05pE56P
RELAX における球状 RFP プラズマ研究の現状と今後の展開
Recent Status and Future Plans
in Spherical RFP Plasma Research in RELAX
三瓶明希夫, 政宗貞男, 田中裕之, 西村香苗, 石井豪, 上羽涼太, 小寺良典, 比村治彦,
秋山毅志 1 , 大舘暁 1 , 水口直紀 1 , 川端一男 1 , 江尻晶 2 , 小口治久 3 , D.J. Den Hartog4 ,K.J. McCollam4 , R. Paccagnella5
Akio SANPEI, Sadao MASAMUNE, Hiroyuki TANAKA, Kanae NISHIMURA, et al.
京都工繊大, 核融合研 1 , 東大 2 , 産総研 3 , Univ. Wisconsin4 , Consorzio RFX5
Kyoto Inst. Tech., NIFS1 , Univ. Tokyo2 , AIST3 , Univ. Wisconsin4 , Consorzio RFX5
The reversed ﬁeld pinch (RFP) is a compact, high-beta magnetic conﬁnement concept, characterized
by self-organization of the magnetic conﬁguration and dominated by magnetic ﬂuctuations excited byresistive magnetohydrodynamics (MHD) instabilities. An equilibrium analyses have shown that mode
rational surfaces are less densely spaced by lowering the aspect ratio (A = R/a) of the RFP conﬁguration,
where R (a) is the major (minor) radius of the plasma column, therefore simpler MHD mode dynamics isexpected in low-A RFP. It is also expected that trapped particle fraction also increases in low-A. Moreover,in low-A equilibrium, self-induced bootstrap current tends to increase [1], which may reduce the electric
current needed from an external source. In order to study these attractive characteristics of low-A RFP
conﬁguration, experimental study has been carried out in REversed ﬁeld pinch of Low Aspect eXperiment(RELAX) [2] device with R=0.51 m/a=0.25 m, A = 2. The device is operated with a 4 mm SS vacuum
vessel (ﬁeld penetration time τw < 3 ms). Recent experimental results have shown that low-A RFP plasmas
have been obtained with plasma current Ip of up to 125 kA and loop voltage Vl of down to ∼30 V, discharge
duration of up to 3.5 ms, electron density in the range from 0.2 to 2.0 × 1019 m− 3 .
Recent progress in RELAX includes feedback control of ﬁeld errors and stabilization of a resistive wall
mode. The feedback experiments have been conducted to stabilize a speciﬁc mode (m=1/n=2) by using
limited number of the power supplies, resulting in increased plasma current with longer discharge duration
up to ∼3.5 ms [3]. The visible light and the soft X-ray (SXR) diagnostic with high-speed camera haverevealed simple helix structure in shallow reversal discharge [4]. Existence of a large magnetic island has
been suggested from the SXR images with a computed tomography technique and a ﬁeld line tracing
technique. Thomson scattering measurement has shown that the central electron temperature is around
100 eV at plasma current of 50-80 kA, increasing with Ip [5]. A super-sonic gas puﬃng system to control
the plasma pressure proﬁle has been constructed and initial experiments have been started [6]. Details of
the above-mentioned characteristic behaviors of low-A RFP plasmas will be discussed.
This work is performed by a Grant-in-Aid for Scientiﬁc Research from MEXT (No. 24740368) and NIFS
Collaborative Research Programs (NIFS13KLPP033, NIFS13KNTT022, NIFS13KLEH033).

[1] S. Shiina et al., Phys. Plasmas. 12, 080702 (2005).
[2] S. Masamune et al., J. Phys. Soc. Jpn. 76, 123501 (2007).
[3] R. Kodera et al., poster 05pE59P in this conference.
[4] A. Sanpei et al., IEEE Transaction Plasma Science 39, 2410 (2011).
[5] R. Ueba et al., poster 05pE58P in this conference.
[6] G. Ishii et al., poster 05pE57P in this conference.