Numerical studies of transport mechanisms in RFX-mod low magnetic chaos regimes

The magnetic topology of quasi-single helicity (QSH) regimes, in the reversed field pinch experiment RFX-mod, is characterized by a helical magnetic structure surrounded by partially chaotic field lines. In this paper we investigate the transport mechanisms within these helical structures, in particular, the particle diffusion due to the residual magnetic chaos. The diffusion coefficients have been estimated for different plasma temperatures, collisionality and magnetic perturbation amplitudes. In the typical high temperature QSH regimes of RFX-mod, ambipolar transport between ions and electrons is found to be ensured by the non-zero but very low level of secondary modes and by the reduced collisionality. Trapped ions give the largest contribution to particles diffusion, while those passing are almost well confined within the helical structure. In the best QSH experimental performances, ion diffusion coefficients computed by numerical simulations are not affected significantly by the residual magnetic chaos and are very close to those expected in a regime with only helical conserved surfaces.