SOL effects on the pedestal structure in DIII-D discharges*

Analysis with the SOLPS code suite (Schneider R. et al 1992 J. Nucl. Mater. 196 810; Wiesen S. et al 2015 J. Nucl. Mater. 463 480–4; Bonnin X. et al 2016 Plasma Fusion Res. 11 1403102) explains the differences in pedestal structure associated with different ion drift directions in DIII-D. Core transport models predict that fusion power scales roughly as the square of the pressure at the top of the pedestal, so understanding the effects that determine pedestal structure in steady-state operational scenarios is important to help project scenarios developed in DIII-D to ITER and other devices. Both experiments and modeling indicate that scrape off layer (SOL) conditions are important in optimizing the pedestal structure for high-beta steady-state scenarios. The SOLPS code is used to provide interpretive analysis of the pedestal, and SOL conditions are used to examine the nature of flows and fueling on the pedestal structure; including the effect of drifts in the fluid model. This analysis shows that flows driven by the ion drift outwards when the drift flows toward the x-point, in a single-null divertor configuration (favorable direction for reduced H-mode power threshold), and they drift inwards when the drift flow is away from the x-point (unfavorable direction). It is hypothesized that these flows decrease the density gradient of the pedestal in the favorable direction, thereby stabilizing the kinetic ballooning mode (KBM) and increasing the pedestal width. Comparisons of the pedestal structures in similarly shaped DIII-D steady-state plasmas confirm this change, showing increased density pedestal width, lower peak density and lower separatrix density with the favorable drift direction. The pedestal temperature is higher at a lower density, resulting in an increased pedestal pressure, which indicates that the increased particle flux does not significantly degrade energy confinement. Modeling cases with constant drift direction, but with a change between the more open lower divertor and more closed upper divertor, show that there is increased fueling inside a pedestal with a more open geometry. The pedestal fueling rate for both attached and detached cases is always lower when there is a more closed divertor geometry than in cases when there is a more open divertor geometry.