Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.

The Gyrotron at 50: Historical Overview

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.

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

A small signal theory is presented of the harmonic gyrotron oscillator with a slotted resonator. The magnetron-type wall structure has the effect of creating a strong harmonic field near the electron orbits and, as a result, greatly reducing the beam voltage and start-oscillation power required for harmonic operations. Characteristics of operation and methods of optimization are studied and numerically illustrated. It is shown that at a given harmonic the startoscillation beam power is lower at a lower beam voltage.

Theory of harmonic gyrotron oscillator with slotted resonant structure

For pt.I, see ibid., vol.20, no.3, p.155-162 (1992). Based on an analytical study of the stability problems of gyrotron traveling wave amplifiers (gyro-TWTs), an extremely high power second-harmonic gyro-TWT has been designed, evaluated and optimized with a self-consistent nonlinear numerical simulation code. The design, which is based on the magnetron-injection-gun (MIG)-type beam, is presented. Using a 100 kV, 25 A MIG beam with alpha =1 and an axial velocity spread of 5%, nonlinear self-consistent analysis of a three-stage second-harmonic gyro-TWT amplifier predicts a peak output power of 533 kW, peak efficiency of 21.3% and a 7.4% saturated bandwidth, which verifies the theoretical predictions that a stable harmonic gyro-TWT can generate power levels an order of magnitude higher than those possible from a fundamental gyro-TWT. It is shown that the positioning of the electron beam is very important. A multistage structure is used to recover the loss in gain resulting from shortening the interaction sections to ensure stability. >

High-power harmonic gyro-TWT's. II. Nonlinear theory and design

This paper provides a critical examination of traditional theoretical treatments of alternating current (ac) space‐charge effects in vacuum electronic devices. By treating several simple examples, it is found that the commonly made decomposition of the first‐order field into a ‘‘circuit part’’ and a ‘‘space‐charge part’’ is ambiguous and misleading. In at least one case, this terminology has led to a formulation that double counts the effect of space charge in the beam–circuit interaction. In other cases, the ‘‘space‐charge term’’ in the dispersion relation−equivalently Pierce’s space‐charge parameter (QC)−has been improperly or incompletely evaluated. The implications for gyrotrons, peniotrons, free‐electron lasers, Smith–Purcell‐type generators, and crossed‐field devices are addressed. The space‐charge effects are briefly discussed in nonlinear theories and in particle simulations. For the models examined, it is found that the most useful and convenient representation for the ac fields is one based on an eigenfunction expansion of the ac magnetic field, even when that field is not itself important to the electron dynamics.

A review of the ac space‐charge effect in electron–circuit interactions

The operation of a millimeter-wave harmonic gyrotron is described in which the interaction is between large-orbit axis-encircling electrons and cylindrical cavity TEnll modes. Efficiencies up to 15% have been measured for moderate harmonic interactions and multi-kW power levels have been attained at the tenth harmonic of the cyclotron frequency. The concept allows the magnetic field of the gyrotron to be reduced by an order of magnitude, thereby making a submillimeter-wave gyrotron feasible.

A. Kupiszewski

Papers

Operation of a millimeter-wave harmonic gyrotron