(EX/P1-14) Steady-State Operation of ICRF Heated Plasma in the Large Helical Device

T. Mutoh1), R. Kumazawa1), T. Seki1), K. Saito1), H. Kasahara1), Y. Nakamura1), S. Masuzaki1), S. Kubo1), Y. Takeiri1), T. Shimozuma1), Y. Yoshimura1), H. Igami1), T. Watanabe1), H. Ogawa2), J. Miyazawa1), M. Shoji1), N. Ashikawa1), K. Nishimura1), M. Osakabe1), K. Tsumori1), K. Ikeda1), K. Nagaoka1), Y. Oka1), H. Chikaraishi1), H. Funaba1), S. Morita1), M. Goto1), S. Inagaki1), K. Narihara1), T. Tokuzawa1), R. Sakamoto1), T. Morisaki1), B. J. Peterson1), K. Tanaka1), H. Nakanishi1), M. Nishiura1), T. Ozaki1), F. Shimpo1), G. Nomura1), C. Takahashi1), M. Yokota1), Y. P. Zhao3), J. G. Kwak4), S. Murakami5), H. Okada5), H. Yamada1), K. Kawahata1), N. Ohyabu1), O. Kaneko1), K. Ida1), Y. Nagayama1), K. Y. Watanabe1), N. Noda1), A. Komori1), S. Sudo1), O. Motojima1)
 
1) National Institute for Fusion Science, Toki, Japan
2) Graduate University for Advanced Studies, Hayama, 240-0163 Japan
3) Institute of Plasma Physics, Academia Sinica, Hefei, 230031, P.R.China
4) Korea Atomic Energy Institute, Daejeon,305-600, Korea
5) Kyoto University, Kyoto 606-8501, Japan

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 0.8×1019m-3 and 2 keV, respectively, at a 1.3GJ discharge (#53776) and 0.4×1019m-3 and 1 keV at a 1.6GJ discharge (#66053). The average input power was 680 kW 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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