Abstract
A dual-frequency gyrotron capable of operation in the TE28,7 cavity interaction mode at 140 GHz, or in the TE22,5 mode at 104 GHz, has been developed for use in electron cyclotron heating in the W7-X stellarator at IPP Greifswald. The gyrotron incorporates an internal converter design that has been numerically optimized to convert either of the two operating modes into a high-quality Gaussian output beam. During short-pulse factory testing, the gyrotron produced 900 kW at 140 GHz, and 520 kW at 104 GHz. After delivery to IPP, the gyrotron was conditioned to long-pulse operation at 140 GHz, demonstrating 30-minute pulses at several power levels up to 811 kW, and producing ten consecutive ten-minute pulses at 811 kW as well. After long-pulse capabilities were demonstrated at 140 GHz, IPP requested an analysis of the feasibility of operating the gyrotron (without internal modification) in an additional mode with a frequency near 175 GHz. Several potential interaction modes were evaluated to determine the required operating parameters for excitation of these modes, and to assess the expected interaction efficiency, output power, internal diffraction losses, and output beam quality. The most promising modes appear to be the TE33,9 (173 GHz) and the TE34,9 (176 GHz), which should generate 400-500 kW of RF in a suitable magnet capable of producing the necessary 7.1 T field required for operation at these higher frequencies. Because the existing gyrotron’s internal converter was not optimized for these modes, however, internal losses are expected to be higher than usual (up to 7%), and the output beam pattern will require external phase-correction in order to produce a Gaussian beam. A feasibility analysis for such external phase correction has been performed, demonstrating that a high-quality beam can be recovered using numerically synthesized external phase-correcting mirror surfaces.
Highlights
Gyrotrons are vacuum electron devices that make use of the cyclotron resonance maser instability to convert DC power into millimetre-wave radiation
Localized electron cyclotron heating of fusion plasmas makes use of millimetre-wave beams injected with a frequency matching the electron cyclotron frequency for the magnetic field at the desired heating location in the plasma
The gun and cavity were optimized for performance at the 140 GHz operating frequency, employing the TE28,7 interaction mode, with simulations indicating that an 80 kV accelerating voltage and a beam current of 40 A should yield over 900 kW of output power, for a peak magnetic field value of 5.54 T
Summary
Gyrotrons are vacuum electron devices that make use of the cyclotron resonance maser instability to convert DC power into millimetre-wave radiation. Localized electron cyclotron heating of fusion plasmas makes use of millimetre-wave beams injected with a frequency matching the electron cyclotron frequency for the magnetic field at the desired heating location in the plasma. Communications & Power Industries (CPI) has designed and fabricated a 2-frequency gyrotron capable of operation at either 140 GHz or 104 GHz, for use in plasma heating in the W7-X fusion stellarator at IPP Greifswald. After testing of the gyrotron, IPP requested an analysis of the feasibility of operating the gyrotron at a higher frequency near 175 GHz, for possible use in plasma diagnostics. This paper summarizes (a) the design of the gyrotron, (b) the demonstrated operation of the gyrotron during factory testing and during acceptance testing at the customer site, and (c) the results of the feasibility study evaluating the possibility of operation at ~175 GHz
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