The new vessel complex for the RFX-mod2 experiment: An effective synergy between fusion research and technological development

The RFX-mod experiment (formerly RFX [1]) is the largest Reversed Field Pinch [2] device in operation, that proved the feasibility of active stabilization of MHD instabilities (Resistive Wall Modes) [3], by enclosing the plasma in a combination of a passive stabilizing shell and a real-time controlled network of saddle coils [4], as originally conceived by J.D. Lawson [5] and later proposed by C.M. Bishop [6]. The core of the experiment was the toroidal vacuum vessel (Inconel 625, Rmajor = 2.0 m, rminor = 0.5 m, thickness = 30 mm), surrounded by a Copper shell (3 mm thick) for the passive stabilization of the MHD instabilities, both enclosed in a toroidal support structure (AISI 304 L, 47 mm thick) embedding a set of 4 × 48 saddle coils for the active MHD control (Fig. 1). The flexibility of the RFX-mod device allowed exploring magnetic configurations at different levels of the safety factor [7]. In RFP regimes, especially at high plasma currents, transitions to improved confinement helical states [8], similar to theoretical and numerical predictions [9], have been observed and characterized. Thanks to active control, stable very-low q (edge q<2) ohmic tokamak discharges have been routinely obtained [10]; moreover, ultra-low q regimes have been studied [8]. H-mode in tokamak plasmas have been obtained by means of a polarized insertable electrode [11]. The properties of RFP plasmas in RFX-mod have been found to be influenced in several ways by the residual MHD instabilities (Tearing Modes), whose amplitude and phase non-linear dynamics are strongly influenced by the characteristics of the toroidal complex containing the plasma. In particular, the very high resistivity of the Inconel vacuum vessel (actually the highest among all RFP devices) was such that Tearing Modes were locked to the wall in all plasma current regimes explored by RFX. RFX-mod active control allowed mitigating the localized interaction due the bulging induced by wall locking of tearing modes and very low plasma current campaigns (Ip<150kA) revealed spontaneous fast rotating tearing modes regimes. On the other hand, the high proximity of the vessel plays an important role in the very-low q ohmic tokamak operations [8]. Having identified the limitations posed by its toroidal complex [12], a substantial modification, of the RFX experiment has been proposed, named RFX-mod2 being the second major modification since its original design. The implementation of the proposed machine modification, involving the components of the whole vessel complex (Fig. 1), has been developed in virtue of an industrial innovation project co-funded by an Italian local authority (Regione Veneto) in the framework of the 2014-2020 European Regional Development Fund. The project, aimed at the development of technologies and innovation of industrial processes for the manufacturing of equipment for energy and environment, has been carried out in partnership between Consorzio RFX (research institution in charge of the conceptual design) and three manufacturing industries with specific competences necessary for the development of the detailed design: o Vacuum vessel and UHV components manufacturing processes (Zanon Pressure Equipment srl, now Brembana & Rolle spa). o Material surface treatments (Alca Technology srl). o Metal additive manufacturing (Sisma spa).