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

Gyrodevice oscillators and amplifiers (or gyro-oscillators and gyro-amplifiers) are being utilized in a variety of applications where high power levels are required at millimeter-wave frequencies. Gyro-oscillators, developed primarily for magnetic fusion research applications, have achieved power levels near 1 MW for pulse durations in excess of 1 s at frequencies above 100 GHz. Continued work on these devices should enable them to achieve continuous-wave operation at multimegawatt power levels at frequencies in the 100-GHz to 200-GHz range, thereby meeting the requirements of planned magnetic fusion experiments. The development of gyro-oscillators for fusion experiments has led to the utilization of the devices in several industrial applications, such as ceramic sintering and metal joining. Activities in this area involve adapting the oscillators to the industrial environment where reliability, efficiency, and ease of operation are paramount. Gyro-amplifiers are being developed for applications requiring phase coherence and instantaneous bandwidth, such as in linear accelerators and millimeter-wave radar. Impressive results from X-band to W-band already suggest the promise of these devices. Potential new applications and novel gyrodevice design approaches continue to attract the attention of researchers around the world.

Characteristics and applications of fast-wave gyrodevices

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

In this introduction to the Special Issue on vacuum electronics, the history, operating principles, and recent technological trends of vacuum electronic devices are reviewed. The development of microwave power tubes is described and improvements in these devices over the past quarter of a century are highlighted. These improvements have been substantial and have been driven by modern high-power applications and by advances in materials and computational science. The second part of this paper describes the advent of a new class of vacuum electronics generator as involving relativistic electron beams and interaction with fast waves (e.g., gyrotrons, free-electron lasers). These new types of generators are opening the electromagnetic spectrum beyond the microwave region (i.e., millimeter-wave, infrared, ultraviolet, and even X-ray) for applications of high-power, coherent generators of electromagnetic radiation.

Vacuum electronics at the dawn of the twenty-first century

The development of small-orbit gyrotrons operating at voltages /spl les/100 kV is reviewed. Gyrotron oscillators have been developed to produce unprecedented 200-kW average power levels at frequencies spanning the range of 28-140 GHz with current work aimed at achieving 1-MW average power. They are widely used in plasma-heating studies and are the natural choice for material processing in the millimeter-wave region. Gyrotron amplifiers have exceeded the peak power limits of more conventional amplifiers at both 35 and 94 GHz, and have been used in a few radars. Gyro-amplifiers under development have been designed to surpass both the peak power and the average power limits of conventional amplifiers, and are anticipated to be widely accepted in millimeter-wave radar systems. Gyrotron amplifiers operating at voltages /spl sim/0.5 MV that are being evaluated for accelerator applications were reviewed in this journal in 1996 and are not included in this review paper,.

A quarter century of gyrotron research and development

The gyrotron is a new type of micrwave tube capable of producing high power output at millimeter wavelengths. Oscillator results have been described in recent Soviet publications This paper describes work in progress at Varian Associates, Inc. to develop an amplifier of the gyroklystron type to deliver 200 kW cw at 28 GHz. Considerable progress has been made with amplifier stability to the point that amplifier gains of up to 40 dB have been measured in a pulsed experimental amplifier. Current effort is concerned with improving efficiency. A pulsed oscillator is also described which produced 248 kW peak power at 28 GHz with 34% efficiency. A cw oscillator is under construction. Areas for future R and D are discussed. These include gyro-TWT amplifiers with increased instantaneous bandwidth (5 to 10%) and operation at harmonics of the cyclotron frequency to reduce the magnetic field requirements.

Gyrotrons for high power millimeter wave generation

Pulsed and CW gyrotron oscillators have been designed and constructed for use in electron cyclotron resonance heating in plasma fusion experiments. The tubes are designed for 200 kW output at 28 GHz with beam input of 80 kV and 8 A. The pulsed design has been operated at duty factors of 5% and pulse lengths up to 40 msec. The CW design has produced output of 200 kW CW with an efficiency of 50%. It also operated with 52% efficiency at an output level of 170 kW CW. The tubes are designed for power output in the TE 02 circular electric mode in oversize (2.5 inch diameter) circular waveguide. Some investigations of mode purity of the output will be described. The design and operation of waveguide components such as bends and mode filters for use with the gyrotrons will be discussed.

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200 kW pulsed and CW gyrotrons at 28 GHz

The gyrotron is a microwave tube which employs the electron cyclotron maser interaction to produce high power output at millimeter wavelengths. It has important and growing applications for heating of plasmas in controlled thermonuclear fusion experiments. The Varian 60 GHz gyrotron has recently generated microwave power in excess of 200 kW during CW operation, with excellent dynamic range and operating stability. This is the highest average power ever produced by a microwave tube in the millimeter wave region. A description of the gyrotron design and test results are presented.

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First 200 kW CW operation of a 60 GHz gyrotron

For conducting very high microwave power at very high frequencies, circular waveguide transmitting a circular-electric-field mode is used. The vacuum-tight window of an electron tube is often the element with lowest power-handling capability. The inventive window has two dielectric plates with a space between them. There is a gap in the waveguide inner wall through which a dielectric fluid is circulated between the plates to cool them. The gap leads to a region containing wave-absorbing material such as water to absorb modes other than the circular-electric-field mode.

Circular electric mode microwave window

Recent experiments have been performed to determine the ultimate power capability of a 28 GHz 200 kW CW gyrotron design A power output of 342 kW CW was measured in these tests with an efficiency of 37% Progress in the development of 60 GHz 200 kW pulsed and CW gyrotrons is discussed An output of 200 kW with 100 msec pulse length has been achieved with the pulsed design

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J. Shively

Papers

Gyrotrons for high power millimeter wave generation

200 kW pulsed and CW gyrotrons at 28 GHz

First 200 kW CW operation of a 60 GHz gyrotron

Circular electric mode microwave window

Gyrotron oscillators for fusion heating