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(EX6/7) Energy Confinement and Thermal Transport Characteristics of Net-Current Free Plasmas in Large Helical Device

H. Yamada1), K. Y. Watanabe1), K. Yamazaki1), S. Murakami1), S. Sakakibara1), K. Narihara1), K. Tanaka1), M. Osakabe1), K. Ida1), N. Ashikawa2), P. de Vries1), M. Emoto1), H. Funaba1), M. Goto1), H. Idei1), K. Ikeda1), S. Inagaki1), N. Inoue1), M. Isobe1), S. Kado1), O. Kaneko1), K. Kawahata1), K. Khlopenkov1), A. Komori1), S. Kubo1), R. Kumazawa1), S. Masuzaki1), T. Minami1), J. Miyazawa1), T. Morisaki1), S. Morita1), S. Muto1), T. Mutoh1), Y. Nagayama1), N. Nakajima1), Y. Nakamura1), H. Nakanishi1), K. Nishimura1), N. Noda1), T. Notake3), T. Kobuchi2), Y. Liang2), S. Ohdachi1), N. Ohyabu1), Y. Oka1), T. Ozaki1), R. O. Pavlichenko1), B. J. Peterson1), G. Rewoldt4), A. Sagara1), K. Saito3), R. Sakamoto1), H. Sasao2), M. Sasao1), K. Sato1), M. Sato1), T. Seki1), T. Shimozuma1), M. Shoji1), H. Sugama1), H. Suzuki1), M. Takechi1), Y. Takeiri1), N. Tamura2), K. Toi1), T. Tokuzawa1), Y. Torii3), K. Tsumori1), I. Yamada1), S. Yamaguchi1), S. Yamamoto3), M. Yokoyama1), Y. Yoshimura1), T. Watari1), K. Itoh1), K. Matsuoka1), K. Ohkubo1), I. Ohtake1), S. Satoh1), T. Satow1), S. Sudo1), S. Tanahashi1), T. Uda1), Y. Hamada1), O. Motojima1), M. Fujiwara1)
 
1) National Institute for Fusion Science, Toki, Japan
2) Department of Fusion Science, Graduate Univ. for Advanced Studies, Hayama, Japan
3) Department of Energy Engineering and Science, Nagoya University, Japan
4) Princeton Plasma Physics Laboratory, Princeton Univ. Princeton, USA

Abstract.  The energy confinement and thermal transport characteristics of net-current free plasmas in the much smaller gyro-radii and collisionality regimes than before have been investigated in the Large Helical Device (LHD). The inward shifted configuration that is superior from the theoretical aspect of neoclassical transport has revealed a systematic confinement improvement on a standard configuration. The improvement of energy confinement times on the international stellarator scaling 95 occurs with a factor of 1.6±0.2 for an inward shifted configuration. This enhancement is primarily due to the broad temperature profile with a high edge value. A simple dimensional analysis involving LHD and other medium sized heliotrons yields strongly gyro-Bohm dependence ( $ \tau_{\mathrm{E}}^{}$$ \Omega$ $ \propto$ $ \rho^{*-3.8}_{}$) of energy confinement times. It should be noted that this result is attributed to comprehensive treatment of LHD for systematic confinement enhancement and that the medium sized heliotrons have narrow temperature profiles. The core stored energy still indicates the dependence of $ \tau_{\mathrm{E}}^{}$$ \Omega$ $ \propto$ $ \rho^{*-2.6}_{}$ when data only from LHD is processed. The local heat transport analysis of dimensionally similar discharges except for $ \rho^{*}_{}$ suggests that the heat conduction coefficient lies between Bohm and gyro-Bohm in the core and changes towards strong gyro-Bohm in the peripheral region. Since the inward shifted configuration has a geometrical feature suppressing the neoclassical transport, confinement improvement can be maintained in the collisionless regime where the ripple transport is important. The stiffness of the pressure profile coincides with enhanced transport in the peaked density profile obtained by pellet injection.

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IAEA 2001