Reduced particle and heat transport with quasisymmetry in the Helically Symmetric Experiment

Measurements of particle and heat transport have been made in the Helically Symmetric Experiment [F. S. B. Anderson et al., Fusion Technology 27, 273 (1995)]. Experimental differences in the density and temperature profiles are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The electron temperature is higher in the QHS configuration, due to a reduction in electron thermal diffusivity that is comparable to the neoclassical prediction. The density profile in plasmas with the symmetry broken is measured to be hollow, while in QHS plasmas the profile is centrally peaked. Calculations of the radial particle flux using the DEGAS code [D. Heifetz et al., J. Comput. Phys. 46, 309 (1982)] show that the hollow profile observed with the symmetry broken is due to neoclassical thermodiffusion. Thermodiffusion is reduced in the QHS configuration, resulting in a peaked density profile.