Internal transport barrier triggered by neoclassical transport in W7-AS

The three-dimensional magnetic configuration of a stellarator offers two sp ecific mechanisms for a transition to improved particle and energy confinem ent. One route goes through the so-called electron-root confinement regime, which leads to a reduction of neoclassical transport via strong radial ele ctric fields. In this Letter evidence for a second route is presented. It o pens due to the layer of a strongly varying radial electric field which is present in the transitional region from neoclassical electron to ion-root c onfinement. This type of improvement acts on turbulent transport.