Electron energy transport in the presence of rational surfaces in the Wendelstein 7-AS stellarator

Abstract

Electron energy transport in the Wendelstein 7-AS stellarator exhibits a strong sensitivity to the boundary value of the rotational transform when the magnetic shear is low. Pronounced confinement maxima close to the low order rational t values correlate with a low density of other rational numbers in these favourable t ranges. The t dependence is lost if magnetic shear is increased by large plasma currents. The hypothesis is proposed that, at rational flux surfaces, electron energy transport is increased above the already anomalous level and that magnetic shear reduces the additional enhancement. An empirical model that incorporates these features into a simple expression for the electron heat conductivity can reproduce the experimental dependences over a wide range of external control parameters, e.g. the rotational transform and the net plasma current. Except for the explicit shear dependence, this model bears a remarkable similarity to the RTP model, which successfully describes the electron transport barriers near low order rational q surfaces in RTP and other tokamaks.