Influence of the magnetic configuration on the high-field side scrape-off layer at ASDEX Upgrade and the role of the secondary separatrix

Abstract

In tokamaks, radial transport is ballooning, meaning it is enhanced at the outer side (low-field side). This work investigates the effect of the magnetic configuration on the inner (high-field side) scrape-off layer. Our experiments involved L-mode and H-mode discharges at ASDEX Upgrade, in which we scanned the magnetic configuration from a lower to an upper single-null shape, thus varying the location of the last flux surface connecting the inner to the outer scrape-off layer (secondary separatrix). We show that the secondary separatrix determines the width of the inner scrape-off layer, meaning that the density is much lower in the region that is magnetically~disconnected from the outer scrape-off layer, outside the secondary separatrix. Furthermore, we observe that the large density often seen in the inner divertor drastically decreases as the separation between the primary and secondary separatrices falls below a particular value. This value is different for L-mode and H-mode plasmas and closely matches the power decay length measured at the outer midplane. We also show how the inner scrape-off layer density is smaller in an upper single-null than in a lower single-null, when the ionic grad-B drift points down. This difference is likely caused by reversing the $\EXB$ drifts in the active divertor when switching the active X-point from the bottom to the top. We further observe that the neutral density in the lower divertor also correlates with the plasma shape and the high-density region in the inner scrape-off layer. During the shape scans analyzed here, the inner divertor remained partially detached throughout, with transitory reattachment modulated by ELM activity in H-mode. This work provides novel experimental data that can be leveraged to further the modeling capabilities and understanding of scrape-off layer physics in highly shaped plasmas.