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A new paradigm for RFP magnetic self-organization: results and challenges

This paper reports the most recent experimental results on quasi-single helicity (QSH) reversed field pinch (RFP) plasmas. QSH is considered a key element towards the full experimental realization of the theoretically predicted single helicity (SH) RFP. The SH RFP, where an individual resistive kink mode and its harmonics drive the dynamo electric field, is predicted to have superior confinement performance with respect to the standard multiple helicity (MH) state. Magnetic chaos is in fact strongly reduced in the SH RFP, which therefore retains all the positive features of the RFP configuration without the problems connected with the magnetic turbulence typical of the MH scenario. Data from the RFX-mod device, presented here, provide a more complete description of QSH states, indicate a positive synergy between the growth of the dominant resistive mode and the decrease in the secondary modes (with reduction of magnetic chaos and hints of confinement improvement outside the helical domain), and show a promising scaling with plasma current. Initial experiments on active control of QSH states in RFX-mod are presented.