Design of the RFX-mod2 First Wall

RFX-mod2 is the last upgrade of the reversed field pinch machine operated at Consorzio RFX. A significant modification consists of the replacement of the first wall tiles, proposed as a key factor for the improvement of the gas density with the reduction of the hydrogen retention, and designed in coherence with the magnetic front-end modification that foresees the tiles supported by the existing MHD passive stabilising shell. The main choices in the design of the new first wall tiles are the polycrystalline graphite as bulk material, the use of the existing fixing keys with fastening bayonets, and the tile width that shall be less than the diameter of the larger port holes to allow remote handling operations for the maintenance of the first wall; the latter fix the number of tiles to 2016 as in the original configuration. The expected decrease of the plasma-wall interaction determined the first wall design with expected power densities up to 50 MW/m2 considering the deformation of the last magnetic surface in both reversed field pinch and tokamak configurations. At the other side, the need of sensor integration and shielding of the passive stabilising shell from the plasma addressed the tile thickness and surface extension. Local prominences have been modelled on tiles, based on visual inspections of actual surfaces after previous operations, in order to limit the plasma in regions far from openings and supporting structures. The tile resistant section has been increased coherently with all the interfaces and constraints, so decreasing the maximum stress at 3.5 MPa calculated from finite element analysis that simulates the operating condition. This low stress level together with a measurement of the experimental loads during next RFX-mod2 operations could qualify the use of extruded graphite for a further first wall change in the future. Indeed, extruded graphite is considered attractive given its high directional thermal diffusivity (about 50% better then polycrystalline graphite) to enhance the heat transmission and so improving the gas density control, and the low stress induced may allow this mechanically less performing grade of graphite.