Connections between RFP, Tokamak and Stellarator physics as highlighted in 3D nonlinear MHD modelling

The understanding of fusion plasmas benefits from investigating the same phenomena in different magnetic configurations such as the tokamak, the stellarator, and the reversed - field pinch (RFP) [1 - 4] . Here, the same approach is applied i n the framework of nonlinear 3D MHD modelling . We first focus on sawtoothing, i.e. , the quasiperiodic magnetic relaxation of RFP and tokamak plasma s . Sawtooth ing manifests itself together with magnetic reconnection events , and with the formation of current sheets and of dynamo electric fields . Such processes are relevant to laboratory and astrophysical plasmas as well. In both tokamak and RFP configurations , sawtoothing is quenched leading to long - l a sting helical states whenever visco - resistive dissipation is increased [ 5 ,6 ] or small helical mag netic perturbations (MPs) are applied [ 7 ,8 ]. A similar effect of MPs has been observed in the RFX - mod device [ 9 ]. We secondly consider the topological properties of partially ergodic magnetic field s , which characterize helical states in the RFP [7] . Barriers to the stochastic diffusion of field lines (the so - called cantori also studied in the tokamak configuration [10] ) are diagnosed in the weakly stochastic core of the helical RFP [ 1 1 ] , which could explain experimentally observed internal transport barriers. C hains of magnetic island s naturally occur at the RFP edge [1 2 ] . Such chains may be exploited to realize a so - called island divertor, which may alleviate plasma - wall interaction s like in the stellarator [3]. Numerical results are provided by two MHD codes , successfully benc hmarked against one another [ 6 ].