LIBS characterization of the Frascati Tokamak Upgrade (FTU) limiter tiles after the experimental campaign with fusion plasma

In this work we present the results of the LIBS surface mapping and depth profiling of the Frascati Tokamak Upgrade (FTU) tiles of the poloidal limiter, made of a Molybdenum alloy (TZM) and removed from the reactor after its last experimental campaign with fusion plasma (late 2019). The measurements were performed by using a spectrometer array that covers the range 200-780 nm and a high-resolution spectrometer (0.1 Å at 500 nm) covering a spectral window of 10 nm. By applying a depth profiling procedure, the contamination of the tiles by B, Ca, Cr, Fe, K, Li, Mn, Mo, Ni, Ti was observed in their surface layers. Contamination with H, O, N was instead observed in deeper layers after some laser shots. B was attributed by the preliminary boronization procedure of the first wall (FW), carried out with deuterated diborane (D2B6), although D was not detected on the surface. Li was detected as deposited layer coming from previous experiments with liquid metals limiters in FTU. Ca and K are believed to come from the manipulation of the tiles after FTU decommissioning or as residual impurities of the deposited together with Li. H, O and N, from exposure of the tiles to the environmental gases, Mo and Ti were detected as main constituents of the tiles but also as eroded material during exposure to fusion plasma. Cr, Fe, Mn and Ni as eroded materials from the vacuum vessel (VV). In all cases, the surface concentration and thickness of the contaminated layer strongly varies across the TZM tile, showing a tendency of major impurity accumulations in areas more distant from the reactor's equatorial section.