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Description
Power crossing the last closed flux surface is guided toward the divertor plates, where it deposits on a small area (wetted area), which in a reactor-scale device would result in an enormous heat flux. For given technical constraints, the maximum wetted area achievable in axisymmetric tokamaks is determined by the power decay length ($\lambda_{q}$) [1]. For Wendelstein 7-X (W7-X), the divertor concept relies on large magnetic islands formed at the plasma boundary to govern exhaust. The field-line pitch angle ($\theta_i$) within the boundary island is approximately two orders of magnitude smaller than that at the tokamak boundary, resulting in enhanced perpendicular transport comparable in magnitude to parallel transport [2]. Due to the non-axisymmetric distribution of heat loads and the complex situation of plasma surface interaction, the power decay length $\lambda_q$ as used in tokamaks, seems inappropriate to characterize the heat load in W7-X.
In W7-X, all ten divertor units are monitored by infrared thermography systems [3]. The 3D implicit anisotropic heat-diffusion solver DELVER (Divertor Energy Load Versatile EstimatoR) [4] has been developed to calculate the heat flux on the water-cooled divertor surfaces [5].
In this study, we introduce a novel approach to quantify the heat load in W7-X by defining an effective area parameter, $A_i$. This parameter is obtained by tracing magnetic field lines to map the heat flux distribution from the divertor targets to a selected poloidal cross-section [6, figure 12 therein]. Unlike previous estimations of wetted areas directly on the divertor plates [7]—which are strongly affected by the specific geometry of W7-X—our method decouples plasma transport effects from geometric influences such as divertor shape and field-line incidence. This separation is essential for gaining deeper physics insights and for enabling reliable extrapolation to reactor-scale stellarators. The analysis is performed for a range of plasma parameters with an aim to establish a scaling law for $A_i$ in terms of relevant parameters, such as $\theta_i$, as well as boundary plasma conditions.
References:
[1] T. Eich et al., Nucl. Fusion 60 (2020) 056016.
[2] Y. Feng et al., Nucl. Fusion 46 (2006) 807.
[3] M. Jakubowski et al. Rev. Sci. Instrum. 89(2018), 10E116.
[4] S. Thiede et al., submitted to Rev. Sci. Instrum.
[5] Y. Gao et al., Nucl. Fusion 59 (2019) 066007.
[6] Y. Gao et al., Nucl. Fusion 60 (2020) 096012 (14pp).
[7] H. Niemann et al., Nucl. Fusion 60 (2020) 084003.