Evidence of formation of lithium compounds on FTU tiles and dust

Highlights: • XPS, ATR, SIMS, XRD, ND characterization of the surface the toroidal limiter tiles and from dust of FTU machine. • LiO, LiOH, Li₂CO₃ formation. • Results suggest Li₂CO₃ formation during venting and decomposition during disruption. • The Gibbs free energy vs surface temperature support the Li₂CO₃ formation/decomposition. Since 2006 lithium as an advanced plasma facing material has been tested on the Frascati Tokamak Upgrade (FTU). Lithium in the liquid phase acts both as plasma facing component, i.e. limiter, and plays also a role in plasma operation because by depositing a lithium film on the walls (lithization) oxygen is gettered. As in all deposition processes, even for the lithization, the presence of impurities in plasma phase strongly affects the properties of the deposited film. During the 2008 campaigns of FTU it was observed a strong release of carbon dioxide (during disruptions), resulting in successive serious difficulty of operation. In order to find the possible reactions occurred, we have analyzed the surface of two tiles of the toroidal limiter close to the Liquid Lithium Limiter (LLL). The presence of molybdenum oxides and carbides suggested that the surface temperatures could have exceeded 1000 K, likely during disruptions. lithium oxides and hydroxides have been found on the tiles and in the dust collected in the vessel, confirming the presence of LiO and LiOH and a not negligible concentration of Li₂CO₃ especially at the LLL location. On the basis of the above results, we propose here a simple rationale, based on a two reactions mechanism, which can explain the formation of Li₂CO₃ and its subsequent decomposition during disruption with release of CO₂ in the vessel. Admitting surface temperatures above 1000 K during a disruption, relatively high partial pressures of CO₂ are also predicted by the equilibrium constant for Li₂CO₃ decomposition.