DC glow discharge conditioning of remote areas in fusion devices

Various methods are used for wall conditioning of the vacuum chamber of magnetic fusion devices. Among them, direct current glow discharges in deuterium or helium are used to control the contamination of the plasma by impurities coming from the walls. However, the relatively high energy of the ions on the plasma facing components and the difficulty to extend the glow discharge in remote areas could reduce their attractiveness. We present the results from tests performed in a dedicated reactor, equipped with a narrow cylindrical duct with seven Langmuir probes. A heated cathode, biased at -40 Volts with respect to the walls, is placed inside the duct. The influence of the pressure on the penetration of the discharge into the tube is shown and discussed. Significant values of ion current and potential in the tube are obtained for pressures two orders of magnitude higher than those currently used in present Tokamaks. The effect of establishing the glow discharge between the anode and the heated cathode was studied. The heated cathode currents can be tuned in such a way that the current measured by the probe increases whereas the potential drop between the glow discharge and the wall decreases and the energy of the ions which are impinging onto the surfaces is reduced.