A substantial modification of the toroidal complex of the RFX experiment, named RFX-mod2, is currently under completion, involving the whole core system of the machine and in particular the vacuum vessel, the entire plasma facing components and a wide set of in-vessel diagnostic systems [1, 2]. Novel technological solutions have been developed to satisfy challenging requirements in terms of electrical insulation and vacuum compatibility of in-vessel components and stringent geometrical constraints to comply with interfaces to existing machine components, in particular external coils and diagnostic systems. Such a development has been performed in collaboration with local industries in the framework of an industrial innovation project co-funded by Regione Veneto (POR-FESR 2014-2020, Regional Operational Program for the European Regional Development Fund). More recently the RFX experiment, being classified as a national high priority Research Infrastructure, has been awarded with additional funds in the framework of the Italian 'National Recovery and Resilience Plan' (Next Generation EU) for further improvements on plasma diagnostics and auxiliary plants, which are presently in progress. This contribution will present an overview of the technological solutions implemented and tested for the realization of the new components integrated in the RFX-mod2 machine complex. Moreover, a summary of the new systems under development and an update of the installation phase, planned to be completed by the end of 2024, will be reported.

The RFX-mod2 device, the upgraded version of the previous RFX-mod with a modified magnetic boundary, is presently under realization and will start to be operated in 2024. Significant upgrades of the diagnostic capabilities have been proposed and are under development. These include a largely increased number of in-vessel magnetic and electrostatic sensors, a new fast reciprocating manipulator for the exploration of the edge plasma in a wide range of experimental conditions, the improved Thomson scattering and soft X-ray diagnostics system for a detailed determination of the behavior of the electron temperature profile, new dedicated systems for the space and time resolved determination of x-ray spectra and neutron rate, a diagnostics made on reflectometric analysis for real-time determination of plasma position, two diagnostics devoted to the imaging of light impurities and influxes behavior along with arrays of Halo current sensors. These diagnostic upgrades will be accompanied by a significant effort on the control the electron density by means of proper treatment of plasma facing components with in-vessel fixed electrodes distributed along the machine. The described advancements will allow a deeper understanding of physics phenomena in the wide variety of magnetic configurations, including the Tokamak, the Reversed-Field Pinch and the Ultra-low q, which can be produced in RFX-mod2 thanks to its flexibility and unique MHD control capabilities.

Plasma position reflectometry (PPR) is a microwave radar technique that aims to provide information on the plasma column position and shape by monitoring the edge density profiles at several poloidal positions in Tokamaks. The PPR in-vessel diagnostic equipment is compact, non-invasive and intrinsically resistant to neutron radiation. For this reason, after some initial tests on ASDEX-Upgrade, a partial PPR system is being designed on DTT in view of DEMO, where long pulses and high neutron fluxes are likely to jeopardize the long-term reliability of the internal magnetic sensors. On RFX-mod2 (R=2.0 m, a=0.49 m), the upgraded version of the previous RFX-mod device, a simplified PPR scheme, consisting of four bistatic reflectometric units, has been conceived to test some of the issues related to a plasma position control on a fusion reactor. However, its integration in the machine had required different innovative solutions. This contribution is focused on the technological aspects linked, in particular, to the installation of the antenna pair and the waveguide system in the high field side section of RFX-mod2. Waveguides, insulated through the application of a ZrO painting, will be routed in between the vacuum vessel and the conductive shell to a vertical port. The severe constraints in terms of physical space available guided the antennas design: a hoghorn antenna model was first numerically modeled and, due to the complex geometry, produced through metal additive manufacturing; then, a post-production surface treatment allowed achieving a surface with characteristic roughness and conductivity comparable to traditional manufactured antennas. A bench-test is finally carried out to assess the overall system performance. Peculiar issues, being faced for PPR high field side subsystem on larger fusion devices, are finally briefly reviewed in light of the experience gained during this specific realization.

This paper discusses the electrical insulation of a copper structure, which will be installed in the RFX-mod2 machine, in the presence of a weakly ionized plasma. The effectiveness of aluminum oxide (alumina) coating in preventing electrical discharges was evaluated through tests conducted both in air and in a weakly ionized plasma environment. It was shown that copper samples with alumina surface deposition sustained a voltage of about 2 kV in air but showed dielectric breakdown in the presence of plasma. SEM analysis revealed an irregular microstructure with a high degree of porosity and large internal cavities, which tend to reduce the effective coating thickness. New copper prototypes with more compact and less porous alumina deposition showed effective electrical insulation in the presence of plasma, highlighting how porosity and compactness are important factors in ensuring electrical insulation in the presence of plasma. The use of other materials, such as paints, silicones and resins, to insulate the inner part of holes in the copper structure, due to limitations in applying alumina coatings with plasma spray techniques, is also discussed.

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).

The design of the RFX-mod2 experimental fusion device requires a copper shell close to the plasma to aid in stability and magnetic confinement of the plasma [1,2]. This conductive structure, only 3mm thick, placed around the plasma, must have electrical discontinuities in both the poloidal and toroidal directions, so as to allow the penetration of electromagnetic fields into the plasma region. These gaps are conceived to avoid the formation of net poloidal and toroidal currents in the copper shell during the experiment phases. Furthermore, the shell was designed with an overlapped region at the poloidal gap in order to minimize induced field errors. The loop voltage, that is the electromotive force induced by external coils which sets up and supports the plasma current, can reach values up to 400 V, during operations in the Reversed Field Pinch magnetic configuration. Besides, if a fast termination occurs, i.e. rapid loss of the plasma magnetic confinement, these values can rapidly rise up to 1.5 kV. Therefore, intense electric fields can be generated between the shell edges, only a few millimetres apart, along the overlapping region. Furthermore, considering that the stabilizing copper shell, placed inside the vacuum chamber, is exposed to low temperature weakly ionized plasma, the formation probability of harmful electric arcs is high. In order to avoid the formation of arcs, the copper shell will be coated with a ceramic layer made of aluminium oxide (alumina), applied by means of atmospheric plasma spraying. An experimental apparatus was prepared in laboratory, with the aim of reproducing the conditions expected at the plasma edge close to the copper shell. It consists of a vacuum chamber in which a helium plasma is produced, generated by an incandescent tungsten filament and a DC power supply. The alumina-coated copper samples are polarized, applying a pulsed voltage up to approximately 2 kV. The electrical tightness of the insulating layer and the possible formation of electrical discharges on the alumina surface were verified. We observed that in some samples, in which the breakage of the dielectric layer occurred, the thickness of the ceramic layer was less than that required (100 µm) and was characterized by an irregular structure with high porosity and large cavities of tens of microns. Moreover, these cavities generate a network of interconnected fractures forming an almost continuous porosity, thus reducing the effective thickness of the alumina. The electrical insulation has been significantly improved by creating alumina deposits with better compactness and reduced porosity. The latter parameters have been validated for having an effective electrical insulation in the presence of plasma. In this contribution, we present the electrical insulation performance of alumina coated samples with different thickness and porosity levels. Furthermore, the copper samples with alumina coating were analysed, both in section and on the surface, by means of a scanning electron microscope (SEM).