Progress towards sustainment of internal transport barriers in DIII-D

Neutral beam heated discharges with internal transport barriers (ITB) have been observed in most of the world's major tokamaks including DIII-D, TFTR, JT-60U and JET. Improved core confinement has been observed over a range of q profiles, including negative central magnetic shear (NCS) and weak positive shear. In discharges with an L-mode edge, the duration of ITBs is generally limited to a few energy confinement times (τe) by low-n MHD activity driven by the steep core pressure gradient or by qmin passing through low-order rational values. In order for ITBs to be useful in achieving a more compact advanced tokamak, the radius of the ITB must be expanded and the pressure gradient must be controlled in order to optimize βN and the bootstrap alignment. In this paper, the authors discuss the results of recent experiments on DIII-D to produce and sustain ITBs for longer pulse lengths. Three techniques are evaluated: (1) reduction of neutral beam power (PNBI) and plasma current to form a weaker ITB which can be sustained; (2) use of an ELMing H-mode edge to help broaden the pressure profile and improve MHD instability; (3) modification of the plasma shape (squareness) to achieve smaller ELMs with a lower density pedestal at the edge