Explosive eruption cycles in a rotating Z-pinch

TL;DR

This paper models the cyclic formation and collapse of an MHD pedestal in a Z-pinch, revealing a repeating process similar to ELM cycles in fusion devices, driven by metastability and energy optimization.

Contribution

It introduces a first-principles approach to predict the energy and plasma ejection in Z-pinch MHD pedestal cycles using combinatorial optimization.

Findings

The MHD pedestal is metastable and undergoes cyclic collapse and rebuild.

Energy ejection during collapse can be predicted from first principles.

The cycle resembles ELM behavior in tokamak fusion devices.

Abstract

A transonic shear flow directed along magnetic field lines can linearly stabilize a steep pressure gradient in a confined magnetohydrodynamic (MHD) plasma. In Z-pinch geometry, we show that, like the edge pedestal in tokamak devices, this transport barrier -- which we call the ``MHD pedestal'' -- is metastable, i.e., unstable to finite-amplitude displacements of flux tubes. We simulate the slow formation of an MHD pedestal in a heated and sheared Z-pinch, which collapses on reaching a critical height, expelling an order-unity fraction of the confined thermal energy. The MHD pedestal then rebuilds and the process repeats, in a manner analogous to the ELM cycle seen in fusion experiments. We show that the available energy of the metastable equilibrium, and the most energetically favorable amount of ejected plasma, can be calculated from first principles via combinatorial optimization of flux-tube interchanges.