Simulations of edge configurations in quasi-helically symmetric geometry using EMC3-EIRENE

Simulations of the edge of a quasi-helically symmetric (QHS) stellarator with geometry based on the Helically Symmetric eXperiment (HSX) are performed using the coupled codes EMC3-EIRENE. The standard configuration of HSX has an island structure outside the separatrix, corresponding to the 8/7 resonance. In addition to the standard configuration, two other configurations are examined: one with small islands outside the separatrix corresponding to the 16/15 resonance, and one with large islands corresponding to the 4/4 resonance. Using EMC3-EIRENE, density scans are employed, while scaling input power linearly with density, in order to determine the transition point from a low- to a high-recycling regime. The small island and the standard cases show markedly similar behaviour, but the large island configuration transitions to high-recycling and detached regimes at significantly lower plasma densities. Reducing the perpendicular diffusion coefficients creates behaviour more consistent with two-point model predictions by reducing the role of perpendicular transport through edge islands and reducing friction loss from counter-streaming parallel flows. When carbon impurities are added, the large island configuration exhibits a large increase in radiated power, while the two configurations with smaller islands do not.