The electron cyclotron maser (ECM) is based on a stimulated cyclotron emission process involving energetic electrons in gyrational motion. It constitutes a cornerstone of relativistic electronics, a discipline that has emerged from our understanding and utilization of relativistic effects for the generation of coherent radiation from free electrons. Over a span of four decades, the ECM has undergone a remarkably successful evolution from basic research to device implementation while continuously being enriched by new physical insights. By delivering unprecedented power levels, ECM-based devices have occupied a unique position in the millimeter and submillimeter regions of the electromagnetic spectrum, and find use in numerous applications such as fusion plasma heating, advanced radars, industrial processing, materials characterization, particle acceleration, and tracking of space objects. This article presents a comprehensive review of the fundamental principles of the ECM and their embodiment in practical devices.

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The electron cyclotron maser

Physics and technology issues of importance to the high-gain gyrotron traveling wave amplifier (gyro-TWT) are investigated in theory and experiment. The gyro-TWT is known to be highly susceptible to spurious oscillations, especially in high gain operations. In the current study, oscillations of various origins are classified and characterized with detailed theoretical modeling. They are shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Knowledge of these processes leads to the concept of an ultra high gain scheme which employs distributed wall losses for the suppression of spurious oscillations. A proof-of-principle Ka-band gyro-TWT experiment stable at zero drive has produced 93 kW saturated peak power at 26.5% efficiency and 70 dB gain, with a 3 dB saturated output power bandwidth of 3 GHz. The saturated gain is more than 30 dB beyond that previously achieved.

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Theory and experiment of ultrahigh-gain gyrotron traveling wave amplifier

Free‐electron laser (FEL) theory and experiments are reviewed. The physical mechanism responsible for the generation of coherent radiation in the FEL is described and the fundamental role of the ponderomotive wave in bunching and trapping the beam is emphasized. The relationship of the FEL interaction to the beam–plasma interaction is pointed out. Various FEL operating regimes are discussed. These include the high‐gain Compton and Raman regimes, both with and without an axial guiding magnetic field. The linear and nonlinear regimes are examined in detail, with particular emphasis on techniques for achieving efficiency enhancement. The quality of the electron beam used to drive FEL’s is a critical factor in determining their gain and efficiency. The subject of electron beam quality, for different accelerators, is discussed. Key proof‐of‐principle experiments for FELs in an axial guiding magnetic field, as well as those driven by induction linacs, rf linacs, electrostatic accelerators, and storage rings, are reviewed. Finally, the requirements on wigglers and resonators are discussed.

A review of free‐electron lasers

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.

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

First-order magnetron injection gun (MIG) design trade-off equations are given for use in obtaining an adequate starting point for gun simulations on the computer. These equations are based on conservation of angular momentum and adiabatic electron flow and they contain relativistic corrections for high voltage design. The equations provide considerable insight into the basic physical processes by which gyro-cyclotron beams are formed and controlled. They have also proven very useful in design feasibility studies of high power MIGs. Two examples of MIG designs are given in which these equations were utilized.

Magnetron injection gun (MIG) design for gyrotron applications

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.

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

Characteristics and optimum operating parameters are determined for a new type of high-power high-efficiency generator of millimeter waves known as a gyrotron traveling wave amplifier. In the example consided, wave amplification results from the interaction of a TE/sub 01/ waveguide mode with the fundamental cyclotron harmonic of an electron beam. The parameter optimization involves the determination of the point of maximum device efficiency as a function of beam density, beam energy, beam positioning, and external magnetic field for the output power required. An analytical linear theory and a numerical simulation code form the basis of theoretical calculations. As a result of the extensive survey in parameter space, the peak efficiency in the beam frame has been found to exceed 70 percent. This result has been applied to the specific design of a 35-GHz amplifier with output power ~340 kW, a power gain of 2 dB/cm, and a laboratory frame efficiency of 51 percent.

Characteristics and Optimum Operating Parameters of a Gyrotron Traveling Wave Amplifier

Analysis of stimulated scattering of a high‐frequency incident pump wave from an unmagnetized relativistic electron beam is presented The backscattered radiation frequency can be enhanced by the factor 4γ20 over the incident pump frequency, where γ0 is the relativistic factor of the electron beam The linear growth rates associated with the wave‐wave and wave‐particle modes of scattering, in the high gain limit, are examined for a number of different pump amplitude regimes Estimates for the scattering efficiency are presented for the wave‐wave scattering process

Stimulated backscattering from relativistic unmagnetized electron beams

The requirements of an electron-beam source suitable for use in cyclotron resonance masers are discussed. The beam source of preference, a magnetron injection gun with its emission temperature limited, is studied by use of a numerical simulation code. This computer code includes the effect of external magnetic fields, the beam's azimuthal self-magnetic field, and space charge in a self-consistent manner. By use of this program, a prediction of gun operation, particularly as it affects gyrotron performance, can be made. Of particular interest are the effects of varying such control parameters as the magnitude of the electric and magnetic fields, the shape of the electric and magnetic fields, and the current. It is demonstrated that, for a particular optimized gun design, the gun operation is very sensitive to both the applied electric and magnetic fields at the cathode. However, the gun shows no strong dependence on other factors such as the temperature-limited beam current or the magnetic-field taper used to compress the beam.

An investigation of a magnetron injection gun suitable for use in cyclotron resonance masers

The characteristics of a 35-GHz oscillator operating with the TE/sub 01/ circular waveguide mode are described. The device produced 147 kW, with an efficiency of 31 percent at 100 kW. The total radiated energy was 2 kJ/pulse. The spectral coherence appears to be equal to those of other high-quality microwave tubes. The mode purity is greater than 95 percent.

A.T. Drobot

Papers

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

Characteristics and Optimum Operating Parameters of a Gyrotron Traveling Wave Amplifier

Stimulated backscattering from relativistic unmagnetized electron beams

An investigation of a magnetron injection gun suitable for use in cyclotron resonance masers

Spatial and Temporal Coherence of a 35-GHz Gyromonotron Using the TE/sub 01/ Circular Mode