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.