Sawtooth Oscillation in Current-Carrying Plasma in the Large Helical Device

Abstract

Sawtooth oscillations have been observed in current-carrying helical plasmas by using electron-cyclotron-emission diagnostics in the Large Helical Device. The plasma current, which is driven by neutral beam injection, reduces the beta threshold of the sawtooth oscillation. When the central q value is increased due to the plasma current, the core region crashes, and, when it is decreased, the edge region crashes annularly. Observed rapid mixture of the plasma in the limited region suggests that these sawtooth crashes are reconnection phenomena. Unlike previous experiments, no precursor oscillation has been observed.

ECCD-induced sawtooth crashes at W7-X

“…ECCD experiments have been conducted at W7-X in OP1.1 and OP1.2 campaigns. In such experiments, repetitive crashes of the central electron temperature were observed: these crashes have similar characteristics as sawtooth oscillations observed in tokamaks [11] and, under certain conditions, in current carrying stellarators [12][13][14]. While small sawteeth can be beneficial, countering or avoiding impurity accumulation [15,16], large amplitude crashes can be detrimental for plasma performances.…”

Influence of sawtooth oscillations on fast ions in a stellarator

“…There is solid experimental evidence of this redistribution due to the crash in many tokamaks [1][2][3][4][5][6][7][8], as well as theoretical models of the process [9][10][11]. The sawtooth-like instability in stellarators was first reported in Large Helical Device (LHD) [12] using Neutral Beam Current Drive (NBCD). There, the sawtooth crashes were reported with the inversion radius corresponding to the rational surface ι/2π = 1/2 (q = 2).…”