The first experimental assessment of low-Z impurity transport in a neutral beam heated, high-confinement H-mode plasma sustained in a low-field, low-aspect ratio spherical tokamak, was performed at the National Spherical Torus Experiment (NSTX). The injected impurities penetrate to the core on a hundred millisecond time scale, indicating a low core particle diffusivity (≲1 m2 s−1) in good agreement with the values predicted by neoclassical transport theory. In addition, a fixed q-profile magnetic field scan that showed reduced impurity penetration at high fields is also reported. This result suggests that anomalous ion particle transport associated with turbulent long-wavelength electrostatic instabilities must be largely suppressed in the NSTX core.
Confinement and local transport in the National Spherical Torus Experiment (NSTX)