K. Felch

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.

Design and operation of magnetron injection guns for a 140 GHz gyrotron

Two magnetron injection electron guns with different electrode geometries have been designed and constructed for use on a 140 GHz gyrotron oscillator. Computer simulations have been performed, detailing the behavior of both designs with variations of all relevant parameters. The first of the two designs has been incorporated into the gyrotron and has been operated up to a beam voltage of 80 kv and a beam current of 7.5 a. With this beam power, the gyrotron has emitted a peak power of 100 kw at 140 GHz, for 1.0 - 1.5 µs pulses. This represents the highest frequency at which a power level of 100 kw has ever been achieved on a gyrotron. In addition to the successful rf results, the gun operates into the highest magnetic mirror ratio (25:1), and the highest cavity and cathode magnetic fields (54 kG and 2.2 kG, respectively) of any magnetron injection guns.

Simulation of space-charge effects on velocity spread in gyro devices

The effects of space-charge field on velocity spread in the drift tube regions of gyro devices which use either magnetron injection guns (MIG) or Pierce gun/wigglers (PW) are investigated using computer simulation codes. It is shown that changes in velocity spread occur while the beam makes a transition from an essentially coherent laminar state to an incoherent mixed state.

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.

GYROTRON OSCILLATORS FOR FUSION HEATING

Recent experiments have been performed to determine the ultimate power capability of a 28 GHz 200 kW CW gyrotron design. A power output of 342 kW CW was measured in these tests with an efficiency of 37%. Progress in the development of 60 GHz 200 kW pulsed and CW gyrotrons is discussed. An output of 200 kW with 100 msec pulse length has been achieved with the pulsed design.

Window-temperature measurements on a high-power gyrotron with an internal, quasi-optical converter

Window-temperature measurements on a double-disc sapphire window have been performed during a series of high-power, long-pulse tests of a 110 GHz, internal-converter gyrotron using a 10-/spl mu/m, infrared camera. Details of the infrared-imaging setup will be discussed and the results obtained will be presented. From these measurements, loss-tangent and film-coefficient values used in calculations can be verified.

Design considerations for a 1 MW CW gyrotron with an internal converter

Varian is carrying out the development of high-power, CW gyrotrons at frequencies ranging from 100-140 GHz.Recent experiments, carried out at a frequency of 110 GHz, resulted in the generation of output powers of 500 kW for 2.5- second pulses and 1 MW for 1 ms pulse durations.1 The output mode of this tube was a whispering-gallery mode, based onthe TE22,2 mode employed in the interaction cavity. Current design activity is aimed at producing a 1 MW CW gyrotron atthe same frequency, but with a gaussian output mode structure. This type of output mode is desirable for low-losstransmission in a corrugated waveguide or mirror transmission line. In addition to the change in output coupling, the cavitymode will be changed to the TE22,6 mode. The higher order cavity mode is consistent with higher power or higherfrequency requirements that will be addressed in subsequent development activities.

Initial tests of a high power, broadband, harmonic gyroTWT

The design of a high power, 95 GHz 3rd harmonic gyrotron amplifier with wide bandwidth is described and test results are presented on a scaled, proof-of-principle experiment. The fabrication of a complete tube is nearly complete, capable of 10% duty operation, with predicted performance of 80 kW peak power at an efficiency of 23% and a bandwidth of 3% about 95 GHz.<<ETX>>

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>>