Designing high efficiency glow discharge cleaning systems

Glow Discharge Cleaning (GDC) is one of the best established techniques used in present fusion devices in order to remove deuterium and impurities from the plasma facing components. At least in its simplest form, with Direct Current (DC) power supply, GDC operation is not compatible with the presence of a magnetic field in the vessel volume, that inhibits the discharge, hence in superconducting coils devices DC-GDC cannot be used for inter-shot conditioning; nevertheless DC-GDC is implemented also in ITER, as fundamental tool for wall conditioning after maintenance with vessel venting and for tritium recovering after D-T campaigns. The effectiveness of the wall conditioning is closely connected to the ion current impinging on its surface, given that in the process the wall plays the role of discharge cathode and undergoes ion sputtering. It was already found [1,2] that in toroidal fusion devices, where the cathode area is typically 2-4 order of magnitudes larger than the anode area, the current does not spread uniformly over the wall, hence cleaning efficiency is not everywhere the same. The mitigation of such non-uniformity pattern is a key issue in order to get good conditioning of the plasma facing wall. In the present work we report about dedicated experiments conducted at RFX-mod toroidal fusion device, devoted to enlighten the effect of different anodes configuration on the profile of ion current density at the wall, jwall. RFX-mod (R=2m and a=0.459m) is very well suited for the purpose since its wall, the cathode, is circularly shaped and uniformly covered by tiles, without discrete limiters or any other protruding structure: the regular shape of the plasma boundary gives high sensitivity in detecting the effect of any parameter on the jwall profiles and allows a direct comparison of the measures to models, based necessarily on simplified geometry. Moreover, RFX-mod is equipped with an extensive set of electrostatic probes, organized in a uniformly distributed toroidal array of 72 probes and a poloidal one of 7 probes, for the full reconstruction of the ion current profile. The jwall pattern has been measured with either one or two working anodes, with anodes of different size and material and with anodes placed at different insertions with respect to the wall, from the center of the poloidal section down to the wall edge. A working pressure scan was also performed. The role of the anode area has been quantified and jwall non-uniformity has been characterized in terms of ratio of anode and cathode areas. Anode discretization and size were also found to impact on the V-I characteristic curve of the discharge. Experimental results shown here were found to be in good agreement with models of glow plasmas found in literature and constitute the basis of the project for a new GDC system for the RFX-mod machine upgrade.