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Wall conditioning of the vacuum chamber is a key factor for the operation and the
performance of magnetic fusion devices. Various types of methods are used to clean in situ contaminated surfaces in today’s equipments. Among them, DC glow discharges in a through flow of deuterium or helium are routinely used to limit the flux of impurities coming from the walls, to control the desorption of hydrogen and hydrogen isotopes (hence the isotopic ratio in the plasma) and to minimize tritium inventory in surface layers of plasma facing components. In particular, glow discharges in Deuterium or in Oxygen have been proved to
be efficient for tritium removal. In glow discharge conditioning (GDC), one or more anodes are located inside the vacuum chamber of the Tokamak. The grounded wall surfaces are thus exposed to a flux of energetic ions. Typical Deuterium and Helium pressures range around 0.4 Pa, the discharge voltage is
around 500 Volts. In Tore Supra, a discharge current of 6A over an inner surface area of 100 m2 is used. However, the relatively high energy of the ions on the PFCs and the difficulty to extend the glow discharge in remote areas could reduce the attractiveness of GDC. Some tests were performed in a dedicated tank, 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 first shown and discussed. Significant values of current and voltage 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. An overall discussion on the most promising method to improve the conditioning of remote areas during GDC will be finally given.
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