Computational Study of Parallel Flows in High Density Plasmas in HSX*

E × B shear flow plays an important role in reducing turbulence and generating transport barriers. Quasisymmetric stellarators have an advantage over conventional stellarators in that the reduced parallel viscous damping could lead to large plasma flow shear. The Helically Symmetric eXperiment (HSX) is a quasihelically symmetric stellarator with constant |B| in the helical direction. Because parallel viscous damping is small, neutrals play a significant role in HSX for damping in the symmetry direction as well as damping of the parallel flow. Here, we present a study of the expected parallel flow velocity during high density operation in HSX which will be possible due to the installation of a new 70 GHz gyrotron. The increase in density results in a decrease of the atomic hydrogen neutral density by a factor of 5 in the upgraded plasmas from 1.0 × 10¹⁰ cm⁻³ to 0.2 × 10¹⁰ cm⁻³ at r/a = 0.44. Initial results based on a version of PENTA that has been adapted to include neutral damping [1], indicate the parallel flow in HSX increases roughly by a factor of 3 at r/a = 0.44. Results will be presented showing how the profile of the parallel flow in the upgraded plasma compares to that of the 28 GHz gyrotron at 1 T as well as plans to measure the parallel flow using the CHERS system [2].

[1] T.J. Dobbins et al., Nuclear Fusion 59, 046007 (2019)

[2] S.T.A. Kumar et al., Nuclear Fusion 57, 036030 (2017)