Hydrogen Retention and Release in the RFX-mod Graphite First Wall

RFX-mod is a toroidal experiment (major/minor radius 2/0.46 m) for the confinement of thermonuclear Hydrogen plasmas in the Reversed Field Pinch magnetic configuration. The first wall is entirely covered by tiles made of polycrystalline graphite and it bears a strong thermal power deposition that can locally reach values of the order of tens MW/m2 and strong particle fluxes from the plasma of about 1021 - 1022 m2/s. Hydrogen retention in the graphite of the wall has a direct impact on the operation of RFX-mod. In fact, during the discharges, plasma density is entirely sustained by particle in-fluxes from the wall and the control of Hydrogen retention is the way to control plasma density during experiments. With the aim both to improve our control capability and to study the more general issue of the behaviour of graphite as first wall material for future fusion experiments, we analysed the Hydrogen retention and release under different conditions of the RFX-mod wall. We measured the Hydrogen that is left in the wall during the plasma operation and that is removed during the wall cleaning treatments (baking and Helium Glow Discharge Cleaning (GDC) sessions). Local but more detailed information was obtained by the analyses of graphite samples that have been inserted inside the vessel up to the first wall envelope surface in three different toroidal-poloidal positions by means of dedicated manipulators. Some tiles have been also removed and analysed, giving information about particles absorption under long-term exposure to plasma and cleaning treatments. We found that during operation a progressive accumulation of Hydrogen in the wall occurs and the effectiveness of the cleaning treatments routinely applied is discussed. The differences in the rate of accumulation on clean graphite and on graphite that has been coated with Boron or Lithium are shown and discussed.