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Description
The divertor plays a crucial role in plasma and particle exhaust.
After heating the surrounding plasma, He ions need to be diverted through the scrape-off layer into the divertor chamber.
Here plasma neutralizes on divertor targets in high-flux regions, known as strike lines.
In Wendelstein 7-X (W7-X) this exhaust is accomplished via an island divertor, where resonant magnetic islands intersected by the divertor target plates form the scrape-off layer and private flux region.
In W7-X’s standard configuration one dominate strike line per divertor module is typically observable.
However, with increased plasma beta $\beta$, defined as the ratio between the plasma pressure and magnetic pressure $(\beta = p_{plasma}⁄(B^2⁄(2\mu_0)))$, a secondary strike line appears $\left[1\right]$.
This additional strike line may be linked to changes in divertor shadowing or to variations in perpendicular and localized transport effects.
The strike lines of experiment programs with the available range of $\beta$ were evaluated.
Their poloidal and toroidal dimensions and magnitudes were characterized.
To investigate the underlying mechanisms, a simulation of the strike lines, using EMC3-Lite $\left[2\right]$, is being performed.
The simulation is compared to experimental results, measured via the infrared and H$_\alpha$ imaging diagnostic systems $\left[3, 4\right]$.
The study examines different $\beta$ effects on the magnetic topology and key parameters, including the perpendicular diffusion coefficient, to assess their influence on secondary strike line formation and to identify the driving physical processes.
$\left[1\right]$ Y. Gao et al 2024 Nucl. Fusion 64 076060
$\left[2\right]$ Y. Feng et al 2022 Plasma Phys. Control. Fusion 64 125012
$\left[3\right]$ J. Marcin et al 2018 Review of Scientific Instruments 89 10E116
$\left[4\right]$ T. Kremeyer et al 2025 JINST 20 T04003\