[Table of Contents]

Plasma and Fusion Research

Volume 3, S1001 (2008)

Regular Articles

Stability and Confinement Studies of High-Performance NBI Plasmas in the Large Helical Device Toward a Steady-State Helical Fusion Reactor
Yasuhiko TAKEIRI, Akio KOMORI, Hiroshi YAMADA, Kazuo KAWAHATA, Takashi MUTOH, Nobuyoshi OHYABU, Osamu KANEKO, Sinsaku IMAGAWA, Yoshio NAGAYAMA, Kiyomasa WATANABE, Takashi SHIMOZUMA, Katsumi IDA, Ryuichi SAKAMOTO, Masahiro KOBAYASHI, Akio SAGARA, Kenichi NAGAOKA, Mikiro YOSHINUMA, Satoru SAKAKIBARA, Yasuhiro SUZUKI, Masayuki YOKOYAMA, Shigeru SUDO, Osamu MOTOJIMA and the LHD Experimental Group
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
(Received 25 November 2007 / Accepted 7 March 2008 / Published 5 August 2008)

Abstract

Recent progress in plasma performance and the understanding of the related physics in the Large Helical Device is overviewed. The volume-averaged beta value is increased with an increase in the neutral beam injection (NBI) heating power, and it reached 5.0% of the reactor-relevant value. In high-β plasmas, the plasma aspect ratio should be controlled so that the Shafranov shift would be reduced, mainly to suppress transport degradation and the deterioration of the NBI heating efficiency. The operational regime of a high-density plasma with an internal diffusion barrier (IDB) has been extended, and the IDB, which was originally found using the local island divertor, has been realized in the helical divertor configuration. The central density was recorded as high as 1 × 1021 m-3, and the central pressure reached 130 kPa. Based on these high-density plasmas with the IDB, a new ignition scenario has been proposed. This should be a scenario specific to the helical fusion reactor, in which the helical ripple transport would be mitigated. A low-energy positive-NBI system was newly installed for an increase in the direct ion heating power. As a result, the ion temperature (Ti) exceeded 5.2 keV at a density of 1.2 × 1019 m-3 in a hydrogen plasma. Transport analysis shows improvement of ion transport, and the Ti-increase tends to be accompanied by a large toroidal rotation velocity of the order of 50 km/s in the core region. The plasma properties in the extended operational regime are discussed from the perspective of a steady-state helical fusion reactor.

Keywords

Large Helical Device, MHD stability, Shafranov shift, internal diffusion barrier, ion transport, toroidal rotation, impurity hole, helical fusion reactor, neoclassical transport, anomalous transport, core electron-root confinement

DOI: 10.1585/pfr.3.S1001

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This paper may be cited as follows:

Yasuhiko TAKEIRI, Akio KOMORI, Hiroshi YAMADA, Kazuo KAWAHATA, Takashi MUTOH, Nobuyoshi OHYABU, Osamu KANEKO, Sinsaku IMAGAWA, Yoshio NAGAYAMA, Kiyomasa WATANABE, Takashi SHIMOZUMA, Katsumi IDA, Ryuichi SAKAMOTO, Masahiro KOBAYASHI, Akio SAGARA, Kenichi NAGAOKA, Mikiro YOSHINUMA, Satoru SAKAKIBARA, Yasuhiro SUZUKI, Masayuki YOKOYAMA, Shigeru SUDO, Osamu MOTOJIMA and the LHD Experimental Group, Plasma Fusion Res. 3, S1001 (2008).