Prevention of resistive wall tearing mode major disruptions with feedback

TL;DR

This paper investigates how resistive wall tearing modes can lead to major disruptions in plasma devices and demonstrates that feedback control can prevent these disruptions by emulating an ideal wall, based on the $q_{75} < 2$ criterion.

Contribution

It introduces a feedback method to emulate an ideal wall, preventing major disruptions caused by RWTMs when the $q_{75} < 2$ criterion is met.

Findings

RWTMs cause major disruptions when $q_{75} < 2$.

Feedback can emulate an ideal wall and prevent disruptions.

The $q_{75} < 2$ criterion is confirmed through simulations and database analysis.

Abstract

Resistive wall tearing modes (RWTM) can cause major disruptions. A signature of RWTMs is that the rational surface is sufficiently close to the wall. For $(m,n) = (2,1)$ modes, at normalized minor radius $\rho = 0.75$, the value of $q$ is $q_{75} < 2.$ This is confirmed in simulations and theory and in a DIII-D locked mode disruption database. The $q_{75} < 2$ criterion is valid at high $\beta$ as well as at low $\beta.$ A very important feature of RWTMs is that they produce major disruptions only when the $q_{75} < 2$ criterion is satisfied. If it is not satisfied, or if the wall is ideally conducting, then the mode does not produce a major disruption, although it can produce a minor disruption. Feedback, or rotation of the mode at the wall by complex feedback, can emulate an ideal wall, preventing major disruptions. The $q_{75}$ criterion is analyzed in a linear simulations, and a simple geometric model is given.