The tearing mode locking–unlocking mechanism to an external resonant field in EXTRAP T2R

Abstract

The tearing mode (TM) locking and unlocking process due to an external resonant magnetic perturbation (RMP) is experimentally studied in EXTRAP T2R. The RMP produces a reduction of the natural TM velocity and ultimately the TM locking if a threshold in the RMP amplitude is exceeded. During the braking process, the TM slows down via a mechanism composed of deceleration and acceleration phases. During the acceleration phases, the TM can reach velocities higher than the natural velocity. Once the TM locking occurs, the RMP must be reduced to a small amplitude to obtain the TM unlocking, showing that the unlocking threshold is significantly smaller than the locking threshold and that the process is characterized by hysteresis. Experimental results are in qualitative agreement with a model that describes the locking-unlocking process via the balance of the electromagnetic torque produced by the RMP that acts to brake the TM and the viscous torque that tends to re-establish the unperturbed velocity.

Tearing mode dynamics and locking in the presence of external magnetic perturbations

“…The only observed cases of TM unlocking are in relation to a large sawtooth crash, where the mode amplitude is decreased and energy is redistributed between the central m ¼ 1 modes and m ¼ 0 modes at the reversal surface. Hysteresis in the lockingunlocking of a TM to a static RMP has previously been observed in EXTRAP T2R reversed-field pinch 26,30 and in tokamaks. 57,58 In RFX-mod, the hysteresis is avoided by feedback suppression of the TM amplitude at the shell surface.…”

“…12,19 Furthermore, after TM wall-locking, hysteresis caused by reduction of viscous torque and increase in the tearing mode amplitude has been experimentally observed. 26,30 Previous works in RFPs have mainly focused on the interaction between a single harmonic RMP and a TM of corresponding resonance. However, in tokamaks, the number of RMP coils that can be placed outside the machine-wall is limited by other equipment, such as ports for neutral beam injection (NBI) and radio frequency (RF) heating.…”

Effects of resonant magnetic perturbation on the instability of single tearing mode with non-shear flow

“…In recent decades, researchers found from experiments that the resonant magnetic perturbation (RMP) can also suppress the tearing mode instability. [15][16][17][18] However, the physical mechanism of the suppressing effect of RMP on the tearing mode instability is not very clear. [15][16][17][18] In this paper, we investigate the effects of RMP on the single tearing mode with flow in a two-dimensional (2D) geometry based on the magnetohydrodynamics (MHD) model.…”

“…[15][16][17][18] However, the physical mechanism of the suppressing effect of RMP on the tearing mode instability is not very clear. [15][16][17][18] In this paper, we investigate the effects of RMP on the single tearing mode with flow in a two-dimensional (2D) geometry based on the magnetohydrodynamics (MHD) model. In order to better study the suppressing effect of RMP, we use a non-shear flow to eliminate the suppressing effect of the shear flow on the tearing mode instability.…”

Control of 3D equilibria with resonant magnetic perturbations in MST

“…One aspect of interest is the effect RMPs have on the dynamics of the tearing modes, particularly plasma rotation [20] and locking [21][22][23][24]. In this paper we describe the use of RMPs in the Madison Symmetric Torus (MST) [25] to control both the orientation of 3D structures and the rotation of 2D plasmas.…”