Heat pulse propagation and anomalous electron heat transport measurements on the optimized stellarator W7-X

G Weir ,M Beurskens ,U Höfel ,N B Marushchenko ,J Geiger,A Dinklage ,J Alcusón ,G Fuchert,M Hirsch,S Bozhenkov ,J Schilling ,E Pasch,O Grulke ,S Äkäslompolo ,S Lazerson ,T Stange ,A Langenberg ,Matt Landreman ,E R Scott ,P Xanthopoulos ,Y Turkin ,N Pablant ,T Klinger

doi:10.1088/1741-4326/abea55

Abstract

The optimized stellarator Wendelstein 7-X (W7-X) is designed to have an approximately quasi-isodynamic magnetic configuration with reduced neoclassical transport in comparison to a classical stellarator, and turbulent transport is expected to be a significant source of anomalous heat transport across the plasma minor radius. The ion temperature gradient driven mode and the trapped electron mode (TEM) are thought to be responsible for the ion-scale turbulence in W7-X plasmas with volume averaged pressure below 1%. In this work, the electron temperature gradient driven turbulence is shown to be a good candidate for the explanation of the observed electron heat flux, in the inner plasma region where the density gradient is weak (in the outer region, a relatively stronger density gradient would drive additional TEM turbulence). The experimental electron heat transport measured during electron cyclotron resonant heating power and plasma density scans is compared to neoclassical predictions, and the stiffness in the electron heat transport measured during transient transport experiments is presented in three common magnetic configurations of W7-X. In low-⟨β⟩ plasma discharges, the stiffness in the electron heat flux, quantified by the ratio of the heat pulse to power balance diffusivity, , is measured to be less than 2, and trend downwards with increasing collisionality.

Highlights

Turbulent processes are expected to dominate the electron energy and particle transport in magnetically confined fusion devices that are optimized to have low neoclassical transport
Profile resiliency is the manifestation of stiffness in the electron heat flux, and gyrokinetic modeling indicates that stiffness in tokamak devices is driven by turbulent microinstabilities such as the electron temperature gradient (ETG) driven mode, the ion temperature gradient (ITG) driven mode and the trapped electron mode (TEM)
The electron thermal diffusivity and the stiffness in the electron heat flux have been measured in matched profile experiments and in collisionality-scans in all three magnetic configurations

Summary

Introduction

Turbulent processes are expected to dominate the electron energy and particle transport in magnetically confined fusion devices that are optimized to have low neoclassical transport. Profile resiliency is the manifestation of stiffness in the electron heat flux, and gyrokinetic modeling indicates that stiffness in tokamak devices is driven by turbulent microinstabilities such as the electron temperature gradient (ETG) driven mode, the ion temperature gradient (ITG) driven mode and the trapped electron mode (TEM) (see, for instance, [6]). These instabilities are expected to be reduced in W7-X plasmas with sufficiently large pressure. Heat pulse propagation measurements with varying electron collisionality are described in section 3 to assess the transition between TEM and ETG/ITG mode dominated electron heat transport, and the possible stabilization of TEM-driven turbulence with approximate quasi-isodynamicity

Magnetic configuration effects in Wendelstein 7-X

Matched profile experiments in three magnetic configurations

Heat pulse propagation in matched profile experiments

Stiffness measurements in collisionality scans

Findings

Summary and conclusions