Speaker
Description
Dust particles are recognized as a significant safety issue in the future fusion devices. When present in large quantities they may create an operational risk for the fusion device. Even small amounts, mobilized during a discharge, can cause additional radiation and influence the plasma performance. Dust particles can be observed by cameras, but these events are hard to interpret. In the 2024/25 campaign many dust events were observed during plasma operation in ASDEX Upgrade (AUG). These may be resulting from the in-vessel work during the installation of the new upper divertor, issues arising from the newly installed components or due to the new plasma operation scenarios. During the 2024/25 campaign, damages on the outer divertor tiles caused tungsten (W) melting, and as a result tungsten droplet production. Melting was also observed on the W coated protection tiles at the midplane and on the edges of the tiles at the new upper divertor. Besides melting, W particles also originate from arcing, sputtering and subsequent re-erosion.
Post-mortem analysis of the dust samples offers classification of dust particles. At AUG silicon (Si) wafers mounted in a protection box are used to collect dust mobilized by the plasma. Five dust boxes had been installed at the low and high field side of the midplane and in the divertor region. After exposure, the wafers were measured by an automated procedure using scanning electron microscopy, allowing identification of thousands of particles by size and composition [1].
For comparison samples installed at the same positions for the 2022 and 2025 campaign are used. The size distribution was determined for particles with different W content. Typically, the W containing particles also show oxygen and carbon content. The size distribution is limited by the magnification used for analysis and the number of pixels used to select a particle. Analysis of a surface area of 2 x 2 $\mathrm{mm}^2$ shows that, for the 2022 campaign, a total of 2700 articles were found in the 0.1 to 300 $\mu\mathrm{m}$ size range, whereas in the 2025 campaign, 7800 particles are observed. Using the size distribution similar amounts were found for particles below 0.3 $\mu\mathrm{m}$ and bigger than 3 $\mu\mathrm{m}$ during both campaigns. In the range between 0.3 and 3 $\mu\mathrm{m}$, up to 4 times more particles are produced in 2025. These results are discussed in the view of different tungsten dust production mechanisms.
[1] M.Balden et al., Nucl. Fusion, 54 (2014) 073010