Modulated heat pulse propagation and partial transport barriers in chaotic magnetic fields

TL;DR

This study uses advanced numerical simulations to show that in chaotic magnetic fields, partial transport barriers such as islands and Cantori influence heat pulse propagation, aligning with experimental observations.

Contribution

It introduces a Fourier-based Lagrangian-Green's function numerical method to accurately model heat pulse propagation in chaotic magnetic fields, revealing the role of partial barriers.

Findings

Chaotic magnetic fields with intermediate stochasticity exhibit transport barriers.

High-order islands and Cantori act as partial barriers to heat pulse propagation.

Magnetic island points significantly influence heat pulse amplitude distribution.

Abstract

Direct numerical simulations of the time dependent parallel heat transport equation modeling heat pulses driven by power modulation in 3-dimensional chaotic magnetic fields are presented. The numerical method is based on the Fourier formulation of a Lagrangian-Green's function method that provides an accurate and efficient technique for the solution of the parallel heat transport equation in the presence of harmonic power modulation. The numerical results presented provide conclusive evidence that even in the absence of magnetic flux surfaces, chaotic magnetic field configurations with intermediate levels of stochasticity exhibit transport barriers to modulated heat pulse propagation. In particular, high-order islands and remnants of destroyed flux surfaces (Cantori) act as partial barriers that slow down or even stop the propagation of heat waves at places where the magnetic field connection length exhibits a strong gradient. Results on modulated heat pulse propagation in fully stochastic fields and across magnetic islands are also presented. In qualitative agreement with recent experiments in LHD and DIII-D, it is shown that the elliptic (O) and hyperbolic (X) points of magnetic islands have a direct impact on the spatio-temporal dependence of the amplitude of modulated heat pulses.