Enhanced confinement with increased extent of the low magnetic shear region in tokamak plasma

TL;DR

This paper demonstrates that increasing the extent of the low magnetic shear region in tokamak plasmas significantly enhances magnetic field line confinement by forming wider internal transport barriers, as shown through Hamiltonian symplectic map models.

Contribution

It introduces a bounded symmetric tokamap model and shows how broader low-shear regions improve confinement by creating larger KAM tori barriers.

Findings

Wider low-shear regions lead to larger internal transport barriers.

Vanishing shear profiles improve magnetic confinement.

The extent of the low-shear region correlates with the size of the confinement barrier.

Abstract

The Hamiltonian representation of magnetic field lines enables one to study their confinement properties in tokamaks through the use of symplectic maps such as the symmetric tokamap and its bounded version, the latter being introduced here. In this time-independent purely magnetic framework, we observed the drastic improvement in the confinement of magnetic field lines produced by the local vanishing of the shear profile. This amounts to a non-twist condition that notably acts in the same way the safety profile being (non-strictly) monotonic or having a reversed shear. We single out the effect of the amount of flatness of the safety profile in the vicinity of its zero shear point. All other things being equal, the beneficial effect of the vanishing of the shear profile is shown to be increased if the radial extent of the low-shear region is increased. To be specific, the low-shear region induces the formation of a belt of robust KAM tori acting as an internal transport barrier whose width is all the larger as the extent of the low-shear region is broad.