Evolution of high [beta][sub [ital p]] plasmas with improved stability and confinement

Experiments to explore the long-time evolution of noninductive, high [beta][sub [ital p]] plasmas in the DIII-D tokamak [[ital Plasma] [ital Physics] [ital and] [ital Controlled] [ital Nuclear] [ital Fusion] [ital Research], 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159], have identified a new, quiescent, high performance regime. The experiments were carried out at low current (400--800 kA) with medium power neutral beam injection (3--10 MW). This regime is characterized by high [ital q][sub 0] ([gt]2) and moderate [ital l][sub [ital i]]([similar to]1.3). It is reached by slow relaxation of the current profile, on the resistive time scale. As the profiles relax, [ital q][sub 0] rises and [ital l][sub [ital i]] falls. When [ital q][sub 0] goes above 2 (approximately), magnetohydrodynamic (MHD) activity disappears, and the stored energy rises. Most dramatic is the strong peaking of the central density, which increases by as much as a factor of 2. The improved central confinement appears similar to the PEP/reversed central shear/second stable core modes seen in tokamak experiments, but in this case without external intervention or transient excitation. At high current, a similar, but slower relaxation is seen. Also notable in connection with these discharges is the behavior of the edge and scrape-off layer (SOL). The edge localized modes (ELM's) as seen previously, are small and very rapid (to 1 kHz). The SOL exhibits high density ([ge]1[times]10[sup 19] m[sup [minus]3]), which shows little or no falloff with radius. Also the power deposition at the divertor surface is very broad, up to four times the width usually seen. This regime is of particular interest for the development of steady-state tokamak operating scenarios, for the Tokamak Physics Experiment (TPX), and following reactors.