W. M. Black

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.

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.

Transformation of the magnetic energy of a rotating relativistic beam into plasma energy

An intense rotating relativistic electron beam of energy 500 kV and current 40−50 kA is injected into neutral gas of pressure 200−700 mTorr, in the presence of an external magnetic field B0≃800 G. It is observed that the ratio of the magnetic field on axis ΔBz(r=0) to B0 is ΔBz(r=0)/B0≃2.2. The average magnetic energy density is about 2.5×1016 eV cm−3. Spectroscopic results show that most of this energy is transferred to the plasma through Joule heating.

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.

Studies Of Radiation Growth And Emission Spectrum Of A Millimeter-Wave Free-Electron Laser Operating In The Collective Regime

Frequency-resolved measurements of emission from a superradiant millimeter-wave free-electron laser using an intense relativistic electron beam have demonstrated high power, good efficiency, high gain, and broad gain bandwidth, and broad tunability, all in good agreement with theoretical predictions for operation in the collective regime.

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 Results On Relativistic Gyrodevices

Experimental work at the Naval Research Laboratory has proceeded on three different relativistic gyrodevices, the cyclotron autoresonance maser (CARM), the three cavity phase-locked gyrotron oscillator, and the high power free running gyrotron oscillator. Experimental results relating to these projects will be presented.

High Power Multi-Cavity Phase-Locked Gyrotron Oscillator Experiment

A high-peak-power 35 GHz multi-cavity phase-locked gyrotron based on a solid 1 MeV, 100 Amp, 30 nsec electron beam has been designed. The required beam transverse momentum is produced by a helical wiggler and increased to a beam pitch ratio 04.75 by magnetic field compression. The locking signal, provided by a 20 kW magnetron, drives a slotted cylindrical TE 111 prebunching cavity. The azimuthal phase bunching of the beam, is enhanced by means of a second passive bunching cavity. The TE121 output cavity, which will self-oscillate in the absence of beam prebunching, is expected to produce output powers of -10 MW and have a locking frequency bandwidth of 0.1 percent.