T. A. Hargreaves

Initial operation of a higher-power quasi-optical gyrotron

Results from the initial operation of a high-power quasi-optical gyrotron based on the 90-kV, 50-A Varian VUW-8144 electron gun are reported. The output power and efficiency have been measured for a resonator mirror separation of 19.4 cm with a magnetic field of 4.95 T, corresponding to resonator output coupling of 1.9%, and for a resonator mirror separation of 21.4 cm with a magnetic field of 4.7 T, corresponding to a resonator output coupling of 3.1%. Operation was multimoded with 3-6 modes excited in the range of 125-130 GHz for the 4.95-T magnetic field. A peak efficiency of 15% at an output power of 161 kW was obtained for a gun voltage of 93 kV and a current of 12 A. A peak-output power of 364 kW at an efficiency of 10% was obtained at a voltage of 95.6 kV and 37.5 A. >

Operating characteristics of a continuous-wave-relevant quasioptical gyrotron with variable mirror separation

Results from a quasioptical gyrotron experiment with a 20–28 cm mirror separation are presented, showing operation at powers up to 150 kW and efficiencies up to 12%. The output coupling would be varied from 0.4%–3% by changing the mirror separation and operating frequency. Operation was obtained over frequencies ranging from 95–130 GHz by changing the axial magnetic field, limited on the low end by waveguide cutoff in the diagnostics and at the high end by the maximum magnetic field achievable. The output power varied by approximately a factor of 2 over this range. Frequency variation of 4% was achieved by varying only the electron gun voltage; however, the output power also varied substantially due to the fact that the electron beam power was changing dramatically. Efficiency optimization by variation of output coupling and tapering of the magnetic field has been demonstrated. Regions of single‐mode operation at powers up to 125 kW have been characterized and compared to recently developed theory.

Tilted resonator experiments on a quasioptical gyrotron

Abstract Tilting the resonator axis slightly with respect to the magnetic field axis in the quasioptical gyrotron (QOG) is predicted greatly to enlarge the parameter space available for stable, single-mode operation. Greater interaction efficiency is also predicted for single-mode operation. A resonator with a 2° tilt has been tested on the QOG experiment at the Naval Research Laboratory. The operation of this resonator has been compared to an untilted, but otherwise identical, resonator. Multimode operation of the two resonators was very similar, in contrast to predictions. However, an output power of 600 kW was produced at an efficiency of 8% and a frequency of 120 GHz. At lower power, efficiencies above 12% were observed. The efficiency of single-mode operation appears to be raised significantly by tilting the resonator axis.

Depressed collector experiments on a quasioptical gyrotron

A simple, single‐stage depressed collector has been tested on the quasioptical gyrotron (QOG) experiment at the Naval Research Laboratory (NRL). This is the first application of a depressed collector to a high‐power gyrotron, and was relatively easily accomplished due to the natural separation of the electron beam and the output radiation in the QOG. Collector efficiencies as high as 50% and overall efficiencies up to 16% were observed. The output power reached 431 kW with an overall efficiency of 13% and a collector efficiency of 41%. The collector efficiency was limited in this experiment due to interception of approximately 15% of the electron beam on an undepressed section of the beam guide, a problem readily correctable with a small change in the beam guide dimensions. If this part of the electron beam was collected at the collector potential, the overall and collector efficiencies would increase to 16% and 55%, respectively. The maximum collector efficiency would increase to 58% (at high output power) and the best overall efficiency would increase to 20% (at lower output power).

Operation Of A Quasi-Optical Gyrotron With Variable Output Coupling

Results from a quasi-optical gyrotron experiment with a 20-28 cm mirror separation will be presented showing operation from 95-130 GHz at powers up to 150 kW and efficiencies up to 11%. The output coupling can be controlled by varying the mirror separation. Efficiency optimization by variation of output coupling and by tapering the magnetic field has been demonstrated.

Ohmic effects in quasioptical resonators

Several properties of the Fabry-Perot-type open resonator used in the quasioptical gyrotron (QOG) and the quasioptical induced resonance electron cyclotron (IREC) maser are derived. The electric fields of the normal modes are given for the general case of the resonator axis tilted with respect to the direction perpendicular to the electron beam axis. The ohmic quality factor and the power dissipated in the mirrors are derived, as is the energy stored in the resonator. The time dependence of the mirror heating, relevant for pulsed experiments, is also derived. The formulae are applied to an example of current relevance, the quasioptical IREC maser resonator.

Cold tests of quasi-optical gyrotron resonators

Cold tests which have been performed on quasi-optical gyrotron resonators at frequencies near 94 and 120 GHz to measure cavity Q are discussed. The separation between the resonator mirrors was varied between 0.15 and 0.35 m. Quality factors ranging from 10000 to 100000 are shown. Good agreement is shown between the measured data and values calculated from scalar diffraction theory. The experimental effect of misaligning the mirrors is examined. >

A 0.5 MW quasi-optical gyrotron experiment

Summary form only given. The quasi-optical gyrotron is under development at the US Naval Research Laboratory as a source for fusion plasma heating. Early experiments with a 24-A, 75-kV electron gun produced output powers up to 150 kW and efficiencies up to 12%. Single-mode operation was observed at powers up to 125 kW despite the resonator being highly overmoded. One drawback of this experiment was the use of a resonator with relatively low output coupling (necessitated by the low electron beam power), resulting in a resonator with comparable output and ohmic losses. The ohmic losses can be reduced to a small fraction of the total resonator losses through the combined use of a higher-power electron beam and a resonator with larger output coupling. Such an experiment has been assembled and began producing data in June, 1989. The high-power electron gun has been run at voltages up to 106 kV and currents up 54 A, which produced 515 kW of power at approximately 120 GHz. The output efficiency peak of 15.4% occurred at an electron gun voltage of 88 kV and current of 16 A. corresponding to 220 kW of output power