Progress Towards Sustainment Of Internal Transport Barriers In Diii D

Download or read book Progress Towards Sustainment of Internal Transport Barriers in DIII-D. written by and published by . This book was released on 1998 with total page 5 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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 ([tau]{sub e}) by low-n MHD activity driven by the steep core pressure gradient or by q{sub min} 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 [beta]{sub N} 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 (P{sub NBI}) 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.