The influence of magnetic turbulence on Internal Transport Barriers of Reversed Field Pinch

The onset of Internal Transport Barriers (ITBs) is routinely observed during Quasi Single Helicity States in the Reversed Field Pinch experiment RFX-mod. ITBs appear as steep electron temperature gradients that enclose thermal structures. The characteristics of such thermal structures vary considerably, so that there are both narrow thermal structures enclosed by steep gradients and large thermal structures enclosed by milder gradients. The complexity increases considering that, while large thermal structures develop on helical states with a single helical axis (SHAx), narrow thermal structures can have both a single or double magnetic axis (DAx). In this paper we characterize the behavior of thermal structure width Wth and of ITB gradients, estimated from 84-point Thomson Scattering profiles, and of their helical equilibrium. Furthermore we present data analyses that show as Wth and the ITB gradients have a strong relationship with the magnetic turbulence of secondary modes [1,2]. In particular, as long as the secondary modes decrease, the DAx and SHAx narrow thermal structures widens (Wth increases) and their gradients become increasingly steeper. However, despite the very low value of secondary modes, the SHAx wide thermal structures exhibit gradients clearly milder than the gradients of DAx and SHAx narrow cases. The Wth behavior is quantitatively analyzed introducing the Quasi Separatrix Layer concept, used in astrophysics to explain the occurrence of solar flares in the absence of separatrix. This model provides also a qualitative explanation for the peculiar behavior of gradients.