Design of the new supporting structure for the Passive Stabilizing Shell of RFX-mod2

An upgrade of the RFX-mod experiment (RFX-mod2) is in progress, involving a major change and reconfiguration of the inner components of the machine assembly. One of the most critical components of the assembly is the Passive Stabilizing Shell (PSS), a 3 mm thick toroidal structure (Rmajor = 1995 mm and rminor = 513 mm) made of copper which must be kept as close as possible to the plasma, in order to provide the suitable stabilization of the RFP magnetic configuration. In the present machine configuration, the PSS is wrapped and fastened around the external surface of the vacuum vessel, whereas for RFX-mod2 the PSS will be enclosed within a new wider vacuum vessel to enhance the 'shell-plasma proximity'. For this reason a new supporting structure has been conceived, able to support the shell and the new first wall (an array of 72x28 graphite tiles covering the entire inner surface of the shell) with stringent electromagnetic, thermomechanical and vacuum operating conditions. The new supporting structure is composed of a frame of 72 rings made of a high-performance polymer (Torlon® PAI), which complies with the thermo-mechanical and electrical requirements, reinforced with suitable stainless steel stiffening ribs and fastening bands. Each ring is bolted to the PSS by means of 28 peculiar clamps which, on the internal surface of the shell, provide also the fastening of the first wall tiles. The paper will describe the following design aspects: numerical FEM analyses performed to verify the thermo-mechanical design of the composite system at the expected operating conditions; development of geometrical features for the optimization of the injection moulding process for the manufacture of the ring sectors; experimental tests, in particular dimensional inspections and mechanical strength tests, carried out on prototypes to assess the reliability of the design solution.