Coordinated Gas and Pellet Fueling for Plasma Density Control in KSTAR Under Reactor-Relevant Conditions

Precise regulation of plasma density is critical for sustained and optimized fusion power output in reactor-grade tokamaks. These future tokamaks will utilize both gas puffing and pellet injection as standard fueling mechanisms to achieve the desired performance. However, since the coordinated use of gas and pellets in feedback control remains largely unexplored, it is of utmost importance to develop and validate suitable fueling feedback strategies in existing machines. These strategies need to address the expected challenges associated with gas puffing and pellet injection such as actuation time delays, unknown fueling efficiencies, and mixed continuous-discrete time dynamics. Due to these challenges, advanced control techniques beyond traditional PID will be required. Consequently, a model-based feedback control algorithm has been developed for the KSTAR device, leveraging its reactor-relevant operational capabilities. Such capabilities have been recently enhanced by the implementation of a NERF-mode algorithm to emulate reactor-constrained actuation. The algorithm coordinates gas and pellet actuators to regulate plasma density around desired levels while accounting for actuator constraints. The performance of the controller is examined in simulations, demonstrating robustness in tracking the targets despite actuation time delays and plasma variability, and indicating readiness for experimental testing.