LIBS measurements inside the FTU vacuum vessel by using a robotic arm

Abstract

In this work we report on the Laser-Induced Breakdown Spectroscopy (LIBS) characterization of the Frascati Tokamak Upgrade (FTU) first wall (FW) and toroidal limiter after the spring-summer 2019 experimental campaign. We performed our analysis by using a compact LIBS system mounted on the FTU robotic arm, entering the FTU vacuum vessel (VV) and measuring the FW elements in-situ. The LIBS system was expressly designed and developed for this task and, to our knowledge, it represents the first prototype of LIBS mounted on a robotic arm for analyses inside a tokamak. It allows to perform measurements at (1) atmospheric pressure or (2) in vacuum (down to 10−2–10−3 mbar) or (3) under gas flux (He, Ne, Ar etc.) and is suitable for 4) single pulse or 5) double pulse LIBS measurements. The LIBS analyses revealed the main chemical components of the stainless-steel FTU FW, Mo and Ti from the Titanium-Zirconium-Molybdenum alloy (TZM) tiles of the toroidal limiter. A superficial contamination of Li, originating from past liquid lithium limiter experiments was also clearly revealed. To simulate a LIBS analysis of a component of the ITER divertor region we placed a metallic sample at the bottom of the FTU VV through a sample holder and we measured it with the LIBS system. The stratigraphy of this ITER-like sample, composed of an Al/D superficial layer 3 μm thick (D 5% atomic concentration, Al as proxy for Be) on a W substrate simulates ITER PFCs in the divertor baffle and was characterized with high axial resolution (few hundreds nm for each laser shot) although the D signal from the Dα emission line at 656.1 nm was found strongly interfering with the corresponding Hα emission at 656.28 nm from the environmental hydrogen.