Synchronous oscillation prior to disruption caused by kink modes in HL-2A tokamak plasmas

Abstract

A class of evident MHD activities prior to major disruption has been observed during recent radiation induced disruptions of the HL-2A tokamak discharges. It can be named SOD, synchronous oscillations prior to disruption, characterized by synchronous oscillation of electron cyclotron emission (ECE), core soft x-ray, Mirnov coil, and a D radiation signals at the divertor plate. The SOD activity is mostly observed in a parametric regime where the poloidal beta is low enough before disruption, typically corresponding to those radiationinduced disruptions. It has been found that the m/n = 2/1 mode is dominant during the SODs, and consequently it is the drop of the mode frequency and the final mode locking that lead to thermal quench. The mode frequency before the mode locking corresponds to the toroidal rotation frequency of the edge plasma. It is also found that during SODs, the location of the q = 2 surface is moving outward, and most of the plasma current is enclosed within the surface. This demonstrates that the current channel lies inside the rational surface during SOD, and thus the resistive kink mode is unstable. Further analysis of the electron temperature perturbation structure shows that the plasma is indeed dominated by the resistive kink mode, with kink-like perturbation in the core plasma region. It suggests that it is the nonlinear growth of the m/n = 2/1 resistive kink mode and its higher order harmonics, rather than the spontaneous overlapping of multiple neighboring islands, that ultimately triggered the disruption.

Very Long-Period Pulsations Before the Onset of Solar Flares

“…Actually, the preflare-VLP is much alike the quasi-periodic oscillations in precursor phase of a Tokamaks major disruption (Wesson 1997, Jiang et al 2015. The precursor phase (similar to the preflare phase) lasts for typically 10 ms and the fast phase (similar to the flare rising phase) for about 1 ms at medium-sized Tokamaks.…”

“…The precursor phase (similar to the preflare phase) lasts for typically 10 ms and the fast phase (similar to the flare rising phase) for about 1 ms at medium-sized Tokamaks. Because of electric currents and limited resistivity, the tearing-mode instability can produce the growth of magnetic oscillations of an m=2 mode in the precursor phase of Tokamak major disruptions (Jiang et al 2015).…”

25th IAEA Fusion Energy Conference: summary of sessions EX/S, EX/W and ICC

“…Furthermore, on the basis of a systematic and detailed statistical analysis of a large database, it was also shown that the JET and COMPASS data were consistent with each other in terms of amplitude of asymmetries and their rotation behaviour-an important result that can prove useful for model development and making load predictions for ITER. Characterization studies of the HL-2A tokamak discharges have identified a precursor mode that always appears to precede the onset of disruptions at the density limit or radiation induced disruptions [4]. It is observed as a synchronous oscillation superposed on the soft x-ray (SXR), electron cyclotron emission (ECE), Mirnov and H α radiation line signals.…”

Quasilinear perturbed equilibria of resistively unstable current carrying plasma

“…Moreover, if left uncontrolled, nonlinearly unstable resistive kinks (Jiang et al. 2015) or thermally unstable tearing modes (Gates & Delgato-Aparicio 2012; White, Gates & Brennan 2015) may grow to a large amplitude and trigger disruption. Thus it is of utmost importance to have a thorough understanding of the stability of resistive modes in both the linear and nonlinear phases.…”

“…In tokamaks, tearing modes and resistive kink modes may arise from unfavourable current density profiles which drive reconnection at corresponding resonant surfaces, resulting in the formation of magnetic islands which tend to cause confinement degradation (Chang & Callen 1990). Moreover, if left uncontrolled, nonlinearly unstable resistive kinks (Jiang et al 2015) or thermally unstable tearing modes (Gates & Delgato-Aparicio 2012;White, Gates & Brennan 2015) may grow to a large amplitude and trigger disruption. Thus it is of utmost importance to have a thorough understanding of the stability of resistive modes in both the linear and nonlinear phases.…”