Abstract
Electron energy transport in the Wendelstein 7-AS stellaratorexhibits a strong sensitivity to the boundary value of the rotationaltransform when the magnetic shear is low. Pronounced confinementmaxima close to the low order rational values correlate with a lowdensity of other rational numbers in these favourable ranges. The dependence is lost if magnetic shear is increased by largeplasma currents. The hypothesis is proposed that, at rational flux surfaces, electronenergy transport is increased above the already anomalous level andthat magnetic shear reduces the additional enhancement. An empiricalmodel that incorporates these features into a simple expression forthe electron heat conductivity can reproduce the experimentaldependences over a wide range of external control parameters,e.g. the rotational transform and the net plasma current. Except forthe explicit shear dependence, this model bears a remarkablesimilarity to the RTP model, which successfully describes the electron transport barriers near low order rational q surfaces in RTP and other tokamaks.
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