Plasma–wall interactions and radial electric fields in the reversed field pinch RFX

The radial electric field Er at the edge of the RFX plasma changes sign, from negative (inward) to positive, when an m=0 perturbation is externally applied. Similar behaviour had been previously observed in presence of wall locked (m=1) modes, whereby the much stronger plasma–wall interaction made data interpretation more complex. The findings are compatible with the presence of an m=0, n=1 magnetic island. While in ordinary situations the edge losses of RFX are interpreted as dominated by finite Larmor radius ion effects, inside the island associated with the external perturbation the diffusion of the electrons is enhanced and exceeds the ion loss flux, thus reversing Er.