C. C. Chiu

A study of sub-terahertz and terahertz gyrotron oscillators

We present a theoretical study on some of the key physics issues associated with over-moded, sub-terahertz, and terahertz (THz) gyrotron oscillators. Simulations of a large number of fundamental and second cyclotron harmonic modes yield a broad database for physics observations as well as a test of relevant scaling laws. Linear properties over a broad magnetic field range exhibit a number of interesting trends. Nonlinearly, despite the possibility of multimode excitation, each mode is found to exist as a dominant single mode in a narrow magnetic field range, and a significant fraction of these modes are due to second cyclotron harmonic interactions. The wall resistivity, while a relatively minor concern for sub-terahertz gyrotrons, is shown to play a radically different role in the THz regime. It affords a linear advantage to the harmonic modes while also significantly degrading their output efficiencies. These results are interpreted in terms of the nature of harmonic mode competition and the scaling law...

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.

Harmonic mode competition in a terahertz gyrotron backward-wave oscillator

Electron cyclotron maser interactions at terahertz (THz) frequencies require a high-order-mode structure to reduce the wall loss to a tolerable level. To generate THz radiation, it is also essential to employ cyclotron harmonic resonances to reduce the required magnetic field strength to a value within the capability of the superconducting magnets. However, much weaker harmonic interactions in a high-order-mode structure lead to serious mode competition problems. The current paper addresses harmonic mode competition in the gyrotron backward wave oscillator (gyro-BWO). We begin with a comparative study of the mode formation and oscillation thresholds in the gyro-BWO and gyromonotron. Differences in linear features result in far fewer “windows” for harmonic operation of the gyro-BWO. Nonlinear consequences of these differences are examined in particle simulations of the multimode competition processes in the gyro-BWO, which shed light on the competition criteria between modes of different as well as the sam...

Relativistic performance analysis of a high current density magnetron injection gun

Electron beam quality is essential to the performance of millimeter-wave gyroamplifiers, particularly the gyrotron traveling-wave tube amplifier, which is extremely sensitive to the electron velocity spread and emission uniformity. As one moves up in power and frequency, the quality of the electron beam becomes even more critical. One aspect of the electron beam formation technology which has received relatively little attention has been the performance analysis of the electron beam itself. In this study, a 100 kV, 8 A magnetron injection gun with a calculated perpendicular-to-parallel velocity ratio of 1.4 and axial velocity spread of 3.5% has been designed, tested, and analyzed. It is shown that the equipment precision and a fully relativistic data analysis model afford sufficient resolution to allow a verification of the theoretical predictions as well as a quantitative inference to the surface roughness of the cathode used.

Study of a high-order-mode gyrotron traveling-wave amplifier

Physics and performance issues of a TE 01 -mode gyrotron traveling-wave amplifier are studied in theory. For a high order mode, absolute instabilities on neighboring modes at the fundamental and higher cyclotron harmonic frequencies impose severe constraints to the device capability. Methods for their stabilization are outlined, on the basis of which the performance characteristics are examined in a multidimensional parameter space under the marginal stability criterion. The results demonstrate the viability of a high-order-mode traveling-wave amplifier and provide a roadmap for design tradeoffs among power, bandwidth, and efficiency. General trends are observed and illustrated with specific examples.

Nonlinearly driven oscillations in the gyrotron traveling-wave amplifier

A new phenomenon in the gyrotron traveling-wave amplifier (gyro-TWT) in the form of nonlinearly driven oscillations was reported. A zero-drive stable gyro-TWT is shown to be susceptible to 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.

Fundamental and harmonic mode competition in the gyromonotron

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.

Tradeoff studies of a high-order-mode gyrotron traveling-wave amplifier

Performance of a fundamental cyclotron harmonic, TE01-mode gyrotron traveling-wave amplifiers (gyro-TWTs) is studied theoretically under the marginal stability criterion. The interaction circuit is shown in Fig. 1. As is illustrated in Fig. 2 for the parameters in Table I, absolute instabilities on both lower and higher order modes and at the fundamental and second cyclotron harmonic frequency present severe constraints to the stable operating current. General methods for stabilizing these unwanted oscillations are outlined, on the basis of which the parametric dependence of gain, power, bandwidth and efficiency on the beam velocity ratio, circuit lengths, wall resistivity, and velocity spread is obtained under the marginal stability criterion. The results allow a tradeoff among gain, power, bandwidth, and efficiency to reach a desired design.

Efficiency enhancement in gyrotron backward wave oscillator with a nonlinearly tapered structure

Efficiency enhancement in a gyrotron backward wave oscillator (gyro-BWO) with a nonlinearly tapered waveguides is investigated with a self-consistent code. It is shown that the interaction efficiency can be enhanced significantly to a value as high as 61%. Mechanism of the efficiency enhancement is interpreted physically via phase space diagrams.

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.