Study of the interplay between magnetic shear and resonances using Hamiltonian models for the magnetic field lines

TL;DR

This paper investigates how low magnetic shear influences magnetic confinement in fusion devices, showing that it can both enhance confinement away from resonances and facilitate transport barrier breakdown near resonances, using Hamiltonian models.

Contribution

It provides a unified Hamiltonian framework to analyze the dual effects of low magnetic shear on confinement and resonance behavior in magnetic fusion devices.

Findings

Low shear enhances confinement away from resonances.

Resonances can lower thresholds for transport barrier collapse.

The approach assesses robustness of magnetic states against perturbations.

Abstract

The issue of magnetic confinement in magnetic fusion devices is addressed within a purely magnetic approach. Using some Hamiltonian models for the magnetic field lines, the dual impact of low magnetic shear is shown in a unified way. Away from resonances, it induces a drastic enhancement of magnetic confinement that favors robust internal transport barriers (ITBs) and stochastic transport reduction. When low-shear occurs for values of the winding of the magnetic field lines close to low-order rationals, the amplitude thresholds of the resonant modes that break internal transport barriers by allowing a radial stochastic transport of the magnetic field lines may be quite low. The approach can be applied to assess the robustness versus magnetic perturbations of general (almost) integrable magnetic steady states, including non-axisymmetric ones such as the important single helicity steady states. This analysis puts a constraint on the tolerable mode amplitudes compatible with ITBs and may be proposed as a possible explanation of diverse experimental and numerical signatures of their collapses.