Predicting Negative Triangularity Edge Confinement with a Neutral-Coupled Full-f Gyrokinetic Model

Abstract

Global full-f gyrokinetic simulations coupled to a reduced neutral-fluid model reproduce key DIII-D experimental trends and identify mechanisms underlying improved confinement in negative triangularity (NT) configurations relative to positive triangularity (PT). Using Gkeyll, we simulate DIII-D inner-wall-limited discharges across the confined edge, scrape-off layer (SOL), and sheath boundaries, allowing the edge electric field to evolve self-consistently. Compared with an adaptive recycling-source model, the reduced neutral fluid model improves particle and energy balances and brings the simulated density and temperature profiles into closer agreement with measurements, with minimal additional computational cost. In NT, suppressed turbulent fluxes coincide with a broad, stationary zonal flow inside the last closed flux surface, whereas PT develops transient, fragmented shear layers. Spectral analysis indicates that modified fluctuation phase relationships and enhanced zonal shear both contribute to transport suppression despite broadly comparable fluctuation amplitudes. The larger co-current departure from a locally matched bi-Maxwellian in NT suggests a velocity-selective kinetic process that may help establish the edge radial electric field by modifying flows or current balance. These results identify kinetic and turbulent mechanisms associated with improved NT confinement within an experimentally established ELM-free regime and demonstrate the importance of coupling gyrokinetic plasma dynamics with neutral interactions across the confined plasma and SOL.