Radiation and confinement properties of impurity seeded discharges in the reversed field pinch RFX

The compatibility of the reversed field pinch configuration with a highly radiative mantle at the edge has been proven by injecting neon into high density RFX plasmas: with respect to ordinary non-seeded discharges, energy confinement only marginally deteriorates, and the convection losses onto the plasma facing wall are reduced. Injection of neon into low density plasmas entails instead a significant increase of the effective charge and an increment of the ohmic input higher than the radiated power increase. The associated degradation of energy confinement tE appears not to be peculiar to impurity injection but follows a more general scaling with the heating power Pheat of the type tE µ Pheat-1. The radiation efficiency, defined as the ratio between radiated power and the impurity contribution to effective charge, is found to be 3 to 5 times higher than that in tokamaks. RITM simulations confirm the experimental findings.