The formation and evolution of negative central magnetic shear current profiles on DIII-D

Using the combination of early neutral beam injection, ramping of the plasma current, low electron density and controlled L-H transitions, robust discharges with negative central magnetic shear are reproducibly obtained on the DIII-D tokamak. The effects of these factors on the formation and evolution of the q profile during the initial phase of these discharges are documented. Details of the evolution of the inverted q profile are obtained from measurements of the internal field pitch using a 16-channel motional Stark effect (MSE) system. Time-dependent MSE data are used to directly construct the profile of the toroidal electric field and allow a straightforward calculation of the non-inductive current density profile.