Eliminating Tokamak Disruptions with Feedback

TL;DR

This paper investigates how feedback control and plasma shaping can prevent disruptions caused by resistive wall tearing modes in tokamaks, with implications for ITER disruption mitigation.

Contribution

It demonstrates that active feedback and elongation control can suppress RWTMs and related disruptions, supported by simulations and experimental data from DIII-D and NSTX.

Findings

Feedback prevents RWTM disruptions in simulations.

Elongation influences the critical internal inductance.

Feedback stabilizes high-beta RWTMs in experiments.

Abstract

Many disruptions are caused by resistive wall tearing modes (RWTM). A database of DIII-D locked mode disruptions provides two main disruption criteria, which are shown to be signatures of RWTMs. The first is that the q = 2 rational surface must be sufficiently close the resistive wall surrounding the plasma to interact with it. If active feedback is used, this implies that RWTMs can be prevented from causing major disruptions. This is demonstrated in simulations. The second criterion is that the current profile is sufficiently peaked. This is caused by edge cooling, such as by impurity radiation and turbulence, which suppress edge current and temperature. This implies the disruptions are not caused by neoclassical tearing modes (NTM), because the bootstrap current is also suppressed. The dependence of the critical internal inductance on elongation is given, which suggests that elongation might be used as an actuator to prevent disruptions. At high $\beta,$ resistive wall modes (RWM) can be stabilized with feedback. Feedback also stabilizes high $\beta$ RWTMs, as shown in NSTX data and in simulations. These results suggest that RWTM disruptions in ITER might be prevented using the resonant magnetic perturbation (RMP) coils.