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Description
The design of the RFX-mod device upgrade requires a 3 mm copper shell close to the plasma to achieve improved plasma confinement properties. Although a continuous conductive structure would be preferred as a passive plasma stabilizer, such a conductive structure shall have an electrical discontinuity in both the poloidal and toroidal directions, in order to allow the penetration of electromagnetic fields into the plasma region. These gaps prevent the formation of net poloidal and toroidal currents. Moreover, the shell has been designed with an overlapping region at the poloidal gap in order to reduce the induced field errors.
During Reversed Field Pinch operations, the loop voltage, externally induced to sustain the plasma current, can reach values up to 400 V. These values can rapidly increase up to 2 kV during fast plasma current terminations. Therefore, intense electric fields can develop between the shell flaps, only a few millimetres apart, along the overlapping region. Furthermore, taking into consideration that the copper shell is located inside the vacuum chamber and is therefore exposed to the low temperature plasma of the scrape off layer, the formation probability of harmful electric arcs is high.
In order to avoid arc formation, different types of insulation coating on the copper, able to withstand the applied electric fields in the presence of plasma, are under investigation.
The electrical insulation performances of an alumina coating on a copper substrate 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, compactness, 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 set up in the 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 the use of a hot 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 are floating and biased by a small capacitor bank (0.3÷2 μF). The voltage on the electrodes was applied for 200ms, with a repetition rate of 1Hz.
In this contribution, the results of the experiments, with the aim of studying the conditions for the arc formation in presence of a weakly ionized plasma (ne ~ 10e16 m-3), are presented. In particular, voltage pulses up to 2.5 kV were applied, with a background gas pressure between 10e-3 and 10 mbar. Furthermore, the electrodes were kept both in contact and spaced up to 5 mm. The analysis of alumina degradation, after dielectric cracking in the presence of arc, is also presented.
