Quantification of erosion pattern using picosecond-LIBS on a vertical divertor target element exposed in W7-X. (8th December 2020)

• Record Type: Journal Article
• Authors: Zhao, D.; Yi, R.; Eksaeva, A.; Oelmann, J.; Brezinsek, S.; Sergienko, G.; Rasinski, M.; Gao, Y.; Mayer, M.; Dhard, C.P.; Naujoks, D.; Cai, L.; W7-X team, the
• Is Part Of: Nuclear fusion. Volume 61:Number 1(2021)
• Publication Date: 2020-12-08
• Subjects: W7-X divertor -- erosion pattern -- picosecond LIBS -- quantitative analysis -- layered sample; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/abc408
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: A set of dedicated marker samples consisting of fine-grain graphite as substrate, an interlayer of 0.2–0.4 μ m molybdenum (Mo) employed as marker, and a 5–10 μ m thick carbon (C) marker layer on top were installed in Wendelstein 7-X (W7-X) to investigate locally the C erosion and deposition. In this study, a set of five individual marker tiles, installed in a vertical divertor element of the test divertor unit in half-module 50, and exposed to about 40 min of plasma predominant in the standard magnetic divertor configuration in the first year of divertor operation in W7-X (OP1.2A), were retrieved from the vessel for post-mortem analysis. Picosecond laser induced breakdown spectroscopy (ps-LIBS) was applied on these marker tiles in order to determine the local erosion/deposition pattern caused by plasma impact. The general erosion/deposition pattern on the vertical target element was studied with the aid of depth-profiling by Mo line emission due to ps-LIBS with the number of applied laser pulses (355 nm, 2.3 J cm −2, 35 ps) at one probing location. Several potential asymmetry factors which avoid a perfect layer-by-layer ablation process in the laser ablations are proposed and discussed when a rough layered structure sample with a rough surface is analysed by the ps-LIBS technique. Thereby, a simulation model was developed to correct the measurement error of the ps-LIBS method caused by the non-perfect rectangle profile of the applied laser beam. The depth resolution of the applied ps-LIBS system was determined by quantification of the laser ablation rates of the different layers and the C substrate which were measured utilising profilometry and cross comparison with the thicknesses of the C and Mo marker layers determined by a combined focused ion beam and scanning electron microscopy technique. For the first time, the erosion/deposition pattern on the vertical target was mapped and quantified by ps-LIBS technique. A relatively wide net erosion zone with a poloidal extend of about 200 mm was identified which can be correlated to the main particle interaction zone at the magnetic strike-line of the dominantly applied standard magnetic divertor configuration. At the position of peak erosion, not only 7.6 × 10 19 C atoms/cm 2 but also 2 × 10 18 Mo atoms/cm 2 which results can be extrapolated to total 15 × 10 19 C atoms/cm 2, were eroded due to plasma fuel particle (H, He) and impurity (O, C) ion impact.