Behaviour of electron and ion transport in discharges with an internal transport barrier in the DIII-D tokamak

The article reports results of experiments to further determine the underly ing physics behind the formation and development of internal transport barr iers (ITBs) in the DIII-D tokamak. The initial ITB formation occurs when th e neutral beam heating power exceeds a threshold value during the early sta ges of the current ramp in low density discharges. This region of reduced t ransport, made accessible by suppression of long wavelength turbulence by s heared flows, is most evident in the ion temperature and impurity rotation profiles. In some cases, reduced transport is also observed in the electron temperature and density profiles. If the power is near the threshold, the barrier remains stationary and encloses only a small fraction of the plasma volume. If, however, the power is increased, the transport barrier expands to encompass a larger fraction of the plasma volume. The dynamic behaviour of the transport barrier during the growth phase exhibits rapid transport events that are associated with both broadening of the profiles and reducti ons in turbulence and associated transport. In some but not all cases, thes e events are correlated with the safety factor q passing through integer va lues. The final state following this evolution is a plasma exhibiting ion t hermal transport at or below neoclassical levels. Typically the electron th ermal transport remains anomalously high. Recent experimental results are r eported in which RF electron heating was applied to plasmas with an ion ITB , thereby increasing both the electron and the ion transport. Although the results are partially in agreement with the usual E x B shear suppression h ypothesis, the results still leave questions that must be addressed in futu re experiments.