Development of full-wave simulation codes for synthetic profile reflectometry and reconstruction framework for locally peaked density in the EAST tokamak

An atypical dual-time-delay phenomenon has been identified near the onset of edge-localized modes (ELMs) through profile reflectometry measurements, resulting in a substantial overestimation of the density profiles in the EAST tokamak. This phenomenon poses a challenge to the standard reconstruction framework, which relies on the assumption of monotonic density profiles. To address this issue, a 2D full-wave simulation code (FOX2D) based on the finite-difference time-domain method has been developed to model synthetic reflectometry diagnostics. The simulations successfully reproduce the dual-time-delay behavior and demonstrate that it originates from tunneling and Bragg backscattering effects caused by locally peaked density profiles. To address the reconstruction challenges, a novel database-driven method incorporating full-wave physics has been developed, enabling improved characterization of non-monotonic density structures. The reconstructed results reveal a density peak localized near the pedestal top, characterized by a narrow radial scale of Lr = 0.64 cm and a slight relative density perturbation level of ne/ne ∼ 1.5%. This structure closely resembles the primary ELM precursor filaments observed by other diagnostics. These findings enhance the understanding of reflectometry measurements and provide deeper insights into ELM physics.