Recent Experimental and Modeling Results from the HSX Stellarator

The Helically Symmetric eXperiment (HSX) is an optimized stellarator with a major radius of 1.2 m and a 1 Tesla magnetic field. Previous work has shown that the unique quasi-helical magnetic field configuration of HSX allows for large intrinsic flows in the direction of symmetry [1] and reduces neoclassical transport [2] so that transport is predominantly driven by trapped electron mode turbulence [3].HSX is equipped with a set of auxiliary coils which are used to study the effects of magnetic field geometry on plasma confinement.

Excellent plasma performance has been achieved following an intensive first wall reset, which involved removing coatings deposited on the plasma facing stainless steel wall, combined with a new vessel baking system and extensive glow discharge cleaning. The resulting reduction in impurity content has driven substantial improvements in plasma profiles, including core electron temperatures well above 2 keV and higher overall energy confinement.

During studies with different main-ion species, more pronounced pressure profile peaking has been measured in hydrogen fueled plasmas than with helium fueling. This is consistent with non-linear GENE [4] simulations which predict reduced turbulent heat fluxes in hydrogen plasmas. Although previous work suggested the strong electron temperature peaking observed in the core of HSX is a result of turbulent suppression by ExB flow shear, recent analysis of correlation ECE measurements have shown increased radiation temperature fluctuations in regions with high electron temperature gradients. These measurements show qualitative agreement with non-linear gyrokinetic flux tube simulations with GENE.

[1] A. Briesemeister et al., 2010 Contrib. Plasma Phys. 50 8

[2] J. Canik et al., 2007 Phys. Rev. Lett. 98 8

[3] W. Guttenfelder et al., 2008 Phys. Rev. Lett. 101 21

[4] F. Jenko et al., 2000 Phys. Plasmas 7 1904

[5] M.J. Gerard et al., 2023 Nucl. Fusion 63 056004