Simulation-Based Inference of High Field Side Scrape-Off Layer Filament Characteristics using Profile Reflectometry*

A stochastic scrape-off layer filament model was used to infer probable filament characteristics, such as density, velocity, and size, using high-field side profile reflectometry data on DIII-D. In general, scrape-off layer (SOL) turbulence is characterized by intermittent filament structures which radially propagate from the last-closed flux-surface (LCFS) to the vessel wall. While filaments are primarily measured in the low-field side SOL, they may also be found on the high-field side (HFS), depending on the magnetic geometry. Such filaments could negatively impact the recently installed DIII-D high-field side lower-hybrid current drive experiment due to undesired wave-filament interactions. A HFS reflectometer on DIII-D is used to measure SOL density profiles, and using simulation-based inference (SBI), information about filament parameters from profile reflectometry can be obtained. This technique utilizes a stochastic JAX-based filament model to simulate the reflectometer response to filament structures. Automatic differentiation is used to calculate reflectometer group delay measurements, while vectorization on a GPU dramatically improves computational efficiency. SBI is then used to estimate the posterior of multiple filament parameters such as filament size and density. The technique is applied to DIII-D HFS SOL reflectometry measurements, and predicted impact on HFS LHCD is discussed.