Steven H. Gold

Review of high-power microwave source research

This article reviews the state-of-the-art in high-power microwave source research. It begins with a discussion of the concepts involved in coherent microwave generation. The main varieties of microwave tubes are classified into three groups, according to the fundamental radiation mechanism involved: Cherenkov, transition, or bremsstrahlung radiation. This is followed by a brief discussion of some of the technical fundamentals of high-power microwave sources, including power supplies and electron guns. Finally, the history and recent developments of both high-peak power and high-average power sources are reviewed in the context of four main areas of application: (1) plasma resonance heating and current drive; (2) rf acceleration of charged particles; (3) radar and communications systems; and (4) high-peak power sources for weapons-effect simulation and exploratory development.

Laser‐plasma interaction and ablative acceleration of thin foils at 1012–1015 W/cm2

The interaction physics and hydrodynamic motion of thin‐foil targets irradiated by long, low‐flux Nd‐laser pulses (3 nsec, 1012–1015 W/cm2) are studied experimentally and compared with theoretical models. Laser light absorption is high (80%–90%) and thin‐foil targets are accelerated up to 107 cm/sec with good (20%) hydrodynamic efficiency in the 1012–1013 W/cm2 range. These results agree with a simple rocket ablation model. Details of thermal heat flow, both axially (related to ablation depth) and laterally (related to beam uniformity requirements), are also presented.

Design of a waveguide resonator with ripled wall reflectors for a 100 GHz CARM oscillator experiment

High Q Bragg resonators are studied for application to millimeter wave CARM oscillators driven by electron beams with power greater than 100 MW. ‘Whispering-gallery’ modes are shown to have the highest Q. The two effects that cause reflections, impedance mismatch owing to variations in the wall radius and currents driven in the walls of the reflector sections, are shown to compete, paving the way for anomalously low Qs for some of the other modes. The effect of wall resistance on the operation of the cavity is studied; as the frequency of the radiation is increased to near 250 GHz, wall heating severely limits the average power rating of whispering-gallery devices.

High Peak Power Ka-Band Gyrotron Oscillator Experiment.

A Ka‐ band gyrotron oscillator powered by a 600 kV pulse‐line accelerator has produced approximately 100 MW at 35 GHz in a circular TE62 mode. It has also demonstrated frequency tuning over the range 28 to 49 GHz by operating in a family of TEm2 modes, with the azimuthal index m ranging from 4 to 10, by variation of the guide magnetic field. Operation is in general agreement with the predictions of theory.

High-peak power K/sub a/-band gyrotron oscillator experiments with slotted and unslotted cavities

A K/sub a/-band gyrotron oscillator powered by a compact pulseline accelerator has been operated using oscillator cavities with and without axial slots. The oscillator was operated at high voltage ( approximately 900 keV) and high current ( approximately 500 A) in the approximate frequency range of 20-50 GHz. The use of axial slots has been shown to suppress low-starting-current whispering-gallery modes, in particular, modes of the TE/sub m2/ type, allowing stable operation in a linearly polarized TE/sub 13/ mode. A peak power of 35 MW has been observed at 6% efficiency. >

Study of gain, bandwidth, and tunability of a millimeter‐wave free‐electron laser operating in the collective regime

Frequency-resolved measurements of the emission of a collective free-electron laser operating at millimeter wavelengths have shown emission spectra that agree with theoretical predictions for the collective free-electron laser instability. Broad tunability, moderate emission linewidths, and high single frequency gain have been observed. In addition, adjusting the axial field in the end region of the interaction has been found in some cases to cause a large increase in measured power and efficiency.

Analysis of the deflection system for a magnetic-field-immersed magnicon amplifier

A linear analysis of the electron-beam deflection system in a magnicon amplifier is presented. The system consists of identical cavities, one driven and the remainder passive, separated by a drift space and immersed in an axial magnetic field. The cavities contain a rotating TM/sub 110/ mode. The length of each cavity is pi nu /sub z// omega , and that of the drift space is pi nu /sub z// omega /sub c/, where omega is the RF frequency, omega /sub c/ is the relativistic gyrofrequency in the guide field, and nu /sub z/ is the mean axial velocity of the beam electrons. The linearized electron orbits are obtained for arbitrary initial axial velocity, radial coordinate, and magnetic field. The small-signal gain and the phase shift are determined. The special case where omega /sub c// omega =2 has unique features and is discussed in detail. For the NRL magnicon design, a power gain of 10 dB per passive cavity is feasible. Results from numerical modeling of a magnicon with two passive cavities are presented. Operation of the output cavity at the fundamental and higher harmonics of the input drive frequency is briefly discussed. >

Radiation Growth in a Millimeter-Wave Free-Electron Laser Operating in the Collective Regime.

Frequency-resolved measurements of radiation growth have been performed on a millimeter-wave free-electron laser using an intense relativistic electron beam. These measurements have shown large radiation growth rates (approx. 2 dB/cm) over a broad instantaneous bandwidth (66-90 GHz), in good agreement with predictions of theory for operation in the collective regime. Growth narrowing and saturation effects have also been observed. In addition, a large increase in experimental power and efficiency has been observed to result from tapering the strength of the axial magnetic field in the sense that compensates for kinetic energy extraction from the electron beam. Direct calorimetric measurements indicate the production of greater than or equal to 75 MW centered at 75 GHz with 6% experimental efficiency.

Measurement of rear surface temperatures of laser‐irradiated thin transparent targets

Visible emission measurements from the rear surface of laser‐irradiated thin transparent target foils show a transient light flash followed by secondary light emission due to rear surface heating. This temporal signature is different from that reported in shock experiments in transparent solids. Rear surface temperature determinations versus time on laser‐accelerated CH targets are presented.

Multimode simulation of high frequency gyrotrons

Abstract This paper presents a simulation study of mode competition in highly overmoded gyrotron cavities. The parameters of the study have been selected to correspond to the approximate design parameters of a 280 GHz, 1 MW gyrotron at the Massachusetts Institute of Technology (MIT). The MIT gyrotron is designed to run in the TE+42.7mode, using a 50 A, 84keV electron beam, with a normalized beam radius of 0-6, and a beam α of 1.6. This study addresses: (1) the problem of achieving gyrotron operation in the design mode, and at a value of magnetic detuning sufficient to achieve high efficiency operation; and (2) the mode purity of the final state.