure. With a growing interest in terahertz-wave regime, gyro-BWO should be preferably operated with high-order mode due to the power-handling capability. Then the upcoming difficulty would be the terahertz interaction circuit as well as the severe transverse mode competition.

Rise and Fall Time Behavior of the Gyrotron Backward-Wave Oscillator

Axial modes of the gyro-BWO are characterized by a discrete set of optimum transit angles separated by ~2pi. We show in theory that, during the rise and fall portions of the beam pulse, the frequency of a given axial mode varies in such a way that its transit angle remains at the characteristic value of the mode. Furthermore, in this transient period, axial mode competition is governed by the distinctive asymmetry of the mode profiles rather than by levels of the start-oscillation currents. A Ka-band gyro-BWO experiment has been carried out to examine the rise/fall time behavior. Time-frequency analyses of the output pulses show good agreement with theoretical predictions.