Effect of Nonlinear Energy Transport on Neoclassical Tearing Mode Stability in Tokamak Plasmas

TL;DR

This paper studies how nonlinear changes in electron heat transport affect the stability and abrupt onset of neoclassical tearing modes in tokamak plasmas, revealing critical bifurcation behavior.

Contribution

It introduces a model linking heat transport reduction to mode stability, showing how transport nonlinearities cause abrupt bifurcations in mode onset.

Findings

Transport reduction leads to more abrupt mode onset.

A critical divergence ratio causes a discontinuous bifurcation.

Once triggered, inverse bifurcation requires significant reduction.

Abstract

An investigation is made into the effect of the reduction in anomalous perpendicular electron heat transport inside the separatrix of a magnetic island chain associated with a neoclassical tearing mode in a tokamak plasma, due to the flattening of the electron temperature profile in this region, on the overall stability of the mode. The onset of the neoclassical tearing mode is governed by the ratio of the divergences of the parallel and perpendicular electron heat fluxes in the vicinity of the island chain. By increasing the degree of transport reduction, the onset of the mode, as the divergence ratio is gradually increased, can be made more and more abrupt. Eventually, when the degree of transport reduction passes a certain critical value, the onset of the neoclassical tearing mode becomes discontinuous. In other words, when some critical value of the divergence ratio is reached, there is a sudden bifurcation to a branch of neoclassical tearing mode solutions. Moreover, once this bifurcation has been triggered, the divergence ratio must reduced by a substantial factor to trigger the inverse bifurcation.