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(EX/P4-17) Coaxial Helicity Injection and n=1 Relaxation Activity in the HIST Spherical Torus

M. Nagata¹⁾, T. Oguro¹⁾, Y. Kagei¹⁾, H. Hasegawa¹⁾, K. Kawami¹⁾, N. Fukumoto¹⁾, T. Uyama¹⁾, S. Masamune²⁾, M. Iida²⁾, M. Katsurai^3)
 
1) Himeji Institute of Technology, Himeji, Hyogo, Japan
²⁾ Kyoto Institute of Technology, Kyoto, Japan
³⁾ University of Tokyo, Tokyo, Japan

Abstract.  Coaxial Helicity Injection (CHI) has demonstrated non-inductive current generation of spherical tokamak (ST) and spheromak plasmas on several devices. In order to understand comprehensively the role of the n=1 instability and relaxation on current generation processes in helicity-driven spherical systems, we have investigated dynamics of ST plasmas produced in the HIST device (major radius R = 0.30 m, minor radius a = 0.24 m, aspect ratio A = 1.25, toroidal field B_t < 0.2 T, plasma current I_t < 150 kA and discharge time t < 5 ms in the ST configuration) by decreasing the external toroidal field (TF) and reversing its sign in time. In results, we have discovered that the ST relaxes towards flipped ST configurations through formation of reversed-field pinches (RFPs)-like magnetic field profiles. Surprisingly, it has been observed that not only toroidal flux but also poloidal flux reverses sign spontaneously during the relaxation process. This self-reversal of the poloidal field is thought to be evidence for ``global helicity conservation''. Furthermore, we have first demonstrated that a flipped ST plasma can be successfully sustained by CHI.

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