Turbulence Mechanisms of Enhanced Performance Stellarator Plasmas

P Xanthopoulos,A Alonso,H M Smith,J A Alcusón,G G Plunk,S A Bozhenkov,F Warmer,G Fuchert,C D Beidler,M N Beurskens,P Helander,Y Turkin,M J Pueschel,J Geiger,A Dinklage,J H E Proll,O Ford

doi:10.1103/physrevlett.125.075001

Abstract

We theoretically assess two mechanisms thought to be responsible for the enhanced performance observed in plasma discharges of the Wendelstein 7-X stellarator experiment fueled by pellet injection. The effects of the ambipolar radial electric field and the electron density peaking on the turbulent ion heat transport are separately evaluated using large-scale gyrokinetic simulations. The essential role of the stellarator magnetic geometry is demonstrated, by comparison with a tokamak.

Highlights

The effects of the ambipolar radial electric field and the electron density peaking on the turbulent ion heat transport are separately evaluated using large-scale gyrokinetic simulations
We focus on two phases of the discharge, see Fig. 1: (i) The electron cyclotron resonance (ECR) phase t 1⁄4 ð4 − 5Þ s, during which the effect of the pellets has vanished long after their injection into the plasma; this phase is essentially equivalent to the initial ECR-heated part of the discharge, prior to pellet injection. (ii) The pellet enhanced performance (PEP) phase t 1⁄4 ð1.65 − 1.75Þ s, taking place shortly after the pellet injections
Focusing on the radial location where the electric field reaches its most negative value, corresponding to the ion root solution of the ambipolarity relation [22], we notice that the PEP phase is characterized by both larger normalized electron density gradient a=Lne 1⁄4 ð−a=neÞdne=dr and ion temperature gradient a=LTi compared to the ECR phase

Summary

Introduction

The effects of the ambipolar radial electric field and the electron density peaking on the turbulent ion heat transport are separately evaluated using large-scale gyrokinetic simulations. Such discharges involve transient phases characterized by enhanced performance, following shortly after the injection of cryogenic hydrogen pellets into an electron cyclotron resonance (ECR)-heated plasma [5].

Results

Conclusion