Plasma and Fusion Research

Volume 3, S1047 (2008)

Regular Articles

Abrupt Flushing of High-Density Core in Internal Diffusion Barrier Plasmas and its Suppression by Plasma Shape Control in LHD

Junichi MIYAZAWA, Ryuichi SAKAMOTO, Satoshi OHDACHI, Masahiro KOBAYASHI, Suguru MASUZAKI, Tomohiko MORISAKI, Satoru SAKAKIBARA, Motoshi GOTO, Shigeru MORITA, Ichihiro YAMADA, Kazumichi NARIHARA, Hisamichi FUNABA, Kenji TANAKA, Clive A. MICHAEL, Yasuhiro SUZUKI, Kiyomasa WATANABE, Nobuyoshi OHYABU, Hiroshi YAMADA, Akio KOMORI, Osamu MOTOJIMA and the LHD experimental group

National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan

(Received 15 November 2007 / Accepted 24 January 2008 / Published 25 June 2008)

Abstract

Abrupt flushing of central density occurs in internal diffusion barrier (IDB) plasmas in the Large Helical Device (LHD), where a super dense core (SDC) of the order of 10²⁰ m^-3 is formed inside; this event is called “core density collapse (CDC).” CDC must be suppressed as further increase of the central pressure is inhibited. Since CDC is always accompanied by a large Shafranov shift of the plasma center, it has been supposed that mitigation of the Shafranov shift might affect CDC. Vertical elongation of the plasma shape (κ > 1) is effective in mitigating the Shafranov shift, and κ can be controlled with the quadrupole magnetic field B_Q. To examine the impact of plasma shape control on CDC, B_Q scan experiment has been performed in the LHD. The large Shafranov shift in IDB plasmas is mitigated by increasing κ. As a result, CDC is suppressed and high central β values of approximately 7% have been achieved in vertically elongated plasmas. The optimum κ varies with the magnetic configuration. Beta gradients greater than those at CDC in the κ = 1 configuration are observed without CDC in vertically elongated plasmas.

Keywords

heliotron, pellet injection, IDB, SDC, Shafranov shift, beta limit, equilibrium, elongation, MHD instability

DOI: 10.1585/pfr.3.S1047

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This paper may be cited as follows:

Junichi MIYAZAWA, Ryuichi SAKAMOTO, Satoshi OHDACHI, Masahiro KOBAYASHI, Suguru MASUZAKI, Tomohiko MORISAKI, Satoru SAKAKIBARA, Motoshi GOTO, Shigeru MORITA, Ichihiro YAMADA, Kazumichi NARIHARA, Hisamichi FUNABA, Kenji TANAKA, Clive A. MICHAEL, Yasuhiro SUZUKI, Kiyomasa WATANABE, Nobuyoshi OHYABU, Hiroshi YAMADA, Akio KOMORI, Osamu MOTOJIMA and the LHD experimental group, Plasma Fusion Res. 3, S1047 (2008).