Locked mode disruptions in DIII-D and application to ITER

TL;DR

This paper demonstrates that resistive wall tearing modes (RWTM) cause thermal quenches in tokamaks, with implications for disruption mitigation and safety in ITER, based on data from DIII-D, theory, and simulations.

Contribution

It identifies RWTM as the primary cause of thermal quenches in locked mode disruptions and analyzes their behavior and impact in ITER.

Findings

RWTMs grow to cause complete thermal quenches

RWTM growth time correlates with TQ duration

ITER disruptions may be less severe than previously thought

Abstract

Disruptions are a serious problem in tokamaks, in which thermal and magnetic energy confinement is lost. This paper uses data from the DIII-D experiment, theory, and simulations to demonstrate that resistive wall tearing modes (RWTM) produce the thermal quench (TQ) in a typical locked mode shot. Analysis of the linear RWTM dispersion relation shows the parameter dependence of the growth rate, particularly on the resistive wall time. Linear simulations of the locked mode equilibrium show that it is unstable with a resistive wall, and stable with an ideally conducting wall. Nonlinear simulations demonstrate that the RWTM grows to sufficient amplitude to cause a complete thermal quench. The RWTM growth time is proportional to the thermal quench time. The nonlinearly saturated RWTM magnetic perturbation amplitude agrees with experimental measurements. The onset condition is that the q = 2 rational surface is sufficiently close to the resistive wall. Collectively, this identifies the RWTM as the cause of the TQ. In ITER, RWTMs will produce long TQ times compared to present-day experiments. ITER disruptions may be significantly more benign than previously predicted.