Helical plasma-wall interaction in the RFX reversed-field pinch: toroidal effects, localization and role of sidebands

The magnetic topology of the stochastic edge of a helical reversed-field pinch in the quasisingle helicity state (QSH)\footnote{R. Lorenzini et al., Nature Phys., \textbf{5}, 570574, (2009).}, which is characterized by a dominant tearing mode with helicity m/n=1/7, shows to be deeply influenced by higher harmonics (m±1)/n, with the same n=7, due to the toroidal coupling of the dominant mode with the Shafranov shift. Recent analyses\footnote{M. Agostini et al., Nucl. Fusion, \textbf{57}, 076033, (2017).} show that even a modest amount (~5~mT total) of higher 77 in the modified RFX-mod2 experiment could result in a dramatic reduction of PWI in this device.