Description
We explore an inboard-limited ITB (Internal Transport Barrier) scenario as an alternative advanced operation mode for KSTAR. The key approach is to hold the plasma on the inboard limiter to suppress the H-mode transition during the early phase of neutral beam injection. ITBs were also observed in an up-shifted configuration, which is unfavorable for H-mode access and thus beneficial for reducing impurity accumulation and erosion damage [1].
Following the upgrade of the lower tungsten divertor in 2024, experiments were performed using an unfavorable flipped divertor configuration with a reversed toroidal magnetic field. In this tungsten environment, a ~60% increase in electron density required at least 50% higher neutral beam heating power to achieve ITB formation while facing the lower tungsten divertor.
In the 2025 campaign, plasma shape control was further developed to detach the plasma from the inboard limiter and transition to a fully diverted configuration. To ensure control stability near the separatrix, the plasma control algorithm was switched from an elongated limited phase to a lower-diverted isoflux configuration. Preventing a sudden H-mode transition during the detachment process was found to be critical. Since the inboard-limited shape without diverting is limited in pulse length due to heat flux on the limiter and impurity influx, the diverted shape is inevitable for longer discharge sustainment and future development. In the final experiments, both ITB and ETB (Edge Transport Barrier) were successfully achieved in a fully diverted plasma configuration.
[1] J. Chung, et al., Nucl. Fusion 61 (2021) 126051.