Interaction between magnetic boundary and first wall recycling in the reversed field pinch

The plasma-wall interaction (PWI) in the reversed field pinch is strongly influenced by three-dimensional edge magnetic topology. This is true both in the chaotic regime, where several modes interact producing a toroidally localized bulge, and in the helical state, which helically modulates the edge profiles. In both regimes an external region with poloidal magnetic islands, toroidally distributed, is present. The density behaviour is also determined by wall recycling, since no divertor or limiters are present and therefore the deformation of the magnetic boundary brings the plasma into contact with a large portion of the wall. In RFX-mod, the shape of PWI results in a modulation of the edge flow, which at high density leads to localized accumulation through the onset of convective cells. The accumulation process is favoured by the high hydrogen retention of the graphite wall combined with high power loads (approximate to 10 MW m(-2)) which can be locally deposited on the wall. The analysis of the edge plasma behaviour allows the identification of two density thresholds: the first one corresponds to an upper limit for the spontaneous development of the helical states, the second one is related to the onset of MARFE-like poloidal radiative belts. For an effective control of the plasma density through a reduction of wall recycling, a change of the first wall from carbon to tungsten is foreseen. In preparation for such a change, tungsten laser blow off experiments have been recently performed.