Improved scaling of the scrape-off layer particle flux width by the Bayes theorem on EAST

Abstract

The scaling of scrape-off layer (SOL) power width (λq) is essential for advancing the understanding of particle and heat transport in the SOL. Due to the sparse layout of divertor Langmuir probes (Div-LPs) and probe erosion during long-pulse, high-performance operations on EAST, estimating SOL particle flux width (λjs, used to approximate λq) from the ion saturation current density profile (js) often incurs substantial uncertainty. This study presents a maximum a posteriori (MAP) estimation method based on Bayes' theorem, achieving approximately 30% improvement in fitting accuracy over traditional ordinary least squares. Using this method and the FreeGS equilibrium code, we updated databases from Liu et al., Nucl. Fusion 64 (2024). Revised λjs scalings for L-mode and H-mode in deuterium and helium plasmas demonstrate better regression quality and slightly altered regression results. Unified L-mode and H-mode scalings in deuterium and helium are: λ_js^L = 0.11 L_c^1.06 n_e^0.35 Z^0.32 P_SOL^0.25 p^(-0.26) and λ_js^H = 0.11 L_c^1.28 n_e^0.56 Z^0.36 P_SOL^0.30, where L_c is the average SOL connection length, n_e the line-averaged electron density, Z the charge number, PSOL the power crossing the last closed flux surface, and p the core-averaged plasma pressure. Key findings include: (i) λjs strongly depends on SOL connection length, indicating a machine size dependence absent in the Eich scaling, and (ii) helium λjs is slightly larger than deuterium λjs. Extrapolated scalings suggest λq ~ 6 mm for ITER L-mode (Ip = 12 MA) and ~13 mm for H-mode (Ip = 15 MA).