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(EX2/3) Plasma Confinement Studies in LHD

   
M. Fujiwara , H. Yamada , A. Ejiri , M. Emoto , H. Funaba , M. Goto , K. Ida , H. Idei , S. Inagaki , S. Kado , O. Kaneko , K. Kawahata , A. Komori , S. Kubo , R. Kumazawa , S. Masuzaki , T. Minami , J. Miyazawa , T. Morisaki , S. Morita , S. Murakami , S. Muto , T. Mutoh , Y. Nagayama , Y. Nakamura , H. Nakanishi , K. Narihara , K. Nishimura , N. Noda , T. Kobuchi , S. Ohdachi , N. Ohyabu , Y. Oka , M. Osakabe , T. Ozaki , B. J. Peterson , A. Sagara , S. Sakakibara , R. Sakamoto , H. Sasao , M. Sasao , K. Sato , M. Sato , T. Seki , T. Shimozuma , M. Shoji , H. Suzuki , Y. Takeiri , K. Tanaka , K. Toi , T. Tokuzawa , K. Tsumori , K. Tsuzuki , I. Yamada , S. Yamaguchi , M. Yokoyama , K. Y. Watanabe , T. Watari , R. Akiyama , H. Chikaraishi , K. Haba , S. Hamaguchi , M. Iima , S. Imagawa , N. Inoue , K. Iwamoto , S. Kitagawa , Y. Kubota , J. Kodaira , R. Maekawa , T. Mito , T. Nagasaka , A. Nishimura , Y. Takita , C. Takahashi , K. Takahata , K. Yamauchi , H. Tamura , T. Tsuzuki , S. Yamada , N. Yanagi , H. Yonezu , Y. Hamada , K. Matsuoka , K. Murai , K. Ohkubo , I. Ohtake , M. Okamoto , S. Satoh , T. Satow , S. Sudo , S. Tanahashi , K. Yamazaki , O. Motojima , A. Iiyoshi 
 
National Institute for Fusion Science, Toki, Gifu, Japan

Abstract
The initial experiments of the Large Helical Device (LHD ) have extended confinement studies on currentless plasmas to a large scale (R = 3.9 m, a = 0.6 m). Heating by NBI of 3 MW has produced plasmas with a fusion triple product  of $\rm 8 \times 10^{18} keVm^{-3} s$ at a magnetic field of 1.5T. An electron temperature of 1.5 keV and an ion temperature of 1.1 keV have been achieved simultaneously at the line-averaged electron density of $\rm 1.5 \times 10^{19} m^{-3}$. The maximum stored energy has reached 0.22 MJ with neither unexpected confinement deterioration nor visible MHD instabilities, which corresponds to $\langle \beta \rangle = 0.7\%$. Energy confinement times, reaching 0.17 s at the maximum, have shown a similar manner to the present scaling law derived from the existing medium sized helical devices, but improve on it by 50%. A distinguishing feature of a favorable dependence of energy confinement  time on density remains in the present power density ( $\rm\sim 40kW/m^3$) and the electron density ( $\rm 3 \times 10^{19} m^{-3}$) regimes unlike L-mode  in tokamaks. High edge temperatures of both electrons and ions as high as 200 eV have been observed at the outermost flux surface, which indicate a qualitative jump in performance from the helical devices to date. Spontaneously generated toroidal currents agree with the physical picture of neoclassical bootstrap currents. Change of magnetic configuration due to finite-$\beta$ effect has been well described by the 3-D MHD equilibrium analysis. An escape of particles from the core region leading to a hollow density profile has been observed in hydrogen plasmas, which is mitigated through core fueling with a pellet injection  or in helium discharges.

     

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