Testing of megawatt-class gyrotrons

Abstract

Summary form only given. CPI is developing gyrotrons for fusion plasma heating and current drive at several frequencies. Several of these gyrotrons have undergone initial testing, and are being rebuilt to improve performance. Others are nearing completion of fabrication, with initial testing to begin soon.At 110 GHz, CPI has recently delivered two VGT-8115 gyrotrons. One was delivered to General Atomics for use on the DIII-D tokamak. It generated up to 1.2 MW during short pulse factory testing, but is operated at a somewhat lower power point at General Atomics, to ensure reliability during longer pulses. The following VGT-8115 gyrotron was built for NASA, for use in microwave thermal launch (MTL) experiments. This gyrotron also will be installed at General Atomics, so that it can be used for plasma heating as well as for MTL experiments. At 117.5 GHz, CPI has completed initial testing of the VGT8117 gyrotron (also intended for use on DIII-D), which produced up to 1.8 MW of output power during short-pulse testing, with electrical efficiencies of up to 42%, including the effect of a single-stage depressed collector, with an accelerating voltage of 98 kV and a beam current of 60 A. This gyrotron also exhibited low internal losses, no evidence of parasitic beam tunnel oscillations, and a high-quality Gaussian output beam profile. The gyrotron is currently being rebuilt to address a high-voltage standoff problem that prevented long-pulse operation. At 140 GHz, two VGT-8141 gyrotrons are in fabrication. One, for the EAST tokamak, is nearly complete, and features several design improvements over its predecessor, including improved collector, internal converter, and electron gun designs. The other, for the W7-X stellarator, is also in fabrication, and is nearly identical, but includes an alternative internal converter design that will enable the gyrotron to be operated at a second frequency (104 GHz, in the TE22,5 mode) in addition to the primary 140 GHz TE28,7 mode. Finally, at 170 GHz, the VGT-8170 gyrotron prototype is currently being rebuilt. This gyrotron, which is to be used for transmission line component testing by the US ITER program, generated over 600 kW during short pulse operation, but exhibited lower-than-expected efficiency, and was unable to achieve reliable long pulse operation, due to the onset of sporadic mod-anode current and excessive power absorption in the beam tunnel. The rebuilt gyrotron will employ a diode gun and a modified beam tunnel design to address these limitations. The current status of these programs, available test results, and future plans will be presented.