Abstract
Sawtooth oscillations are often detected in the signals of several diagnostics during tangential neutral beam injection (NBI) heating in the compact helical system (CHS) [K. Matsuoka et al., Plasma Pysics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 441]. The sawtooth crashes are closely related to the q=2 (q is the safety factor) rational surface located on the normalized minor radius ρ∼0.5 and most of the crashes indicate a character of an annular crash. The disturbance of plasma parameters on the magnetic axis depends on the toroidal plasma beta and the size of the q=2 surface. The time scale of the crash is fairly large to be 0.5–1.0 ms which is an order of magnitude larger than that in tokamaks. Two types of m/n=2/1 (m and n are poloidal and toroidal mode numbers, respectively) magnetic fluctuations, which are fishbone-like burst mode (FB) and low frequency ideal/resistive interchange mode (LF mode), grow just before the crash. Those internal structure and time evolution across the crash clearly indicate that the sawtooth crash in NBI heated plasmas of CHS is induced by ideal/resistive interchange mode not by energetic ion driven mode.
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