Damping of radial electric field fluctuations in the TJ-II stellarator

TL;DR

This paper uses drift kinetic equations to simulate the formation of radial electric field shear in TJ-II stellarator plasmas, revealing conditions where zonal flows are undamped and exhibit oscillatory relaxation near the electron-ion root transition.

Contribution

It provides a first-principles simulation of radial electric field shear formation and zonal flow dynamics in stellarators near the transition from electron to ion root.

Findings

Zonal flows are undamped near the transition, facilitating their experimental observation.

Collisionless zonal flow relaxation shows oscillatory behavior consistent with experiments.

The study links neoclassical physics to the transition dynamics in stellarator plasmas.

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 to describe from first principles the formation and physics of the radial electric field shear, which is associated to the transition from electron to ion root. We show that the range of frequencies of plasma potential fluctuations in which zonal flows are experimentally observed is neoclassically undamped in a neighborhood of the transition. This makes the electron root regime of stellarators, close to the transition to ion root, a propitious regime for the study of zonal-flow evolution. We present simulations of collisionless relaxation of zonal flows, in the sense of the Rosenbluth and Hinton test, that show an oscillatory behaviour in qualitative agreement with the experiment close to the transition.