Observation of simultaneous internal transport barriers in all four transport channels and correlation with turbulence behaviour in NCS discharges on DIII-D

Very steep internal transport barriers (ITBs) have been observed in all four transport channels on DIII-D. These ITBs are among the most highly localized (width⩽5 cm), simultaneous core transport barriers observed on any machine to date, and have only been observed in discharges with a negative central magnetic shear (NCS discharges), at power levels above ~8 MW. Profile gradients and scale lengths at the location of the core (ρ~0.3-0.4) transport barriers are similar to those observed at the plasma edge during H-mode, while profiles inside the transport barriers are flat. The spatial location of the transport barriers coincides in all four transport channels, although the temporal evolution of the profiles is different; very steep gradients in the electron density and temperature profiles form after such gradients are first observed in the ion temperature and angular momentum profiles. Turbulence measurements during the evolution of the core particle transport barrier show no decrease in low-wavenumber (0-6 cm-1) turbulence levels in the plasma centre, including within the steep gradient ITB region. Several possible interpretations for this observation are presented.