Transport with reversed shear in the National Spherical Torus Experiment

In the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)], plasmas with strongly reversed magnetic shear, s≡(r∕q)(dq∕dr)<0, in the plasma core exhibit a marked improvement in electron confinement compared to otherwise similar plasmas with positive or only weakly reversed magnetic shear. The q profile itself is determined by the early evolution of the plasma current, the plasma cross section, and the neutral-beam heating power. In the region of shear reversal, the electron thermal diffusivity can be significantly reduced. Detailed experimental investigation of this phenomenon has been made possible by the successful development of a motional Stark effect (MSE) polarimetry diagnostic suitable for the low magnetic field in NSTX, typically 0.35–0.55T. Measurements of the electron and ion temperature, density, and plasma toroidal rotation profiles are also available with high spatial and temporal resolution for analysis of the plasma transport properties.