R. K. Parker

A quarter century of gyrotron research and development

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,.

Experimental study of a Ka-band gyrotron backward-wave oscillator

A gyrotron backward-wave oscillator (gyro-BWO) has been operated and magnetically tuned over the frequency range 27-32 GHz. Tuning by varying the electron beam voltage was effective over a smaller frequency range ( Delta f approximately 1 GHz). Output power was as large as 7 kW, corresponding to a device efficiency of 19%. This large efficiency value was unexpected, and related analysis indicates it may be associated with the nonuniform magnetic field profile in the interaction region. >

Vacuum electronics at the dawn of the twenty-first century

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.

Production of megawatt submillimeter pulses by stimulated magneto‐Raman scattering

Pulses of submillimeter radiation (λ∼400 μm) with a peak power of approximately 1 MW have been generated by illuminating an intense relativistic electron beam with a counterstreaming microwave pump wave (λ∼2 cm). This observation of powerful submillimeter radiation is consistent with a model of stimulated scattering which includes a resonance between the pump wave frequency and the electron cyclotron frequency as well as the occurrence of electrostatic oscillations at the beam plasma frequency.

A linear theory and design study for a gyrotron backward-wave oscillator†

Abstract A linear theory for a gyrotron backward-wave oscillator (gyro-BWO) is developed. The theory solves a reduced one-dimensional Maxwell-Vlasov equation in the form of a linear integro-differential equation using the Laplace transformation. The relative amplitudes among the waveguide modes and beam modes are completely determined and enable one to calculate gyro-BWO start-oscillation conditions. Using this analysis and including velocity spread effects, a design of a millimetre-wave gyro-BWO has been carried out based on the operating parameters of an existing electron gun. Tunability over a range of 86 GHz to 103 GHz is predicted with output power estimated to be ∼ 1 kW.

Broadband operation of a Ka-band tapered gyro-traveling wave amplifier

A wideband low-voltage millimeter-wave gyro-traveling wave tube (gyro-TWT) amplifier operating in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency is under investigation, The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband operation. Experimental results of a wide (33%) instantaneous bandwidth with a small signal gain in excess of 20 dB and saturated efficiency of /spl sim/10% were achieved and shown to be in good agreement with the theory. Reflective instability due to multi-pass effects by mismatches was observed and characterized. Gain and efficiency have been limited by this reflective instability rather than by absolute instabilities which limit the performance of gyro-TWTs with uniform cross-section. The start-oscillation current in terms of the relevant experimental parameters such as the beam velocity ratio (/spl alpha/), magnetic field detuning and reflection coefficient has been measured and compared with theory. Measurements of the phase variation in terms of the RF frequency have shown that the phase varies /spl plusmn/30/spl deg/ from fitted linear phase line. >

Microwave amplification with an intense relativistic electron beam

A linear microwave amplifier has been constructed using an intense relativistic electron beam (5 kA, 1.2 MV) as the active medium. Stable amplification of an externally applied 100‐kW signal at ∼ 3‐cm wavelength has been demonstrated with a spatial growth rate of 1 dB/cm and an over‐all gain in this single‐stage device of 16 dB. The amplifier appears to be a very wide band device having a bandwidth extending over several gigahertz. The measured characteristics of the amplifier have been compared with theoretical predictions for the relativistic electron cyclotron instability. Measurements of both the variation of amplifier gain with magnetic field strength and the linear spatial growth rate are in reasonable agreement with theoretical predictions.

Development and testing of a high-average power, 94-GHz gyroklystron

The development of a 10-kW average power, 94-GHz gyroklystron amplifier is described. This average power was obtained with 11% radio frequency (RF) duty factor and 92-kW peak power in the TE/sub 01/ circular cavity mode. The instantaneous bandwidth was 420 MHz, and the efficiency was 33.5%. Low-duty-factor testing also yielded a peak power of as much as 115 kW with 600-MHz instantaneous bandwidth. This development effort was carried out over the past three years and represents record average power performance in an amplifier at this frequency.

Design and operation of a collective millimeter-wave free-electron laser

A new free-electron laser experiment has been designed at NRL to operate at millimeter wavelengths using a collective beam-wave interaction. Critical features of the experiment include an apertured diode which provides a low-emittance electron beam, a wiggler magnet with adiabatic entrance and exit, and an operational domain centered around the wiggler-guide field gyroresonance. With the experiment configured as a superradiant amplifier, the effects of the gyroresonance on beam dynamics and the beam-wave interaction have been studied. Measurements indicate a peak power production of 35 MW at 4 mm with an electronic efficiency of 2.5 percent. Aspects of the experimental design are discussed, and the results of a parametric study of the power dependence on the fields are presented. Detailed calculations (both analytic and computational) have been performed to analyze the linear and non-linear effects in the experiment. The results of these calculations are shown to be in good agreement with laboratory measurements.

An electron synchrotron maser based on an intense relativistic electron beam

An electron cyclotron maser has been operated with an output of ∼50 MW of coherent radiation at a frequency ∼8 GHz. A magnetic field configuration is described which effectively converts streaming electron energy into energy transverse to the streaming direction. The redistribution of beam energy makes available a source of free energy to be converted, via an instability, into radiation at the Doppler‐shifted relativistic electron cyclotron frequency. A nonlinear theoretical analysis shows that the coherent synchrotron radiation is a relativistic negative mass type instability reflecting the fact that the cyclotron frequency is a function of the electron energy. The emission was narrow band with a coherence time ≳50 nsec, and was also spatially coherent being primarily radiated into the TE01 circular waveguide mode. The potential for making a frequency tunable maser by employing a tapered drift tube has also been demonstrated.