Prevention of resistive wall tearing mode major disruptions with feedback

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.