Speaker
Description
Since the installation of the actively cooled lower tungsten (W) divertor in 2023, KSTAR has conducted four experimental campaigns (>7,200 discharges; avg. 6.5–7 MW, ~10 s) addressing critical plasma-surface interactions and optimizing divertor performance. Previously, KSTAR operated with carbon wall for 15 years (~32,800 discharges). This overview synthesizes key physics and engineering results from the campaigns with the W divertor
Initial experiments confirmed that W impurities transported into the core plasma significantly degrade performance, with 2D radiation profiles measured by Infra-Red Video Bolometer (IRVB) revealing a distinct outboard-localized radiation power loss. To rigorously investigate these phenomena, diagnostic capabilities were significantly enhanced, including the development of W-flux monitors and real-time IRVB systems. Additionally, impurity transport codes were utilized to derive W concentration profiles by analyzing 2D radiation, Vacuum Ultraviolet Spectroscopy, and Compact Advanced EUV Spectrometer (CAES) measurements.
A novel “β-kicking” NBI heating scenario was developed. By optimizing NBI power injection immediately after the H-mode transition, this scenario mitigated impurity accumulation and restored plasma performance to levels comparable to the carbon-divertor phase.
Divertor detachment control was extensively explored using neon (Ne) and nitrogen (N) injection. While N injection effectively controlled the ion saturation current measured by Langmuir Probes (LPs) at the striking point on the outer divertor target, high injection rates led to increased W concentration in the core plasma. This trade-off highlights the necessity of optimizing N gas injection rates. Advanced real-time control systems were also deployed, including a feedback algorithm utilizing real-time 2D IRVB data and a UEDGE-based surrogate model. Furthermore, a novel Machine Learning-based virtual diagnostic system, trained on Absolute Extreme Ultraviolet, LPs, and historical IRVB data, demonstrated the capability to predict 2D radiation profiles and control the radiation front without direct real-time IRVB input.
Wall conditioning strategies have evolved to support high-performance operations. Impurity Powder Dropper (IPD) boronization provided a quantitative assessment of W-source reduction at the divertor target. Notably, the IPD system is now integrated into the Plasma Control System, enabling feedback-controlled injection to optimize boronization specifically for long-pulse discharges. In preparation for the full-W wall transition after the 2026 campaign, these advanced conditioning techniques, along with improved ECWC scenarios, are being established as essential tools for ensuring successful plasma start-up and effective wall conditioning.
Finally, material challenges were identified. Localized melting of W monoblocks and structural issues, such as dislodgement of the W plate, occurred. These incidents and implications for future operations are discussed.