Description
Since the 2023 experimental campaign, the KSTAR lower divertor has been replaced with tungsten (W) monoblock components. Following this transition to a metallic wall environment, degradation of pedestal performance was observed compared with the former carbon wall environment. In particular, the thermal pedestal temperature decreased to ~0.3 keV under ~5 MW of auxiliary heating power, whereas values exceeding 0.7 keV were routinely obtained with carbon walls. The line-averaged electron density increased by 30–40%, indicating modified edge particle balance and recycling conditions. Moreover, intense ELM bursts triggered at the L–H transition significantly enhanced the tungsten source, promoted impurity penetration into the confined plasma, and increased radiative losses. These effects prevented robust H-mode sustainment and frequently induced repetitive transition and back-transition dynamics.
To mitigate these issues, we implemented a transition-focused ELM control strategy aimed at increasing ELM frequency while reducing individual ELM energy loss. The onset of plasma shaping for divertor configuration was advanced from 1.5 s to 0.5 s, and the L–H transition timing was correspondingly shifted from after 1.5 s to before 1.0 s. Owing to KSTAR’s relatively slow plasma current ($I_P$) ramp-up rate, this approach enabled the L–H transition to occur at lower $I_P$ during high $I_P$ plasma operation, resulting in smaller ELMs within the same $I_P$ regime. In addition, localized D2 gas injection into the divertor region was employed to regulate ELM frequency and amplitude, thereby reducing tungsten sputtering and maintaining effective impurity exhaust. As a consequence, pedestal performance and global confinement were restored to levels comparable to carbon wall. In discharges exceeding 10 s, controlled gas fueling during the $I_P$ flat-top phase influenced edge/SOL conditions and resulted in a sustained increase in normalized beta ($β_N$), demonstrating the favorable impact of integrated core–edge–SOL optimization in a tungsten divertor environment.