Quantification of erosion pattern using picosecond-LIBS on a vertical divertor target element exposed in W7-X

Abstract

A set of dedicated marker samples consisting of fine-grain graphite as substrate, an interlayer of 0.2–0.4 μm molybdenum (Mo) employed as marker, and a 5–10 μm 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 × 1019 C atoms/cm2 but also 2 × 1018 Mo atoms/cm2 which results can be extrapolated to total 15 × 1019 C atoms/cm2, were eroded due to plasma fuel particle (H, He) and impurity (O, C) ion impact.

Laser-Induced Breakdown Spectroscopy Imaging for Material and Biomedical Applications: Recent Advances and Future Perspectives

“…Two articles were published using these specific setups. In the first one, elemental and quantified erosion images were produced with a spatial resolution of 5 mm from typical graphite tiles used in reactors after being exposed to He/H plasma . These 2D images corresponded to one direction and depth analysis.…”

“…In the first one, elemental and quantified erosion images were produced with a spatial resolution of 5 mm from typical graphite tiles used in reactors after being exposed to He/H plasma. 83 These 2D images corresponded to one direction and depth analysis. 200 shots were produced for the depth, resulting in images in the range of 0 to 350 mm in the poloidal coordinates and down to several μm in depth.…”

Atomic spectrometry update: review of advances in the analysis of metals, chemicals and materials

“…A set of divertor tiles containing a 0.2–0.4 μm Mo interlayer and 5–10 μm C layer on top was installed in Wendelstein 7-X and analysed for erosion and deposition following exposure to around 40 minutes of plasma in the standard magnetic divertor configuration over one year. 166 Vertical erosion and deposition patterns were assessed for the first time using picosecond LIBS. The position of peak erosion was identified (total 15 × 10 19 C atoms per cm 2 ) due to plasma fuel (H, He) and impurity (O, C) ion impact.…”

Plasma-wall interaction studies in W7-X: main results from the recent divertor operations

“…The details of the experimental techniques used for the analysis i.e. Elastic Backscattering Spectrometry (EBS), Nuclear Reaction Analysis (NRA), Focused Ion Beam (FIB) cutting, Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray spectroscopy (EDX) are described in [19], pico-second Laser Induced Breakdown Spectroscopy (ps-LIBS) in [20] and optical reflection measurements in [21].…”