Ceramic coatings for arc prevention between plasma facing components

The design of the RFX-mod2 fusion experimental device requires a 3 mm copper shell close to the plasma to achieve improved plasma confinement properties. Such conductive structure, located around the plasma and aimed to passively stabilize the plasma, shall have an electrical discontinu-ity in both the poloidal and toroidal directions, in order to allow the penetration of electromagnetic fields into the plasma region. These gaps avoid the formation of net poloidal and toroidal eddy cur-rents during transient phases of the plasma confinement experiment. Moreover, the shell has been designed with an overlapping region at the poloidal gap in order to reduce the induced field errors. During operations in Reversed Field Pinch magnetic configuration, the loop voltage, that is the externally induced electromotive force sustaining the plasma current, can reach values up to 400 V. These values can rapidly step up to 1.5 kV during fast plasma current terminations. Therefore, intense electric fields can generate between the shell flaps, only a few millimetres apart, along the overlapping region. Furthermore, taking into account that the stabilizing shell, being located inside the vacuum chamber, is exposed to low temperature plasma, the formation probability of harmful electric arcs is high. In order to avoid arc formation, different kind of insulation coating on the copper, able to with-stand the applied electric fields in the presence of plasma, are under investigation. The electrical insulation performances of ceramic coating on a copper samples are investigated. Several deposition methods have been evaluated, including magnetron sputtering, atmospheric plasma spray and detonation gun spray, which differ in the adhesion to the substrate, compact-ness, porosity and mechanical strength of the deposited material. The last two methods have the advantage of being conducted in air at atmospheric pressure, without the aid of complex vacuum systems. In order to validate the process, an experimental apparatus was prepared in laboratory, aimed to reproduce the expected conditions at the shell gap. It consists of a vacuum chamber in which a helium plasma was generated by means of a hot tungsten filament and a DC power supply. A bias voltage was applied between a copper plate and a cylindrical electrode (Ø 4 mm). The plate side facing the electrode was covered with alumina. The two electrodes were floating and biased by a small capacitor bank (0.3÷2 ?F). The voltage on the electrodes was applied for 200ms, with a repetition rate of 1Hz. In this contribution, the experimental results, aiming to study the conditions for the arc formation in presence of weakly ionized plasma (ne ~ 1016 m-3), are presented. In particular, voltage pulses up to 2.7 kV were applied, with a background gas pressure between 10-3 and 10 mbar. Further-more, the electrodes were kept both in contact and spaced up to 5 mm. Furthermore, the results of similar experiments performed on a mini mock-up, simulating the RFX-mod2 overlapping shell, are presented.