K. R. Chu

A self-consistent field theory for gyrotron oscillators: application to a low Q gyromonotron

This paper presents an application of a self-consistent field theory for gyrotron oscillators to a low Q TE01 mode gyromonotron. In this model, the RF field profile function satisfies a wave equation in which the AC beam current appears as a source. The wave equation is solved simultaneously with the electron equations of motion. The results of calculations are compared with experimental data for the efficiency, starting current, and operating frequency. To facilitate a detailed comparison between theory and experiment, data have been taken over a wide range of beam currents and axial magnetic fields. The gyromonotron studied in this work has a tapered cavity structure to enhance efficiency, and a low Q factor to enhance output power. A strong self-consistent field effect is observed in the behaviour of the starting current.

Gain and bandwidth of the gyro-TWT and CARM amplifiers

Issues concerning the interpretation of gain and bandwidth from the dispersion relation are examined for the gyrotron traveling wave tube (gyro-TWT) and the cyclotron auto-resonance maser (CARM) amplifiers. A general method for the determination of the critical current for oscillation is illustrated. Despite the broad bandwidth predicted for the CARM amplifier by the commonly employed dispersion relation, it is seen in particle simulation that single-particle interaction. Rather than collective amplification, prevails over much of the band. Reasons for the discrepancy are analyzed. >

Theory of electron cyclotron maser interaction in a cavity at the harmonic frequencies

A theory of the cyclotron maser interaction between an annular electron beam and the standing electromagnetic wave in a cavity structure is formulated on the basis of the relativistic Vlasov equation and the Maxwell equations. Detailed analytical expressions for the beam‐wave coupling coefficient, beam energy gain, and threshold beam power have been derived for the fundamental and higher cyclotron harmonics. Physical interpretations of these results and comparison with cyclotron maser interactions in a waveguide structure are presented. Methods of parameter optimization and their applications to experiments are illustrated through numerical examples.

Gyrotron travelling wave amplifier: I. Analysis of oscillations

The operation of the gyrotron travelling wave amplifier is based on the convective cyclotron maser instability. It is found that this convective instability may become absolute (nonconvective) at a sufficiently high current level, resulting in oscillation instead of amplification. This threshold current for the transition depends sensitively on the applied magnetic field. The axial wavelength and the characteristic frequency of oscillation at the onset of absolute instability are given. It is found that momentum spread has virtually no effect on the threshold current. A small amount of resistive wall loss, however, raises the threshold current significantly. Oscillations due to partial reflection at the ends of the system are also examined. Preliminary experimental results on both types of oscillations are reported and are found to be in good agreement with the theory.

Theory and experiment of a 94 GHz gyrotron traveling-wave amplifier

Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because o...

A 30-MW Gyroklystron-Amplifier Design for High-Energy Linear Accelerators

Microwave tubes required to drive future 1-TeV linear colliders have challenging requirements, including 300-MW peak power at wavelengths near 3 cm. It is believed that gyrotrons have good potential for development into the high peak power, high gain, efficient amplifier required for this application. Presented here is the detailed design for a 3-cm gyroklystron amplifier at a peak power level of ~ 30 MW. This power level is approximately three orders of magnitude above the gyroklystron state of the art, and a 30-MW experimental study will be a crucial step toward achieving the 300-MW goal. Novel problems associated with a high-energy (¿ ~ 2) gyrotron beam are considered in detail. A new partially self-consistent nonlinear analysis is described and subsequently used to design the gyroklystron circuit. Based on the TE011 mode in cavities of circular cross section, stable operation is anticipated; the current is well below the threshold at which an unbunched electron beam would excite oscillations in the cavities and the drift spaces are designed to heavily load modes which are not cut off.

Methods of Efficiency Enhancement and Scaling for the Gyrotron Oscillator

It is shown that a gyrotron oscillator operating in a slightly tapered magnetic field can attain an efficiency of ~78 percent, approximately 1.7 times higher than that obtainable in a constant magnetic field. Extensive numerical data have been tabulated and a convenient parameter is introduced to generate numerical efficiency scaling relations through which optimum operating conditions are clearly exhibited. Conditions for reaching the high efficiency operating regime are also studied and numerically illustrated.

Overview of research on the gyrotron traveling-wave amplifier

The gyrotron traveling-wave tube (gyro-TWT) is a millimeter-wave amplifier based on the electron cyclotron maser instability. It is a device of increasing importance because of its power and bandwidth capabilities. The current paper presents a brief overview of the gyro-TWT research over the past quarter of a century. Advances made by different groups employing various schemes are discussed and achieved performances are surveyed.

Theory and Simulation of the Gyrotron Traveling Wave Amplifier Operating at Cyclotron Harmonics

An analytical expresion for the efficiency of the gyrotron traveling wave amplifier is derived for the case of nonfundamental cyclotron harmonic interaction. It scales the efficiency with respect to the modes and parameters of operation. This relation, together with a general linear dispersion relation, also derived in the present paper, gives the characteristics and optimum operation conditions of the gyrotron traveling wave amplifier.

Marginal stability design criterion for gyro-TWTs and comparison of fundamental with second harmonic operation

Abstract Stability properties of both the fundamental and second harmonic gyrotron travelling wave amplifier (gyro-TWT) are examined with multi-mode particle simulations. The second harmonic cyclotron interaction with an axis-encircling electron beam is found to be more stable to oscillations and can yield significantly greater power than the fundamental harmonic gyro-TWT. A multiple stage interaction structure based on a marginal stability criterion is proposed and illustrated with examples of a 128kW fundamental gyro-TWT and a 532 kW second harmonic gyro-TWT, Stable amplification at much higher power levels is in principle possible.