Electrode conditioning for the prevention of DC arc formation within a cold plasma

In the RFX-mod2 experiment presently under construction, devoted to the study of magnetic confinement of fusion relevant plasmas, significant electric fields, in the kV/mm range, are expected to form in between in-vessel conductive plasma facing components during transient plasma current phases (start-up and termination). While such electric fields are of no concern for components in vacuum, the presence of a scrape-off plasma at the edge (electron density ne 1016 ÷1018 m-3, electron temperature Te of few eV) can create the conditions for potentially dangerous arc formation. For this reason part of the plasma facing components (in particular the graphite 'first wall' tiles covering the copper 'stabilizing shell' placed within the plasma chamber) require a proper conditioning technique capable of maintaining the insulation between conductive components even in presence of the scrape-off plasma. An experimental apparatus has been developed in order to test the conditions for the arc formation and prevention between two electrodes immersed in a plasma generated by a hot emitting filament. The results of an extensive experimental campaign will be presented, aimed at demonstrating the possibility of gaining a sufficient electrical conditioning by applying the standard conditioning technique usually employed for higher voltage ranges. It consists of a sequence of high voltage pulses applied to the pair of electrodes with current limitation, in the presence of a background cold plasma with low ionization degree. The experimental procedure is such that the voltage of the pulses is slightly increased when arcing ceases, until the final desired voltage level is achieved (2.5 kV). Different electrode materials have been tested in a variety of plasma conditions in terms of electron density and working gas pressure.