The magnitude of sawtooth crash events in multiple and quasi-single helicity states in a reversed-field-pinch plasma

Factors affecting the magnitude of sawtooth crashes in soft x-ray (SXR) signals (ISXR) have been studied using data from high-Θ (pinch parameter) discharges in the toroidal pinch experiment TPE-1RM20 reversed-field pinch device [Y. Yagi et al., J. Plasma Fusion Research 69, 700 (1993)]. In TPE-1RM20, discharges with a high Θ that are free of large sawteeth (improved high-Θ mode discharges), spontaneously appeared [Y. Hirano et al., Nucl. Fusion 36, 721 (1996)] among typical conventional discharges exhibiting large sawtooth crashes. The reason for the spontaneous appearance of sawtooth-free discharges has been unclear. It is shown that the normalized magnitude of the SXR crashes, [Δ(ISXR)/ISXR], is well correlated with the m (poloidal mode number)=0 mode amplitude in the postcrash phase. It is also shown that Δ(ISXR)/ISXR is smaller when the quasi-single helicity state precedes the crash rather than when the multiple-helicity state precedes the crash, and Δ(ISXR)/ISXR is smaller when Eparallel (the parallel electric field at the plasma surface) is approximately zero in the precrash phase rather than Eparallel being negative