Calculations of neoclassical viscous damping on flux surfaces near magnetic islands in the helically symmetric experiment

Neoclassical viscous damping rates have been calculated for the helically symmetric experiment [F. S. B. Anderson et al. Fusion Technol. 27, 273 (1995)], concentrating on the effects of magnetic surface shape changes due to magnetic islands. The interaction between the main helical spectral component and magnetic surface shape changes caused by an island chain introduces symmetry breaking spectral components on closed surfaces near the island chain. Results indicate that small islands present in the quasihelically symmetric configuration can give rise to an observable but not problematic deviation from quasisymmetry on surfaces near the islands. These islands can be eliminated via a small reduction in the rotational transform. Large islands can be produced by the introduction of the t=4/4 resonance inside the last closed magnetic surface. These islands result in not only a significant reduction in the volume of closed nested magnetic surfaces, but also an elevated level of viscous damping on magnetic surfaces adjacent to the islands.