Main gas particle transport in RFP helical equilibria

Self organized helical states, where the innermost resonant tearing mode grows to large amplitude, routinely appear in all the RFP machines. With a sufficiently large dominant mode, the plasma's original magnetic axis is replaced with a new helical axis, bringing about the single helical axis (SHAx) state. A 3D magnetic topology can thereby occur in a toroidally axisymmetric device and this topology has been shown in RFX-mod (the Italian RFP) to exhibit improved energy confinement features ($\tau _E $ increases more than a factor 3) and beneficial effects on particle confinement, too. In this work, we will compare the main gas particle transport in SHAx plasmas in RFX-mod and MST. This analysis is based in part on multi-chord measurements of the density profile and modeling with the ASTRA code, accounting for the helical topology of the magnetic surfaces in the core.