Understanding the impact of divertor and main chamber ion fluxes on divertor closure in the DIII-D tokamak

The diverted tokamak redirects extreme heat and particles to targets, a plasma-facing component designed for such loads. Here, the local fluxes produce strong particle recycling and sputtering. Recycled neutrals can “leak” into the region between the core and wall, the scrape-off-layer (SOL), impacting plasma performance. Increasing divertor closure can reduce leakage by containing neutrals within the divertor. However, there exists a need to quantify divertor baffle restrictions and understand closure directly from empirical data as opposed to indirectly through modeling.

Our study introduces the Geometric Restriction Parameter (GRP) based on simplifying neutral transport to ballistic pathways. Specifically, it considers the angle subtended between the strike point and target, as well as the outboard baffling. The GRP serves as an indicator of neutral retention in attached conditions, with lower values corresponding to reduced restrictions and increased leakage. Leveraging data from 12 DIII-D discharges characterized by different divertor geometries, plasma conditions, and drift orientations, we utilize Window-Frame Analysis (WFA) and a modeling tool, OEDGE, to gain insights into neutral dynamics in the SOL.

Our analysis shows DIII-D divertor configurations can access a full range of GRP values. Furthermore, we quantitatively assess main chamber fluxes in relation to calculated divertor restriction. Our findings indicate increased neutral leakage with lower divertor restrictions and underscore the impact of drift on neutral particle transport in L-mode. An inverse correlation between structural closure and main chamber flux magnitude shows the significance of divertor restrictions in H-mode. While our empirical values align with OEDGE modeling, it is imperative to note that the model’s validity pertains solely to attached plasma conditions.

In summary, our study shows the divertor closure’s impact on main chamber and divertor fluxes, emphasizing the importance of SOL opacity on closure. In L-mode favorable drift conditions, divertor recycling emerge as the dominant factor with increased restrictions and neutral compression. The WFA exhibits a decreasing flux trend as the GRP increases, contrasting with the lack of discernible trend in unfavorable drift conditions. Additionally, divertor compression estimates, influenced by varying opacity, have implications for GRP estimation, highlighting the need to consider the SOL operating regime.