Dynamics of core transport barrier formation and expansion in the DIII-D tokamak

While core transport barriers have been created in most large tokamaks, including DIII-D [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159], the underlying physics that governs their creation, expansion, and limitations has not been fully elucidated. Although negative central magnetic shear during a discharge aids in the creation of a core transport barrier, the model that has evolved to explain these results includes synergistic effects of magnetic shear and E×B velocity shear as the central elements. In DIII-D, the core barrier initially forms over an interval of several hundred milliseconds during the current ramp, with very low power applied. The barrier subsequently expands outward if the injected power is raised above a threshold, between 2.5 and 5 MW in DIII-D. Electrostatic turbulence reduces as the shearing rate increases to exceed the local turbulence growth rate while the transport barrier expands. Both the existence of the threshold and the barrier expansion with additional power are consistent with the theory.