H. Huey

Gyrotrons for ECH applications

Gyrotron oscillators have served as effective sources for electron cyclotron heating (ECH) applications in the area of magnetic confinement fusion. Successful development programs at frequencies at 28 GHz, 60 GHz, and 140 GHZ, have led to the availability of wide-range gyrotron sources with high-average-power capabilities. Since 1975, over 100 pulsed and CW gyrotrons with typical power levels of 200 kW at frequencies ranging from 28–106 GHz have been used by various fusion laboratories. Present development activity is aimed at providing sources that will generate power levels up to 1 MW CW at frequencies in the range 100–140 GHz for the ECH experiments that are currently being planned. Initial experimental efforts in this area have verified many of the concepts to be employed in forthcoming 1-MW CW test vehicles. Source requirements, that are even more formidable, are foreseen for the next generation magnetic fusion facilities. Frequencies ranging from 200–300 GHz with power generation capabilities of 1–2 MW CW per tube are being considered for these future applications. To this end, various gyrotron designs have been conceived that address these demanding specifications.

A 60 GHz, 200 kW CW gyrotron with a pure output mode†

Abstract For the first time, a Varian 60 GHz gyrotron designed specifically to generate microwaves in a single output mode has been operated at power levels up to 200 kW CW. Using mode-specific directional couplers, measurements of the output mode content indicated that greater than 95% of the microwave output was in the desired TE02 mode with only small percentages in the neighbouring TE01 and TE03 circular electric modes. High output-mode purity is required for the efficient utilization of gyrotrons as high-power microwave sources for electron cyclotron resonance heating (ECRH) in magnetic fusion plasmas.

First 200 kW CW operation of a 60 GHz gyrotron

The gyrotron is a microwave tube which employs the electron cyclotron maser interaction to produce high power output at millimeter wavelengths. It has important and growing applications for heating of plasmas in controlled thermonuclear fusion experiments. The Varian 60 GHz gyrotron has recently generated microwave power in excess of 200 kW during CW operation, with excellent dynamic range and operating stability. This is the highest average power ever produced by a microwave tube in the millimeter wave region. A description of the gyrotron design and test results are presented.

Design considerations in achieving 1 MW CW operation with a whispering-gallery-mode gyrotron

High-power, CW (continuous-wave) gyrotrons at frequencies in the range 100 GHz to 150 GHz are being developed for use in electron cyclotron heating applications. Early test vehicles have utilized a TE/sub 15,2,1/ interaction cavity and have achieved short-pulse power levels of 820 kW and average power levels of 80 kW at 140 GHz. Present tests are aimed at reaching 400 kW under CW operating conditions and up to 1 MW for short pulse durations. Work is also underway on modifications to the present design that will enable power levels of up to 1 MW CW to be achieved.<<ETX>>

Design and operation of a 200 kW CW, 140 GHz gyrotron oscillator

This paper presents significant design issues and a description of the final tube design of a 200 kW CW, 140 GHz gyrotron, presently under development at Varian. Tests on our first two 140 GHz gyrotrons have produced output powers up to 100 kW CW, 150 kW for long pulses and 200 kW for short pulses. The first tube was rebuilt with an improved electron gun and has yielded higher efficiencies than our previous tubes. The 200 kW CW gyrotron design combines the best of the previous designs to provide high efficiency and high output powers at 140 GHz with good dynamic range and stability.

Infrared monitoring of gyrotron windows

A technique for monitoring the gyrotron output window surface temperature with an infrared camera while the gyrotron is in operation has been developed. The IR camera views the window through a perforated waveguide wall, and serves both as a guide for the safe operation at high average power of the tube, as well as an aid in the analysis of new window designs. Window temperatures were studied as a function of a number of parameters, including gun anode voltage, beam current, magnetic field, coolant flow, and load matching. The IR technique is applicable to many types of high average power microwave and millimeter wave tubes. Successful operation of the Varian 60 GHz gyrotron to 214 kW CW was guided by the infrared camera. Analyses on 28, 56 and 60 GHz gyrotrons have led to a number of design changes. A comparison with computer calculations is also presented.

Design considerations for a 100 kW CW, 140 GHz gyrotron oscillator

A gyrotron oscillator capable of generating 100 kW of CW power is currently under development at Varian. The tube is being designed for operation in the TE031ocavity mode with the electron beam located at the second radial electric field maximum in the cavity. The electron beam will be produced by a magnetron injection gun and the 56 kG magnetic field required for 140 GHz operation will be provided by a superconducting magnet. Initial design calculations for the important elements of the tube are reported and the various technology issues of the tube design are discussed.

Recent experimental results on a whispering-gallery-mode gyrotron

The development of gyrotron oscillators capable of generating output powers of 1 MW CW at a frequency of 140 GHz is described. The tubes use a simple, tapered interaction cavity in combination with a hollow electron beam to generate output power in the TE/sub 15.2/ whispering-gallery mode. The first experimental tube was designed to achieve pulse powers of 1 MW and 400 kW CW. In recent tests on this gyrotron, short-pulse power levels of 655 kW, 750 kW, and 820 kW have been obtained at beam voltages of 80 kV, 90 kV, and 95 kV, respectively. During operation at peak power levels at 350-400 kW, the duty factor has been raised to 10% (30 p.p.s., 3.33-ms pulse duration). Pulse durations of 10 ms have also been achieved in the same range of output power. Power losses in various parts of the tube have been measured and are in agreement with theoretical predictions.<<ETX>>

Megawatt Gyrotrons For ECR Heating

Varian is in the process of developing gyrotrons capable of generating power levels of 1 MW CW at a frequency of 140 GHz. The first experimental gyrotron in this program has been designed to generate short-pulse power levels of 1 MW and up to 400 kW CW. In recent tests on this gyrotron, a short-pulse power level of 820 kW was obtained at 21% efficiency. While operating at peak power levels of 350-400 kW, the duty factor was raised to 10% (30 pps, 3.33 ms-pulse duration). Power losses in various parts of the tube have been measured and are in agreement with theoretical predictions. Tests at higher average powers, up to the 400 kW CW goal for the tube, are underway. Design efforts aimed at increasing the average power capabilities of the gyrotron up to 1 MW are in progress.

A 60 GHz, 200 kW CW gyrotron with high output mode purity

For the first time, a Varian 60 GHz gyrotron, designed specifically to generate microwaves in a single output mode, has been operated at power levels up to 200 kW CW. High output mode purity is required for the efficient utilization of gyrotrons as high power microwave sources for electron cyclotron resonance heating (ECRH) in magnetic fusion plasmas. Using mode-specific directional couplers, measurements of the output mode content indicated that greater than 95% of the microwave output was in the desired TE02mode, with only small percentages in the neighboring TE01and TE03circular electric modes. The pure mode CW design uses a 2.5-inch diameter collector. The collector has a magnetic field arrangment which capably avoids excessive heating by distributing the 640 kW CW beam over a sufficient collector area. With pure mode operation, window temperatures are 10°C to 15°C higher than with mixed mode operation.