Neoclassical currents and transport studies in HSX at 1 Tesla

With quasihelical symmetry and a high effective transform, calculations indicate that the Helically Symmetric Experiment (HSX) has small equilibrium currents and low neoclassical transport. Previous results at 0.5 Tesla indicated that particle, momentum and heat transport were all reduced compared to a configuration that mimics a conventional stellarator, in agreement with neoclassical estimates. First experimental results from 1 Tesla operation are presented showing that the small magnitude of the Pfirsch-Schlueter (PS) and bootstrap currents agree with calculations and that the PS current is helical due to the lack of toroidal curvature. With the same ECRH power, electron temperatures up to 2.5 keV at the plasma core were measured in the quasihelically symmetric (QHS) configuration compared to 1.5 keV when the symmetry is intentionally broken (termed 'Mirror'). These results at the higher field continue to demonstrate the expected neoclassical benefits of the quasihelical configuration. In this paper we also present an initial assessment of the effect of quasihelical symmetry on anomalous transport. A modified Weiland ITG/TEM tokamak model shows good agreement with the measured electron temperature profile outside the plasma core, as well as with the global energy confinement time. (author)