Validation of first-principles turbulence simulation in diverted negative triangularity on DIII-D

The GENE-X code was used to perform first-of-its-kind non-linear gyrokinetic (GK) simulations of the entire plasma edge region of diverted negative triangularity (NT) discharges on the DIII-D tokamak, from inside the separatrix to divertor targets and including the X-point. Our results reveal the structure of the DIII-D NT edge, where trapped electron modes (TEMs) are the main driver of turbulent transport and low-frequency resistive MHD-like modes dominate at the outermost edge.

Achieving reliable predictions of plasma profiles in the edge and scrape-off layer (SOL) in future fusion pilot plants requires first-principles turbulence models that are thoroughly validated against experimental data. While substantial progress has been made in predicting core profiles with prescribed edge conditions, the edge itself remains elusive due to its complex physics. NT plasmas are considered viable scenarios in reactors for a variety of reasons, among which are a quiescent edge, free of edge-localized modes, and H-mode grade confinement levels. A thorough understanding of the NT edge is essential for extrapolating performance from current experiments to reactor conditions.

We present first-principles simulations where boundary conditions are fixed and plasma profiles are allowed to evolve freely, resulting in exceptional agreement with experimental data for density, ion- and electron temperature. Our simulations reveal a TEM-dominated edge region, consistent with NT stabilizing turbulent transport. A major milestone is the first validation of edge and SOL ion temperature profile in L-mode turbulence by GK simulations. We investigate the radial electric field and poloidal rotation, confirming a satisfied force balance. Geodesic acoustic modes (GAMs) with frequencies close to analytic theory are identified. Analysis of SOL divertor heat fluxes shows that the heat deposition closely follows the multi-machine regression. Future work will use advanced collision operators in GENE-X to determine properties of resistive modes in the outer edge and SOL and evaluate any impact towards making NT a core-edge integrated solution. The successful validation of our simulation represents a crucial step towards a self-consistent prediction of turbulence, edge profiles, and SOL power fall-off lengths in future devices.