Shaping the edge radial electric field to create shearless transport barriers in tokamaks

TL;DR

This study explores how shaping the radial electric field in tokamaks influences the formation of shearless transport barriers, which can improve plasma confinement by reducing chaotic particle transport.

Contribution

It demonstrates how well-like and hill-like electric field profiles can create and enhance shearless transport barriers at the plasma edge, depending on their parameters.

Findings

Barriers become more resistant to fluctuations with increased depth or height.

Parameter space maps show conditions for barrier existence across electric field configurations.

Fractal structures at the barrier boundary indicate quasi-integrable Hamiltonian dynamics.

Abstract

In tokamak-confined plasmas, particle transport can be reduced by modifying the radial electric field. In this paper, we investigate the influence of both a well-like and a hill-like shaped radial electric field profile on the creation of shearless transport barriers (STBs) at the plasma edge, which are a type of barrier that can prevent chaotic transport and are related to the presence of extreme values in the rotation number profile. For that, we apply an ExB drift model to describe test particle orbits in large aspect-ratio tokamaks. We show how these barriers depend on the electrostatic fluctuation amplitudes and on the width and depth (height) of the radial electric field well-like (hill-like) profile. We find that, as the depth (height) increases, the STB at the plasma edge becomes more resistant to fluctuations, enabling access to an improved confinement regime that prevents chaotic transport. We also present parameter spaces with the radial electric field parameters, indicating the STB existence for several electric field configurations at the plasma edge, for which we obtain a fractal structure at the barrier/non-barrier frontier, typical of quasi-integrable Hamiltonian systems.