Vanishing neoclassical viscosity and physics of the shear layer in stellarators

TL;DR

This paper uses first-principles simulations of the drift kinetic equation to explain the formation of the shear layer in stellarators, revealing how neoclassical viscosity vanishes at a critical density, enabling turbulence and zonal flows.

Contribution

It provides a fundamental, first-principles explanation for the shear layer formation and the vanishing of neoclassical viscosity in stellarators, validated against experimental data.

Findings

Neoclassical viscosity approaches zero at a critical density.

The transition to the shear layer is accurately captured by the model.

Turbulence and zonal flows emerge near the transition.

Abstract

The drift kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows to simulate density ramp-up experiments and describe from first principles the formation and physics of the radial electric field shear layer. The main features of the transition are perfectly captured by the calculation, and good quantitative agreement is also found. The results presented here, that should be valid for other non-quasisymmetric stellarators, provide a fundamental explanation for a wealth of experimental observations connected to the shear layer emergence in TJ-II. The key quantity is the neoclassical viscosity, which is shown to go smoothly to zero when the critical density is approached from below. This makes it possible for turbulence-related phenomena, and particularly zonal flows, to arise in the neighborhood of the transition.