KSTAR PI Controller Stabilizes ELM Frequency at 30 Hz Steps, Halting Exponential Tungsten Core Radiation Rise

The study deployed a PI feedback loop on KSTAR's tungsten lower divertor, actuating D2 puffing to regulate f_ELM in real time. Two-step references were followed across 7-second intervals with mean absolute percentage error of 13%, confirming actuator authority without inducing the performance degradation associated with excessive gas injection. Volume-integrated core radiation remained stable versus reference shots, yet the absence of any puffing produced exponential tungsten accumulation, highlighting the narrow operational window between impurity flushing and fuel dilution.

Tungsten sputtering and neoclassical transport in high-Z divertor configurations pose a central barrier for ITER and DEMO, where uncontrolled ELMs allow impurity accumulation that can quench fusion power. This KSTAR result extends prior open-loop gas pacing studies on JET and ASDEX Upgrade by closing the loop on frequency, directly addressing the longevity requirement for sustained high-performance pulses. The work reveals that active optimization of gas command trajectories is essential; static or manual puffing either under-flushes tungsten or over-dilutes the plasma, eroding Q.

Next steps require integration with real-time tungsten spectroscopy diagnostics and multi-actuator schemes that combine gas puffing with resonant magnetic perturbations. Scaling to ITER-relevant conditions will demand validation on devices with higher triangularity and longer pulse lengths to confirm that controlled ELM frequencies preserve both core confinement and divertor heat flux mitigation simultaneously.