Boundary plasma physics in RFX-mod: Radial electric field and transport topology

The high current regimes of the RFX-mod (R=2m, a=0.46m) reversed field pinch experiment are characterized by the presence of a dominant m/n=1/−7 kink-type mode that moulds the plasma in a 3D topology. This paper describes the strong influence of such global helical state on the scrape-off layer, which becomes helically modulated itself. In particular, the link between the edge magnetic topology and the temporal variation of locally measured quantities (like particle influx, electron density and temperature, toroidal flow and floating potential) is analyzed for different plasma density. The edge radial electric field Er exhibits a periodicity similar to the dominant m/n=1/−7 mode, being more negative in correspondence to the outward magnetic shift. Indications are obtained about the ambipolar nature of the edge radial electric field. This suggests a link with the mechanism of density limit in the RFP, where Er is determined by different electron mobility modulated by m/n=0/1 islands.