Implementation of a radiative emission front position controller in KSTAR with W divertors*

The management of heat loads on PFCs remains a critical challenge in tokamak operation. Previous research has identified controlled detachment with W divertors as a promising approach for heat load mitigation in future fusion devices. This study explores the implementation of a radiative emission front position controller in the KSTAR tokamak following its recent upgrade to W divertors. Building on previous detachment control strategies [1, 2] that utilized various diagnostics and control targets, our approach employs RT-IRVB [3] for tomographic reconstruction of the radiation distribution [4, 5]. A new control system was designed to detect and parameterize the position of the radiative emission front. The radial position of the inboard emission front was used as the control variable, with a PID controller adjusting nitrogen gas injection to maintain the target position. Initial demonstrations showed successful transfer of emission front position data to the PCS and responsive gas valve control. However, repetitive H-L back transitions were observed, attributed to rapid increases in core radiation. These oscillations posed challenges for stable control. A modified scenario was developed to mitigate these oscillations. This work represents an important step in developing real-time emission front position control capabilities for KSTAR with its new W divertors, addressing critical issues of heat flux management for future fusion devices while balancing the need to maintain plasma performance.