Role of explosive instabilities in high-$\beta$ disruptions in tokamaks

TL;DR

This paper demonstrates that high-$eta$ disruptions in tokamaks involve an intrinsically explosive growth of a ballooning finger caused by an unstable mode, leading to rapid plasma ejection and magnetic field stochasticity.

Contribution

It reveals a generic mechanism for explosive growth in high-$eta$ disruptions driven by an n=1 mode with a 2/1 component, without secondary processes.

Findings

Explosive growth of a ballooning finger is demonstrated in nonlinear MHD calculations.

The formation is driven by an n=1 mode with a 2/1 component.

Rapid plasma ejection and magnetic stochasticity occur in Alfvénic time scales.

Abstract

Intrinsically explosive growth of a ballooning finger is demonstrated in nonlinear magnetohydrodynamic calculations of high-$\beta$ disruptions in tokamaks. The explosive finger is formed by an ideally unstable n=1 mode, dominated by an m/n=2/1 component. The quadrupole geometry of the 2/1 perturbed pressure field provides a generic mechanism for the formation of the initial ballooning finger and its subsequent transition from exponential to explosive growth, without relying on secondary processes. The explosive ejection of the hot plasma from the core and stochastization of the magnetic field occur in Alfv\'enic time scales, accounting for the extremely fast growth of the precursor oscillations and the rapidity of the thermal quench in some high-$\beta$ disruptions.