Density limit, radiation and magnetic topology in the reversed-field pinch

In this paper we analyze the density limit in the reversed-field pinch machine RFX-mod, whose upgrades (in particular, the new feedback control system\footnote{S.Martini and the RFX team, Nucl. Fusion \textbf{47}, 783 (2007).}) have greatly ameliorated plasma-wall interaction issues. In fact, when $n/n_G < 0.35$ (with $n_G$ the Greenwald density), there is no signature of enhanced radiation outside the regions of the residual localized plasma-wall interaction. On the contrary, when $n/n_G > 0.35$ a localized enhancement of the radiation is observed, not necessarily associated to the region of maximum plasma-wall interaction. This localized radiation has the shape of a poloidal ring, and appears in correspondence to edge magnetic islands, originated from the MHD $m=0$ modes ($m$ is the poloidal mode number). Besides the local decrease of particle diffusivity $D$ associated to the magnetic islands, the presence of highly radiating rings can be related to a reduction of the turbulent edge transport\footnote{see P.Scarin, this conference.}, which takes place approximately at the same values of $n/n_G$. In this respect, the microscopic cause of the density limit could be similar to the MARFE phenomenon in Tokamaks.