Measurements of reduced neoclassical particle and electron heat transport with quasisymmetry in the HSX stellarator

The theoretical neoclassical transport in the quasihelically symmetric configuration (QHS) of the HSX stellarator is greatly reduced compared to a conventional stellarator. The symmetry can be intentionally spoiled using a set of auxiliary coils. Profiles of electron temperature and density have been measured with a Thomson scattering system. With central heating, the density profile is centrally peaked in QHS, while for non-symmetric plasmas the profile is hollow as has often been observed in conventional stellarators. The experimental particle flux has been inferred using 3D neutral gas modeling coupled to absolutely calibrated Halpha measurements. In the core of the plasma the neoclassical particle flux in the non-symmetric configuration is comparable to the experimental flux. This neoclassical flux is dominated by particle flux driven by the temperature gradient, showing that the flattening of the density profile without symmetry is caused by neoclassical thermodiffusion. In QHS, the neoclassical particle flux (including thermodiffusion) is reduced, leading to a peaked density profile. For the same launched power, the central electron temperature in the QHS configuration is significantly higher than that in the configuration without symmetry. Transport analysis has been performed on discharges in which the power has been adjusted to match the temperature profiles in the two configurations. The same absorbed power profile is used for both configurations, and the total absorbed power is measured using the Thomson scattering system. The resulting electron thermal diffusivity in the core increases from ∼2.5 m2/s in QHS up to ∼4 m 2/s as the symmetry is broken; this difference is comparable to the difference in neoclassical transport between the configurations.