Design and testing of a dual-frequency 104/140 GHz megawatt-class gyrotron for fusion plasma heating

Summary form only given. CPI's existing 140 GHz, 900 kW, long-pulse gyrotron design has been adapted to enable operation in an alternative mode at 104 GHz, in addition to the primary 140 GHz mode. The dual-frequency version of this gyrotron has been developed for use in plasma heating experiments at the W7-X stellarator at IPP Greifswald. The gyrotron has been fabricated, and factory testing is currently in progress (as of February, 2016). The gyrotron employs a diode magnetron injection gun to generate an annular electron beam which can be tuned to excite either the TE
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or the TE
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eigenmode of the gyrotron's cylindrical interaction cavity. The TE
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mode at 140 GHz is generated with a beam accelerating voltage of 80 kV, a beam current of up to 45 A, a cavity magnetic field of 55.3 kG, and a collector depression voltage of up to 25 kV. By reducing the magnetic field in the interaction region to 39.7 kG, lowering the beam accelerating voltage to 60 kV, and adjusting the magnetic compression ratio to obtain the optimal beam size, the TE
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mode at 104 GHz can be excited in the interaction cavity, instead of the TE
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mode at 140 GHz. The dual-frequency gyrotron employs an internal mode converter system consisting of a dimpled-wall launcher and three steering and phase-correcting mirrors. These components have been numerically optimized to yield a high-quality Gaussian TEM
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output beam exiting the gyrotron window, for either interaction mode. Preliminary test results have demonstrated short pulse operation at about 920-950 kW in the 140 GHz mode. At 104 GHz, the desired mode has been observed, and optimization of operating parameters is in progress with the goal of generating about 450 kW in the 104 GHz mode. Vacuum conditioning of the gyrotron, and optimization of the collector magnet coil settings to most effectively distribute the residual power of the spent election beam on the collector surface, are also in progress, in order to enable long pulse operation. An overview of the gyrotron design features, the current status of the system, and available test results will be presented.