A new type of internal transport barrier (ITB) has been observed in JT-60U reversed shear discharges. It accompanies clear electron temperature and density pedestals and significant reduction of effective thermal diffusivities of electrons and ions ( ${\chi }_e^{\mathrm{eff}}$ and ${\chi }_i^{\mathrm{eff}}$); ${\chi }_e^{\mathrm{eff}}$ sharply drops by a factor of 20 within 5 cm while ${\chi }_i^{\mathrm{eff}}$ is smaller than the conventional neoclassical value by a factor of 4 or more. The ratio of ion temperature to electron temperature was less than 1.5 inside the ITB. The ITB lies in the negative shear region and extended beyond 60% of the plasma minor radius. High density plasmas with high confinement have been obtained with the formation of ITB.

• Received 29 May 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.2377