Scrape Off Layer (SOL) currents and their role in divertor sheath drop and heat flux redistribution in W7-X

In magnetically confined fusion plasmas, scrape-off layer (SOL) currents are commonly observed and have been shown to influence the heat flux profile. Although extensively studied in tokamaks, a systematic investigation of these currents in the three-dimensional SOL geometry of stellarators is still lacking. In Wendelstein 7-X, parallel SOL currents are routinely measured with the divertor Langmuir probes. We present a parametric study of these currents under varying heating power and line-integrated density. Substantial parallel currents are observed in long connection-length regions under attached conditions. The current measured at the divertor and near midplane tend to become more negative with increasing density. Stellarator-symmetric locations on the upper and lower divertors collect currents of the same polarity, suggesting that the current due to the pressure dependent non-ambipolar drifts dominate over thermoelectric current.

At low densities, a strong up-down asymmetry is seen in the divertor current profiles, which appears to be linked to island-poloidal ExB drifts that modify the SOL pressure profiles and, consequently, affect the current profiles. The SOL currents modify the floating potential and sheath drop between plasma and the target voltage, affecting electron flux. Accounting for them reveals a heat flux redistribution in the SOL. The results show the importance of including SOL currents in sheath and heat-flux models for predictive stellarator divertor design