Negotiating with magnetic self-organization in confined plasmas

Magnetically confined plasmas of fusion interest displayphenomena that find several analogies in astrophysicsand other complex systems. This is the case of toroidalpinches where magnetic self-organization molds theplasma into a peculiar helical shape when the ratio ofplasma current to toroidal magnetic flux exceeds the socalled Kruskal-Shafranov limit. In this case, a core kinkinstability ("sawtoothing" and/or "snake") tends todevelop in Tokamaks 1. Similarly, a global helical shape(so-called Quasi Single Helical, QSH, regimes) forms inReversed Field Pinch experiments (RFP)2,3, helicallymodulating the plasma up to the edge 4,5. MHD modelinghas been largely successful in capturing the basicfeatures of such a phenomenon 6,7,8, quite evading thefamous Taylor's relaxation theory for RFP 9,10. Nearlyperiodic relaxation events involving current sheetreconnection can be observed 6,9, together with amagnetic chaos healing effect when the helical states arerobustly achieved11,8. The latter effect is presently thebest candidate to explain the formation of internalelectron transport barriers2 observed in RFP helicalregimes. In particular, "hidden" magnetic field linestransport barriers have been recently detected inexperimental-like numerical simulations, which areassociated with "fine" topological structures like Cantorisets or Lagrangian Coherent Structures 12. Aftersummarizing these general features, we here discuss therecent successful MHD prediction of alternative helicalregimes, obtained by seed edge magnetic perturbationswith suitable choice of helical pitch. A first set of RFXmod experiments substantially confirms modelingpredictions 13. The new helical regimes obtained asplasma response to edge Magnetic Perturbations arepredicted to favor magnetic chaos healing in the case ofnon-resonant seeds. After first indications obtained inRFX-mod experiment 13, we expect to validate modelingpredictions concerning transport properties in themodified device RFX-mod2 starting operation in 2020.The device, characterized by reduced plasma-wall/feedback coils distance, will provide efficient control actionin order to negotiate at best with RFP helical magneticself- organization.