Resistive wall mode and neoclassical tearing mode coupling in rotating tokamak plasmas

TL;DR

This paper models the interaction between resistive wall modes and neoclassical tearing modes in rotating tokamak plasmas, revealing a coupled mode that can trigger NTMs without a threshold, and how rotation can mitigate this effect.

Contribution

It introduces a coupled RWM-NTM model in rotating tokamak plasmas, showing mode interactions and the impact of plasma rotation on mode stability.

Findings

Coupled RWM-NTM mode accelerates NTM growth without a threshold.

Presence of a resistive wall influences mode dynamics.

Increased rotation can suppress mode coupling effects.

Abstract

A model system of equations has been derived to describe a toroidally rotating tokamak plasma, unstable to Resistive Wall Modes (RWMs) and metastable to Neoclassical Tearing Modes (NTMs), using a linear RWM model and a nonlinear NTM model. If no wall is present, the NTM growth shows the typical threshold/saturation island widths, whereas a linearly unstable kink mode grows exponentially in this model plasma system. When a resistive wall is present, the growth of the linearly unstable RWM is accelerated by an unstable island: a form of coupled RWM-NTM mode. Crucially, this coupled system has no threshold island width, giving the impression of a triggerless NTM, observed in high beta tokamak discharges. Increasing plasma rotation at the island location can mitigate its growth, decoupling the modes to yield a conventional RWM with no threshold width.