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 possibility of stimulating new quasi-single helicity (QSH) regimes by allowing a small helical deformation of the magnetic boundary. In particular, QSH states based on non-resonant helicities are predicted to be more resilient to magnetic stochasticity induced by secondary modes. This has motivated a series of successful experiments in the RFX-mod device with applied magnetic perturbations (MPs). We present here recent developments in nonlinear MHD modeling of RFP, highlighting aspects which share similarities between the RFP helical regimes, the tokamak sawtoothing and the stellarator. RFP helical configurations are obtained 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 equilibrium properties, like the helical safety factor or the magnetic well profile. The width of the region 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 of magnetic field lines are diagnosed in the weakly stochastic region surrounding the conserved helical core in numerical QSH states, providing a new tool for the study of Internal Transport Barriers formation at the transition to helical regimes in RFP experiments. Similar to the tokamak configuration, the sawtoothing dynamics of RFP plasmas is mitigated with the application of helical MPs. As for the edge region, the RFP is characterized by m = 0 island chains at the q = 0 reversal surface, which play a role similar to edge islands in tokamak and stellarator plasmas.