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Description
The complex 3D magnetic topology of the island divertor in Wendelstein 7-X (W7-X) produces feature rich erosion and deposition patterns on the divertor plates and strongly affects impurity transport in the edge plasma region. While crucial for stellarator optimization towards reactor-relevant configurations, modelling impurity transport is challenging and requires thorough code validation. Here, the gold standard is tracer particle experiments: isotope-labeled impurities are injected at known positions and quantities into repeated, identical discharges to establish erosion, deposition, and migration patterns on plasma-facing surfaces. Components or samples are then removed and undergo ex-situ post-mortem analysis after which the results are compared to model predictions.
Therefore, to explicitly study long-range migration in W7-X, a dedicated tracer injection experiment with isotope separated $^{15}$N was performed: a total of $\sim 1.5 \times 10^{21}$ $^{15}$N atoms were injected via the divertor gas injector module. The $^{15}$N was then collected on graphite samples mounted on the multipurpose manipulator which intersects the same magnetic island as the injection but is separated by a toroidal angle of $\sim 70^\circ$). For comparison with previous $^{13}$CH$_4$ tracer experiments~\cite{Kawan2024}, similar magnetic configurations and plasma parameters were chosen: standard configuration $n_{e,\text{line int.}} \sim 6 \times 10^{19} \text{m}^{-2}$, $T_e \sim 2.3$ keV, and 3.5 MW of pure ECRH heating. The injections started 1 s after plasma initialization and continued until the ECRH power was ramped down. Altogether 27 nearly identical discharges with injection were performed consecutively, for a total plasma duration of 350 s. The radiative power $P_{\rm rad}$ increased from $\sim 1$ MW to $\sim 1.5$ MW during the injection in the initial discharges; a moderate legacy effect due to the repeated nitrogen injection was observed as the initial radiative fraction steadily increased to $\sim 2.5$ MW in the later discharges. Due to a scheduled opening of the plasma vessel right after the experiment, additional plasma facing components were retrieved and it is planed to quantify also their surface $^{15}$N concentration.
This contribution summarizes the experimental procedure and gives an overview of the current state of the post-mortem analysis, especially the measurements of $^{15}$N on the collection samples by nuclear reaction analysis (via the $^{15}$N(p,$\alpha)^{12}$C reaction). Furthermore, a comparison of the $^{15}$N tracer experiment with the previous $^{13}$CH$_4$ tracer experiment is presented, and implications for future optimization of stellarator edge configurations are discussed.
[1] C. Kawan et al., Nuclear Material Energy 39 (2024) 101675.