Pedestal Density Control at KSTAR via Gas Fueling and 3D Field Perturbation

Achieving detachment and edge-localized mode (ELM) suppression requires reliable control of the electron density (n_e) at the pedestal, making it a key operational parameter for core-edge integrated solution in future pilot devices. In this work, we developed and demonstrated a real-time pedestal density control at KSTAR, targeting near ψ_N =0.89, using both resonant magnetic perturbations (RMPs) and main gas fueling [1] as actuators. The radial density profile is reconstructed using a parametric model fitted to line-averaged density measurements from five channels of a two-color interferometer (TCI) [2] and magnetic equilibria [3]. To enable real-time profile reconstruction, a multi-layer perceptron (MLP) is used to accelerate the process. The system identification is derived using a first-order model, with parameters identified from reference discharges. Control gains were determined via pole placement logic. Experiments during the FY25 KSTAR campaign demonstrated that the controller successfully tracks dynamic targets with median and average absolute percentage errors of 1.64% and 2.20%, respectively. This controller enables active pedestal density regulation for scenario exploration and real-time density stabilization during plasma operation. Future efforts will extend this approach to control multiple profile points, supporting the development of advanced core-edge integrated scenarios.

[1] Seo, Seong-Heon, et al. "Korea superconducting tokamak advanced research vacuum and gas puffing system." Review of Scientific Instruments 79.11 (2008).

[2] Juhn, June-Woo, et al. "Multi-chord IR–visible two-color interferometer on KSTAR." Review of Scientific Instruments 92.4 (2021).

[3] Lao, Lang L., et al. "Reconstruction of current profile parameters and plasma shapes in tokamaks." Nuclear fusion 25.11 (1985): 1611.