Speaker
Description
A series of experiments with plasma shaping scans were conducted in WEST and KSTAR illuminating the respective role of lower divertor impurity sourcing (high impurity production and screening) compared to the non-divertor regions (lower impurity production and screening) and the non-intuitive resulting impact on W core contamination. Since the new ITER baseline was presented in 2024, W has become the leading candidate Plasma-Facing Component (PFC) in next-step fusion devices. With extended pulse length relevant for ITER scenario development, PFCs will have to withstand massive cumulative particle fluences leading to significant deposition over long time scales not yet experienced on short pulse devices. The lack of understanding in both managing the release of W into the Scrape-Off Layer (SOL) to maintain performance (avoiding radiative collapse via high-Z core accumulation) and controlling material build-up without creating copious redeposition layers are serious uncertainties in qualifying W as a viable PFC material in future devices. A series of experiments were conducted in KSTAR and WEST to characterize impurities by varying strategically the plasma shape in front of hypothetically high impurity sourcing regions. In KSTAR, the lower X-point height was increased in upper (USN) and lower (LSN) single null to see the effect of the screening efficiency of the lower divertor. The impurity sourcing and core contamination increased with the lower X-point height. In WEST, the primary separatrix driven away from the upper divertor in LSN led to the upper divertor source being strongly reduced (-90% deposited power, -40% radiated power, -60% of W and -80 to -90% of light impurity source) while the lower inner target source significantly increased (+70% heat load, +40% radiated power, +55% of W and +180 to +380% of light impurity source) measured from the recently developed integrated multidiagnostics suite. Reducing the expectedly poorly screened upper divertor impurity source contribution but increasing the strongly screened lower inner divertor source should be beneficial for plasma performance by reducing the W core concentration. However, experimental results showed only a slight decrease of the core radiated power (-6%), while central Te decreased (-11%) and W central concentration increased (+27%). Interpretative modelling with the SOLEDGE3X-ERO2.0 codes will be presented to discuss the non-intuitive experimental results from WEST and KSTAR, highlighting the complex interplay between sources and core pollution.
This work is supported by the U.S. DOE under Grant Number DE-SC0020414.