Real frequency tearing layers with parallel dynamics and the effect on locking and resistive wall modes

TL;DR

This paper investigates how real frequency tearing modes, influenced by parallel dynamics and pressure effects, impact plasma locking and resistive wall mode stability, revealing new effects in simplified models that could affect plasma confinement.

Contribution

It derives a dispersion relation for tearing modes with pressure and curvature effects, demonstrating the occurrence of Glasser-like effects in simplified models and their influence on mode locking and stability.

Findings

Real frequency tearing modes affect locking and mode stability.

Glasser-like effects occur in simplified models with pressure and curvature.

Destabilization thresholds for resistive wall modes are significantly lowered.

Abstract

Tearing modes with real frequencies in the plasma frame (i.e. in addition to the Doppler shift due to $E\times B$ rotation) are of potential importance because of their effect on the locking process. In particular, it has recently been shown [J. M. Finn, A. J. Cole, and D. P. Brennan, Phys. Plasmas (Letters), 22:120701, 2015] that the Maxwell torque on the plasma in the presence of an applied error field is modified significantly for tearing modes having real frequencies near marginal stability. In addition, it is known [J. M. Finn and R. A. Gerwin, Phys. Plasmas, 3:2344, 1996] that resistive wall tearing modes can be destabilized below their no-wall limits by rotation, if the tearing modes have real frequencies near marginal stability. In this paper we first derive the tearing mode dispersion relation with pressure gradient, field line curvature and parallel dynamics in the resistive-inertial (RI) regime, neglecting the divergence of the $E\times B$ drift and perpendicular resistivity. The results show that the usual Glasser effect, a toroidal effect which involves real frequencies, occurs in this simplified model, which ignores perpendicular resistivity and the divergence of the $E\times B$ drift. We also find, using a similar simple model, the surprising result that in the viscoresistive regime with pressure gradient, favorable curvature due to toroidal effects, and parallel dynamics, a similar Glasser-like effect is found. We show that in both regimes the existence of tearing modes with complex frequencies is related to nearby electrostatic resistive interchange modes with complex frequencies. We discuss the effect on locking to an error field and the significant lowering of the threshold for destabilization of resistive wall tearing modes, which can be much more pronounced than the weak effect observed for RI tearing modes without pressure-curvature drive in Finn and Gerwin, 1996.