Speaker
Description
The island divertor concept in Wendelstein 7-X (W7-X) is the leading candidate for future stellarator reactors for handling heat and particle exhaust [1]. However, to date the study of this concept has primarily focused on experimental scale and shown that achieving detachment and high-recycling is more complex than standard tokamak poloidal divertors due to the complex 3D magnetic geometry. Only a few results at reactor-scale exist and these indicate a significant separation of the heat and particle flux channels [2], not observed in W7-X. Further study of heat and particle transport at reactor-scale is necessary to elucidate if this is problematic/beneficial for efficient heat/particle exhaust, respectively. To bridge the gap between experiment and reactor-scale performance, we simulate 3 different scales (1x, 2x, 4x) of W7-X using EMC3-Eirene [3]. The largest scale (4x) roughly corresponds to reactor-scale. For the W7-X standard magnetic configuration, we examine how increasing device size alters the divertor performance of the island divertor. We analyze how device size impacts location shifts of the ionization source and the downstream density and temperature trends in the island divertor for achieving detachment. For 2x scale, detachment is observed with pressure weighted averaged downstream temperatures $\leq10\text{eV}$ at separatrix densities $>3\times10^{-19}\text{m}^{-3}$. Additionally, we simulate Neon impurities to assess if reactor-scale W7-X leads to Ne concentrations compatible with reactor operation [4]. This analysis helps determine if density build-up and impurity retention for W7-X-like geometry is sufficient or needs improvement for a reactor. Finally, the heat and particle flux deposition on the scaled up island divertor configurations are calculated. Shifts in the heat flux distribution are seen with increasing separatrix density. To characterize the scrape-off layer (SOL) power exhaust and heat transport, we apply the approach developed in [5] for estimating the different SOL transport channel widths and examine how these vary with device size. Estimation of SOL transport channel widths has been instrumental for tokamak divertor configurations to connect the main heat transport mechanisms with the resulting divertor heat flux distribution, and is an area of ongoing work for stellarators [5]. This overall reactor-scale analysis will help guide island divertor improvements for future stellarator reactors.
[1] A. Bader et al, J. Plasma Phys. (2025)
[2] Y. Feng, J. Nucl. Mat. 438 (2013) S497-S500
[3] Y. Feng et al, Contrib. Plasma Phys. 44 (2004) 57-69
[4] E. Sytova et al, Nucl. Mater. Energy 19 (2019) 72-78
[5] A. Kharwandikar PhD Dissertation (2025)