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Author(s):
T. Mutoh et al.
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Title:
Steady-State Operation of ICRF Heated Plasma in the Large Helical Device
Date of publication:
Oct. 2006
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Key words:
21 IAEA Fusion Energy Conference , EX/P1-14
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Abstract:
Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. High temperatures of approximately 1-2 keV, and steady-state plasma-sustainment operations have been reported. After the last IAEA conference, the steady state operation regime was greatly extended in the Large Helical Device (LHD). A high-temperature plasma was created and maintained for more than 30 min with a world record injected heating energy of 1.3 GJ in 2004FY, and recently for 54 min with 1.6 GJ in the 2005FY experimental program. The three-dimensional heat-deposition profile of the LHD helical divertor was modified, and during long-pulse discharges it effectively dispersed the heat load using a magnetic-axis swing technique developed at the LHD. A sweep of only 3 cm in the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies (ICRF). By accumulating the small flux of charge-exchanged neutral particles during the long pulse operation, a high energy ion tail which extended up to 1.6 MeV was observed. The long pulse operations lasted until a sudden increase of radiation loss occurred, presumably because of metal wall flakes dropping into the plasma. The sustained line-averaged electron density and temperature were approximately 0.8 x 10^19 m^-3 and 2 keV, respectively, at a 1.3GJ discharge (#53776) and 0.4 x 10^19 m^-3 and 1 keV at a 1.6GJ discharge (#66053). The average input power was 680kW and 490 kW, and the plasma duration was 32 min and 54 min, respectively. These successful long operations show that the heliotron configuration has a high potential as a steady-state fusion reactor.
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