# MST Resistive Wall Tearing Mode Simulations

## TL;DR

This paper investigates resistive wall tearing modes and resistive wall modes in the MST tokamak, showing they could cause disruptions but with longer thermal quench times than in other devices, aligning with experimental observations.

## Contribution

It presents simulations demonstrating the stability characteristics of RWTMs and RWMs in MST, highlighting their potential role in disruptions and the transition between these modes.

## Key findings

- MST is unstable to RWTMs and RWMs.
- Predicted thermal quench times are longer than in other tokamaks.
- Disruptions are likely caused by RWTMs and RWMs.

## Abstract

The Madison Symmetric Torus (MST) is a toroidal device that, when operated as a tokamak, is resistant to disruptions. Unlike most tokamaks, the MST plasma is surrounded by a close fitting highly conducting wall, with a resistive wall penetration time two orders of magnitude longer than in JET or DIII-D, and three times longer than in ITER. The MST can operate with edge q_a < 2, unlike standard tokamaks. Simulations presented here indicate that the MST is unstable to resistive wall tearing modes (RWTMs) and resistive wall modes (RWMs). They could in principle cause disruptions, but the predicted thermal quench time is much longer than the experimental pulse time. If the MST thermal quench time were comparable to measurements in JET and DIII-D, theory and simulations predict that disruptions would have been observed in MST. This is consistent with the modeling herein, predicting that disruptions are caused by RWTMs and RWMs. In the low q_a regime of MST, the RWTM asymptotically satisfies the RWM dispersion relation. The transition from RWTM to RWM occurs smoothly at q_a = m/n, where m,n are poloidal and toroidal mode numbers.