Dynamics of an n=1 explosive instability and its role in high-beta 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 n = 1 mode dominated by an m/n = 2/1 component. It is driven by a large pressure gradient in weak magnetic shear and can appear in various high-beta, 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 2/1 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 finiteamplitude perturbations. Similarities to some high-beta disruptions in reversed-shear discharges are discussed.