Dynamics of an n = 1 explosive instability and its role in high-β disruptions

Abstract

Some low-n kink-ballooning modes not far from marginal stability are shown to exhibit a bifurcation between two very distinct nonlinear paths that depends sensitively on the background transport levels and linear perturbation amplitudes. The particular instability studied in this work is an mode dominated by an component. It is driven by a large pressure gradient in weak magnetic shear and can appear in various high- hybrid/advanced scenarios. Here it is investigated in reversed shear equilibria where the region around the safety-factor minimum provides the low-shear conditions necessary for instability. For a certain range of parameters, a relatively benign path results in a saturated ‘long-lived mode’ (LLM) that causes little confinement degradation. At the other extreme, the quadrupole geometry of the perturbed pressure field evolves into a ballooning finger that subsequently transitions from exponential to explosive growth. The finger eventually leads to a fast disruption with precursors too short for any mitigation effort. Interestingly, the saturated LLM state is found to be metastable; it also can be driven explosively unstable by finite-amplitude perturbations. Similarities to some high-β disruptions in reversed-shear discharges are discussed.