Feedback control of $m=0$ modes in the Reversed Field eXperiment

Recent theory and simulations have highlighted the role of tearing modes with poloidal mode number $m=0$ and $m=1$ in the edge of the reversed-field pinch (RFP)\footnote{G. Spizzo, S. Cappello, A. Cravotta, D.F.Escande \textit{et al.}, Phys. Rev. Lett. \textbf{96}, 025001 (2006).}. These results show that the nonlinear superposition of these modes allows for the formation of a chain of magnetic islands, responsible for a transport decrease in the region where the toroidal field $B_{\phi}$ vanishes and reverses direction. In particular, it has been demonstrated that the reduction of the total $m=0$ island dimension (due to $m=0$ modes and the beating of the $m=1$'s) is beneficial in terms of transport. As a first step to this end, in the RFX-mod reversed-field experiment, we exploited the feedback control of the 12 toroidal sectors used for producing the toroidal field, so as to actively cancel the $m=0$ mode amplitude. Indeed, experimental results show the reduction of $m=0$ fluctuation amplitude, associated to an overall temperature increase, and a reduction and redistribution of plasma-wall interaction due to the mode phase locking.