EFFECTS OF E X B 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 studi es of the relative role of magnetic and E x B sheer in creating core t ransport barriers in the DIII-D tokamak and considers the detailed dyn amics of the barrier formation. The core barriers seen in DIII-D negat ive shear discharges form in a stepwise fashion during the initial cur rent ramp. The reasons for the stepwise formation are not known. Their extremely good shot to shot reproducibility suggests the steps are co nnected to a very reproducible plasma parameter such as the current de nsity profile; however, the simple hypothesis that they occur each tim e q (0) or q(min) crosses an integer value is not consistent with all the data. The data from DIII-D are consistent with previous results th at 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 bar riers in panicle, electron thermal, ion thermal and angular momentum t ransport channels. Shots with strong negative magnetic shear have prod uced the steepest ion temperature and toroidal rotation profiles seen yet in DIII-D. In addition, the E x B shearing rates seen in these sho ts exceed the previous DIII-D record value by a factor of four.