Identification of the helical scrape-off layer structure and transport properties in the edge of HSX

Author / Creator

Akerson, Adrian Robert, author

Available as

Online

Physical

Summary

The first detailed study of edge transport properties in the Helically Symmetric Experiment (HSX) in the quasi-helically symmetric configuration is presented. In this configuration, the edge of HSX...

The first detailed study of edge transport properties in the Helically Symmetric Experiment (HSX) in the quasi-helically symmetric configuration is presented. In this configuration, the edge of HSX is characterized by a prominent 8/7 magnetic island remnant that is marginally intercepted by the vessel, forming a complex divertor-like structure. The goal of this research is to understand the role of this complex magnetic structure on the heat and particle transport, and neutral and impurity sourcing. Detailed comparisons of the measurements to edge simulations using the EMC3-EIRENE 3D plasma fluid and kinetic neutral gas transport model are performed. The plasma structure and related transport in this anisotropic, 3D plasma boundary at HSX was investigated using a moveable Langmuir probe. The measurements show that particle transport is diffusive within the island region and dominantly convective in the SOL region with evidence for low cross-field diffusion coefficient in the SOL. In contrast, the theory/simulation comparison with experimental data suggests that the heat transport is dominated by radial heat diffusion across the SOL domain which exceeds the convective transport. Parallel flow measurements show, for the first time, evidence of counter-streaming ion flows in the edge of a low-shear stellarator. In addition, measurements show a strong flow near the island X-point. This X-point flow pattern and magnitude is not predicted by the EMC3-EIRENE modeling, pointing towards uncertainties in the edge particle source distribution. Electrostatic potential measurements show a potential hill centered at the island O-point. This feature can drive ExB flows and affect cross-field transport which are not modeled in EMC3-EIRENE. The impact of these flows on SOL transport is discussed along with the link to the plasma source/sink relation. These results provide a detailed overview of 3D scrape-off layer transport in HSX, and can inform results from low-shear stellarators with island divertors like W7-X.

Show More