An Enhanced Nonlinear Critical Gradient for Electron Turbulent Transport due to Reversed Magnetic Shear

TL;DR

This paper demonstrates through nonlinear gyrokinetic simulations that reversed magnetic shear significantly enhances the critical gradient for electron turbulence, aiding in the formation of electron internal transport barriers in fusion devices.

Contribution

It introduces the first nonlinear gyrokinetic simulation results showing the impact of reversed magnetic shear on electron turbulence and transport barriers.

Findings

Reversed magnetic shear increases the nonlinear critical gradient to three times the linear value.

Nonlinearly driven off-midplane radial streamers are observed in turbulence.

Magnetic shear effectively triggers and sustains electron internal transport barriers.

Abstract

The first nonlinear gyrokinetic simulations of electron internal transport barriers (e-ITBs) in the National Spherical Torus Experiment show that reversed magnetic shear can suppress thermal transport by increasing the nonlinear critical gradient for electron-temperature-gradient-driven turbulence to three times its linear critical value. An interesting feature of this turbulence is nonlinearly driven off-midplane radial streamers. This work reinforces the experimental observation that magnetic shear is likely an effective way of triggering and sustaining e-ITBs in magnetic fusion devices.