Prevention of resistive wall tearing mode major disruptions with feedback

Resistive wall tearing modes (RWTMs) can cause major disruptions. A signature of RWTMs is that the rational surface is sufficiently close to the wall to interact with it. For (m,n)=(2,1) modes, a RWTM requires normalized minor radius of the rational surface rho(2)(q) >= 0.75, which can also be expressed as q(75) <= 2. Major disruptions can occur when the criterion is satisfied. 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 can be feedback stabilized. If the rho(2)(q) criterion 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 rho(2)(q) criterion depends weakly on the wall radius. A simple geometric model of its dependence on wall radius is given. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).