Effects of velocity shear and magnetic shear in the formation of core transport barriers in the DIII-D tokamak

Core transport barriers can be reliably formed in DIII-D by tailoring the evolution of the current density profile. This paper reports studies of the relative role of magnetic and shear in creating core transport barriers in the DIII-D tokamak and considers the detailed dynamics of the barrier formation. The core barriers seen in DIII-D negative shear discharges form in a stepwise fashion during the initial current ramp. The reasons for the stepwise formation are not known. Their extremely good shot to shot reproducibility suggests the steps are connected to a very reproducible plasma parameter such as the current density profile; however, the simple hypothesis that they occur each time q(0) or crosses an integer value is not consistent with all the data. The data from DIII-D are consistent with previous results that negative magnetic shear facilitates the formation of core transport barriers in the ion transport channel but is not necessary. However, strongly negative magnetic shear does allow formation of transport barriers in particle, electron thermal, ion thermal and angular momentum transport channels. Shots with strong negative magnetic shear have produced the steepest ion temperature and toroidal rotation profiles seen yet in DIII-D. In addition, the shearing rates seen in these shots exceed the previous DIII-D record value by a factor of four.