Observations of enhanced core confinement in negative magnetic shear discharges with an L mode edge on DIII-D

Abstract

Plasma discharges with negative central magnetic shear (NCS) and access to the ballooning second stability regime have been produced on DIII-D with edge plasma conditions similar to those in the standard L mode confinement regime. Confinement enhancement factors up to H identical to tau _E/ tau _ITER-89P~2.5 are obtained while maintaining the L mode edge. Compared with discharges with monotonic q profiles, highly peaked toroidal rotation (f_phi (0) approximately=30-70 kHz), ion temperature (T_i(0) approximately=15-22 keV) and density (n_e(0)/n_e approximately=2.2) profiles are observed. This regime has yielded the highest DD neutron rates observed to date on DIII-D. Steep pressure gradients drive a large bootstrap current, with up to ~75% total non-inductive current drive. These features make this an attractive advanced tokamak operating regime for further study