Abstract
Very steep internal transport barriers (ITBs) have been observed in allfour transport channels on DIII-D. These ITBs are among the mosthighly localized (width⩽5 cm), simultaneous core transport barriersobserved on any machine to date, and have only been observed in dischargeswith a negative central magnetic shear (NCS discharges), at power levelsabove ~8 MW. Profile gradients and scale lengths at the location ofthe core (ρ~0.3-0.4) transport barriers are similar to thoseobserved at the plasma edge during H-mode, while profiles inside thetransport barriers are flat. The spatial location of the transport barrierscoincides in all four transport channels, although the temporal evolutionof the profiles is different; very steep gradients in the electron densityand temperature profiles form after such gradients are first observed inthe ion temperature and angular momentum profiles. Turbulence measurementsduring the evolution of the core particle transport barrier show nodecrease in low-wavenumber (0-6 cm-1) turbulence levels in theplasma centre, including within the steep gradient ITB region. Severalpossible interpretations for this observation are presented.
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