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Density Regimes of Complete Detachment and Serpens Mode in LHD

Junichi MIYAZAWA, Suguru MASUZAKI, Motoshi GOTO, Naoki TAMURA, Ryuichi SAKAMOTO, Byron J. PETERSON , Ichihiro YAMADA , Kazumichi NARIHARA, Kenji TANAKA, Tokihiko TOKUZAWA, Mamoru SHOJI, Hisamichi FUNABA, Satoru SAKAKIBARA, Masahiro KOBAYASHI, Masaki OSAKABE, Shigeru MORITA, Hajime ARIMOTO¹⁾, Katsumi KONDOH¹⁾, Sadayoshi MURAKAMI²⁾, Masaki NISHIURA, Naoko ASHIKAWA, Tomohiro MORISAKI, Kiyohiko NISHIMURA, Hiroshi YAMADA, Nobuyoshi OHYABU, Akio KOMORI, Osamu MOTOJIMA and the LHD Experimental Group

National Institute for Fusion Science

1)

Graduate School of Energy Science, Kyoto University

2)

Department of Nuclear Engineering, Kyoto University

(Received 26 November 2005 / Accepted 10 April 2006 / Published 16 May 2006)

Abstract

In the Large Helical Device (LHD), the hot plasma column shrinks at the high-density regime and complete detachment takes place. Hydrogen volume recombination is observed at complete detachment. This phase is self-sustained under specific experimental conditions and called the Serpens mode (self-regulated plasma edge ‘neath the last-closed-flux-surface). The Serpens mode is achieved after either rapid or slow density ramp up, and either by hydrogen or helium gas puffing. The threshold conditions for complete detachment and the Serpens mode are experimentally documented in the parameter space of heating power and density. The threshold density for the Serpens mode transition increases with ˜ 0.4 power of the heating power. The total radiation is shown to be not adequate to describe the threshold conditions, since it mainly includes the information of very edge region outside the hot plasma column. The operational density limit in LHD, which is sustainable in steady state, has been extended to 1.7 times as high as the Sudo density limit, by applying pellet injection to the Serpens plasmas.

Copyright (c) 2006 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

high-density plasma, detachment, Serpens mode, serpent, volume recombination, density limit

DOI: 10.1585/pfr.1.026

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This paper may be cited as follows:

Junichi MIYAZAWA, Suguru MASUZAKI, Motoshi GOTO, Naoki TAMURA, Ryuichi SAKAMOTO, Byron J. PETERSON , Ichihiro YAMADA , Kazumichi NARIHARA, Kenji TANAKA, Tokihiko TOKUZAWA, Mamoru SHOJI, Hisamichi FUNABA, Satoru SAKAKIBARA, Masahiro KOBAYASHI, Masaki OSAKABE, Shigeru MORITA, Hajime ARIMOTO, Katsumi KONDOH, Sadayoshi MURAKAMI, Masaki NISHIURA, Naoko ASHIKAWA, Tomohiro MORISAKI, Kiyohiko NISHIMURA, Hiroshi YAMADA, Nobuyoshi OHYABU, Akio KOMORI, Osamu MOTOJIMA and the LHD Experimental Group, Plasma Fusion Res. 1, 026 (2006).