Stability impacts from the current and pressure profile modifications within finite sized island

TL;DR

This paper investigates how current and pressure profile modifications within magnetic islands influence their stability, using a perturbed equilibrium approach to assess the effects of various profile changes on island stability in fusion plasmas.

Contribution

It provides a detailed analysis of the stability impacts of current and pressure profile modifications within finite-sized magnetic islands, advancing understanding of disruption mitigation strategies.

Findings

Positive helical current perturbations stabilize islands.

Pressure bumps stabilize small islands, while holes destabilize them.

Pressure effects are complex and depend on island size and asymmetry.

Abstract

The stability (or instability) of finite sized magnetic island could play a significant role in disruption avoidance or disruption mitigation dynamics. Especially, various current and pressure profile modifications, such as the current drive and heating caused by electron cyclotron wave, or the radiative cooling and current expulsion caused by the Shattered Pellet Injection could be applied within the island to modify its stability, thus change the ensuing dynamics. In this study, we calculate the mode structure modification caused by such profile changes within the island using the perturbed equilibrium approach, thus obtain the change of stability criterion $\gD'$ and assess the corresponding quasi-linear island stability. The positive helical current perturbation is found to always stabilize the island, while the negative one is found to do the opposite, in agreement with previous results. The pressure bump or hole within the island has a more complicated stability impact. In the small island regime, its contribution is monotonic, with pressure bump tends to stabilize the island while pressure hole destabilizes it. This effect is relatively weak, though, due to the cancellation of the pressure term's odd parity contribution in the second derivatives of the mode structure. In the large island regime, such cancellation is broken due to the island asymmetry, and the pressure contribution to stability is manifested, which is non-monotonic. The stability analysis in this paper helps to more accurately clarify the expected island response in the presence of profile modifications caused by disruption avoidance or mitigation systems.