Impurity screening in the RFX reversed field pinch

Despite the large input power of tens of MW, in the reversed field pinch RFX, the effective charge can be remarkably small. Z(eff) decreases with density but does not show a dependence on the plasma current up to 1 MA for a given density. It is found that due to the large total particle outflux (2 x 10(23) s(-1)) the energy per particle is kept within reasonable levels. These values, together with the carbon yield and ultimately the impurity concentrations, are comparable with the tokamak case, where typically both power input and particle outflux are proportionally lower by an order of magnitude. Experimental results and Monte Carlo simulations suggest that in RFX the particle screening is due to the short ionisation length of neutrals with a contribution of the finite Larmor radius effect.