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

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

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.

Overview of research on the gyrotron traveling-wave amplifier

The experimental results are reported for a stable second-harmonic gyrotron traveling wave amplifier, which generated a record-breaking 207-kW output power based on the principle that the weaker harmonic interactions are more stable to spontaneous oscillations than at the fundamental, and therefore, capable of generating higher output power. The high-power amplifier was kept completely (zero-drive) stable by employing a mode-selective interaction circuit and web-matched directional input and output couplers, and choosing an amplifier interaction length shorter than the start-oscillation length for gyrotron backward-wave oscillations. The single-stage Ku-band amplifier utilized an 80-kV 20-A /spl upsi//sub /spl perp////spl upsi//sub /spl par//=1.1 electron beam from a magnetron injection gun and yielded an efficiency of 12.9%, an output phase variation of 10/spl deg//kV, a saturated bandwidth of 2.1%, a large-signal gain of 16 dB, and a detuned small-signal gain of 38 dB.

Operation of a stable 200-kW second-harmonic gyro-TWT amplifier

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.

C. C. Lin

Papers

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