Models of Tokamak Disruptions

TL;DR

This paper models tokamak disruptions caused by resistive wall tearing modes, showing how wall conductivity influences disruption severity and providing insights relevant for future devices like ITER.

Contribution

It introduces a combined linear and nonlinear model of RWTMs, revealing the critical role of wall resistivity and edge safety factor in disruption dynamics.

Findings

Ideal walls lead to minor disruptions with no thermal quench.

Resistive walls cause major disruptions with thermal quench at q_a ≤ 3.

Disruption severity sharply transitions at q_a = 3.

Abstract

Disruptions are a serious issue in tokamaks. In a disruption, the thermal energy is lost by means of an instability which could be a resistive wall tearing mode (RWTM). During precursors to a disruption, the plasma edge region cools, causing the current to contract. Model sequences of contracted current equilibria are given, and their stability is calculated. A linear stability study shows that there is a maximum value of edge $q_a \approx 3$ for RWTMs to occur. This also implies a minimum rational surface radius normalized to plasma radius from RWTMs to be unstable. Nonlinear simulations are performed using a similar model sequence derived from an equilibrium reconstruction. There is a striking difference in the results, depending on whether the wall is ideal or resistive. With an ideal wall, the perturbations saturate at moderate amplitude, causing a minor disruption without a thermal quench. With a resistive wall, there is a major disruption with a thermal quench, if the edge $q_a \le 3.$ There is a sharp transition in nonlinear behavior at $q_a = 3.$ This is consistent with the linear model and with experiments. If disruptions are caused by RWTMs, then devices with highly conducting walls, such as the International Tokamak Experimental Reactor (ITER) will experience much milder, tolerable, disruptions than presently predicted.