Progress in theoretical RFP studies: new stimulated helical regimes and similarities with tokamak and stellarator

Recent theoretical studies of the reversed-field pinch (RFP) have demonstrated the possibilityof stimulating new quasi-single helicity (QSH) regimes by allowing a small helicaldeformation of the magnetic boundary. In particular, QSH states based on non-resonanthelicities are predicted to be more resilient to magnetic stochasticity induced by secondarymodes. This has motivated a series of successful experiments in the RFX-mod device withapplied magnetic perturbations (MPs). We present here recent developments in nonlinearMHD modeling of RFP, highlighting aspects which share similarities between the RFP helicalregimes, the tokamak sawtoothing and the stellarator. RFP helical configurations areobtained by applying small helical MPs with resonant (n 7) and non-resonant (n < 7)toroidal periodicities. MPs turn out to be a powerful tool for varying the helical equilibriumproperties, like the helical safety factor or the magnetic well profile. The width of theregion of conserved magnetic surfaces turns out to be the largest for the non-resonant case.By using a method to compute Lagrangian Coherent Structures, barriers to the diffusion ofmagnetic field lines are diagnosed in the weakly stochastic region surrounding the conservedhelical core in numerical QSH states, providing a new tool for the study of Internal TransportBarriers formation at the transition to helical regimes in RFP experiments. Similarto the tokamak configuration, the sawtoothing dynamics of RFP plasmas is mitigated withthe application of helical MPs. As for the edge region, the RFP is characterized by m = 0island chains at the q = 0 reversal surface, which play a role similar to edge islands intokamak and stellarator plasmas.