Rotation in a reversed field pinch with active feedback stabilization of resistive wall modes

Abstract

Active feedback stabilization of multiple resistive wall modes (RWMs) has been successfully proven in the EXTRAP T2R reversed field pinch. One of the features of plasma discharges operated with active feedback stabilization, in addition to the prolongation of the plasma discharge, is the sustainment of the plasma rotation. Sustained rotation is observed both for the internally resonant tearing modes (TMs) and the intrinsic impurity oxygen ions. Good quantitative agreement between the toroidal rotation velocities of both is found: the toroidal rotation is characterized by an acceleration phase followed, after one wall time, by a deceleration phase that is slower than in standard discharges. The TMs and the impurity ions rotate in the same poloidal direction with also similar velocities. Poloidal and toroidal velocities have comparable amplitudes and a simple model of their radial profile reproduces the main features of the helical angular phase velocity. RWMs feedback does not qualitatively change the TMs behaviour and typical phenomena such as the dynamo and the 'slinky' are still observed. The improved sustainment of the plasma and TMs rotation occurs also when feedback only acts on internally non-resonant RWMs. This may be due to an indirect positive effect, through non-linear coupling between TMs and RWMs, of feedback on the TMs or to a reduced plasma-wall interaction affecting the plasma flow rotation. Electromagnetic torque calculations show that with active feedback stabilization the TMs amplitude remains well below the locking threshold condition for a thick shell. Finally, it is suggested that active feedback stabilization of RWMs and current profile control techniques can be employed simultaneously thus improving both the plasma duration and its confinement properties.

Plasma stability theory including the resistive wall effects

“…Information on some of them is collected in table 1. Here, the data for J-TEXT are taken from [122], for JFT-2M from [123], for DIII-D from [13], for LHD from [46], for JET from [1] and [124], for RFX-mod from [106, 120] and for EXTRAP T2R from [39, 125, 126].…”

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

“…For example, in the EXTRAP T2R reversed-field pinch, the ratio of poloidal to toroidal rotation (v h =v / ) increases with minor radius. 56 The increase in plasma-wall interaction during the RMP application, previously observed in MST, 10 is another possible source of plasma braking at the edge.…”

Braking due to non-resonant magnetic perturbations and comparison with neoclassical toroidal viscosity torque in EXTRAP T2R

“…Several m = 1 harmonics are resonant inside the plasma with n = −12 being the first resonant and − n 11 ⩾ non-resonant. Co-rotation between TMs and plasma is assumed in EXTRAP T2R, as experimentally verified in [30]. The plasma velocity profile is obtained by plotting the velocity of each TM at its corresponding resonant radius.…”

“…The TM velocity is measured using the magnetic fluctuations measured with pick-up coils located at 4 poloidal and 64 toroidal positions. The plasma flow is also measured from the chord-integrated oxygen velocity (OV) obtained from the Doppler shift of its spectral emission line [30]. The OV velocity is a line integrated measurement and it is used only to show qualitatively the MP effect on the flow.…”