Numerical analysis of the heat flux characteristics in the helical scrape-off layer of the limiter start up configuration at Wendelstein 7-X

A crucial topic for the stellarator W7-X is the power dissipation by impurities for future island divertor scenarios. The investigation of the related heat flux distribution and profiles including the radial power fall-off length $\lambda_q$ in the 3D stellarator SOL is less straight forward as in toroidally symmetric tokamaks. Studies with the 3D plasma edge transport code EMC3-Eirene predicted a modulation of plasma parameters with $L_C$ and correlated heterogenous heat and particle loads onto the limiters during start-up operation. The relative simple start up geometry at W7-X allows for a detailed analysis of the heat fluxes in separate helical transport channels featuring different $\parallel$ to $\perp$ transport ratios. It is shown that the SOL has two characteristic fall off domains - a near SOL and a far SOL domain which both have different power decay lengths. An increase of $\lambda_q$ with $L_C$ in the order of ~1-1.5cm in the near SOL and ~1.8-2.8cm in the far SOL for a power scan in the range of $P=$0.5-2MW at $n_{LCFS}=2\times$10$^{18}$m$^{-3}$ has been found. First comparisons with IR camera data will be discussed.