An Improved Neoclassical Drift-Magnetohydrodynamical Fluid Model of Helical Magnetic Island Equilibria in Tokamak Plasmas

TL;DR

This paper presents an improved neoclassical drift-MHD model to analyze the impact of ion polarization currents on the stability of magnetic islands in tokamak plasmas, considering different flow regimes and island states.

Contribution

It introduces a self-consistent calculation of pressure and electric potential profiles in a four-field drift-MHD model, accounting for various flow damping regimes and island interactions.

Findings

Polarization current is generally stabilizing or marginally stabilizing.

Stability depends on the ion temperature to density gradient ratio (eta_i).

Different effects observed for freely rotating versus locked magnetic islands.

Abstract

The effect of the perturbed ion polarization current on the stability of neoclassical tearing modes is calculated using an improved, neoclassical, four-field, drift-MHD model. The calculation involves the self-consistent determination of the pressure and scalar electric potential profiles in the vicinity of the associated magnetic island chain, which allows the chain's propagation velocity to be fixed. Two regimes are considered. First, a regime in which neoclassical ion poloidal flow damping is not strong enough to enhance the magnitude of the polarization current (relative to that found in slab geometry). Second, a regime in which neoclassical ion poloidal flow damping is strong enough to significantly enhance the magnitude of the polarization current. In both regimes, two types of solution are considered. First, a freely rotating solution (i.e., an island chain that is not interacting with a static, resonant, magnetic perturbation). Second, a locked solution (i.e., an island chain that has been brought to rest in the laboratory frame via interaction with a static, resonant, magnetic perturbation). In all cases, the polarization current is found to be either always stabilizing, or stabilizing provided that eta_i = d ln T_i/d ln n_e does not exceed some threshold value. In certain ranges of eta_i, the polarization current is found to have have a stabilizing effect on a freely rotating island, but a destabilizing effect on a corresponding locked island.