A. K. Kinkead

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.

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.

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.

Megavolt, multikiloamp Ka band gyrotron oscillator experiment

A Ka‐band gyrotron oscillator experiment using a 1–1.35 MeV, multikiloampere beam from a pulse line accelerator has produced approximately 250 MW at 35 GHz in a circular TE62 mode with a peak efficiency exceeding 10%. Time‐dependent simulation studies have been used to predict the behavior of a high‐peak‐power, short‐pulse gyrotron in this parameter range. The simulations demonstrate the occurrence of such phenomena as hard excitation of the gyrotron as a result of the time dependence of the voltage pulse. The experimental results are in reasonable agreement with the predictions of theory.

High‐voltage millimeter‐wave gyro‐traveling‐wave amplifier

An intense‐beam gyro‐traveling‐wave amplifier experiment has demonstrated an apparent growth rate of 2.2 dB/cm at 35 GHz, with total gain exceeding 30 dB, and an output power of ∼20 MW (±3 dB), corresponding to an efficiency of ∼11%.

Breakdown of the atmosphere by emission from a millimeter‐wave free‐electron maser

Production of an atmospheric pressure air breakdown plasma using the emission from a short‐pulse millimeter‐wave free‐electron maser is used to demonstrate frequency tunability over the range 50–100 GHz, via breakdown standing wave patterns, and very high peak power.

Millimeter-wave gyroklystron amplifier experiment using a relativistic electron beam

A fundamental-mode TE/sub 111/ degrees two-cavity intense-beam gyroklystron amplifier experiment, operating at an accelerating voltage of 1 MV, is reported. The two cavities that were tested are designed to serve as bunching cavities for a high-power output cavity. The two-cavity amplifier has demonstrated a linear gain of 15 dB and an unsaturated output power of approximately 40 kW, with the intracavity gain and power approximately 4 dB higher. The frequency of the second cavity has been found to track the frequency of the driven cavity over a range of 300 MHz around a center frequency of 35 GHz. Stable amplifier operation was achieved with beam currents as large as 150 A and a velocity pitch ratio of 0.36. The stable operating range was limited by spurious oscillation in the TE/sub 112/ degrees mode. Theoretical calculations indicate that higher gains might be attainable if this mode could be suppressed. >

Reply to comments of Bekefi and Fajans

Both the free‐electron laser (FEL) and cyclotron‐maser (CM) instabilities can play a role in wave generation laser experiments employing both an axial guide magnetic field and a transverse wiggler magnetic field. The experimental distinction between these two mechanisms can only be made by comparing measured radiation characteristics with the predictions of either model. For a recent intense beam experiment [Phys. Fluids 26, 2683 (1983)], most of the data are in good agreement with the FEL mechanism, as previously concluded, rather than with the CM instability, and the data taken far from gyroresonance are of unambiguous FEL origin.

Measurement of plasma‐neutralized super‐vacuum currents in a gyrotron configuration

Experimental results are reported on the transport of an electron beam with current in excess of the vacuum space‐charge‐limited value, in a configuration directly applicable to gyrotron oscillators. The vacuum space‐charge limit is circumvented by the introduction of a neutralizing background plasma which is produced by an array of four plasma guns placed immediately downstream of the electron gun anode.

Experimental Investigation Of The Effects Of A Neutralizing Background Plasma On The Operation Of A High Current Gyrotron

We present plans for an experiment to investigate the effects of a neutralizing background plasma on the operation of a high current gyrotron. A neutral plasma filling the electron beam transport and gyrotron cavity regions allows for the propagation of super-vacuum currents and for tuning of the interaction frequency through the neutralizing plasma density. The background plasma is completely ionized before the passage of the high current electron beam, and the plasma, density(in the absence of the electron beam) is a known, measured quantity.