Observations of Heteroclinic Bifurcations in Resistive MHD Simulations of the Plasma Response to Resonant Magnetic Perturbations

TL;DR

This paper reports the discovery of a new class of heteroclinic bifurcations in resistive MHD simulations of spherical tokamak plasmas, highlighting the role of plasma self-organization in magnetic island dynamics.

Contribution

It identifies a novel bifurcation phenomenon in resistive MHD simulations, emphasizing the importance of plasma response in magnetic island behavior.

Findings

Heteroclinic bifurcations occur with increased magnetic perturbation.

Resistive plasma response is crucial for bifurcation development.

New hyperbolic-elliptic fixed points are formed during bifurcations.

Abstract

A new class of static magnetohydrodynamic (MHD) magnetic island bifurcations is identified in rotating spherical tokamak plasmas during single- and two-fluid resistive MHD simulations. As the magnitude of an externally applied non-axisymmetric magnetic field perturbation is increased in these simulations, the internal flux surfaces that make up a sub-set of the resonant helical magnetic islands in the plasma gradually elongate and undergo heteroclinic bifurcations. The bifurcation results in the creation of a new set of hyperbolic-elliptic fixed points as predicted by the Poincar\'e-Birkoff fixed point theorem. Field line calculations without including the resistive MHD plasma response to the applied perturbation field do not undergo this class of bifurcations indicating the importance of plasma self-organization in the bifurcation process.