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Time-resolved simulations of erosion on tungsten-coated divertor plates in the Large Helical Device (LHD) were performed using ERO2.0, a three-dimensional Monte Carlo code for plasma–surface interaction analysis [1]. The temporal evolution of the erosion areas during the experimental campaign was investigated using a new modeling approach that accounts for time-dependent changes in surface composition on the plates. This approach enables more accurate lifetime estimations for tungsten layers.
Before the experimental campaign in FY2019, conventional carbon divertor plates in the closed helical divertor region of one of the ten helical sections were replaced with tungsten-coated plates to evaluate their compatibility with plasma discharges [2]. During the experimental campaign, the tungsten-coated plates successfully suppressed deposition layers, thereby reducing dust particle generation in the divertor region without significant degradation of plasma performance. However, post-campaign inspections of the plasma-facing components revealed erosion of the tungsten coatings, with depletion at strike points on about half of the tungsten-coated divertor plates. This finding poses a critical challenge for maintaining the lifetime of plasma-facing components and highlights the need for further investigation into material migration.
To elucidate the physical mechanism underlying tungsten depletion, ERO2.0 simulations were performed, revealing that the calculated tungsten-erosion profiles matched observations and demonstrated the progressive expansion of depleted areas throughout the experimental campaign. The evolution of depleted areas was driven by tungsten self-sputtering and interactions with intrinsic carbon ions in the peripheral plasma. This analysis identified two impurity transport processes contributing to tungsten erosion:
1. Long-range transport of carbon sputtered from strike points on carbon divertor plates in other divertor regions,
2. Short-range transport of carbon sputtered from depleted areas on the tungsten-coated divertor plates.
Post-mortem surface analysis of a tungsten-coated divertor plate using Glow Discharge Optical Emission Spectrometry (GD-OES) revealed the formation of three distinct areas characterized by different material composition depth profiles:
1. Carbon deposition areas, where carbon is deposited on the tungsten-coated layers,
2. Tungsten-coated areas, where tungsten layers remain on the carbon substrate,
3. Depleted areas, where tungsten layers were removed entirely, exposing the carbon substrate.
To elucidate the mechanisms of formation of the three areas, impurity migration on the tungsten-coated divertor plate is investigated using ERO2.0. It provides valuable insights into the physical processes governing material migration in the tungsten-coated divertor region.
[1] J. Romazanov, et al., Nucl. Mater. Energy 18, 331 (2019)
[2] G. Motojima, et al., Proc. 28th IAEA FEC 2020, IAEA-CN-286/EX/P6-22, Vienna, Austria, May 2021