Neutral beams were injected into low field (B = 0.7-1.0 T) deuterium plasmas in an attempt to destabilize toroidicity induced Alfve'n eigenmodes (TAE modes). When the parallel beam velocity approached the Alfve'n velocity and the volume averaged beam beta exceeded 2%, localized, propagating modes with n = 2-10 were observed. As much as 45% of the beam power was lost as a result of the modes. The threshold for TAE instability is at least one order of magnitude higher than that predicted by Fu and VanDam (Phys. Fluids B 1 (1989) 1949).

ABSTRACT. Although most DIII-D plasmas are stable to the fishbone instability, fishbones are sometimes observed when /3 p £ 1.5 and ^ 5 5 x 10 13 cm" 3 . These bursts are usually of minor significance operationally; however, under one condition, over 50% of the beam power was lost. The angle of beam injection has little effect on the virulence of the instability, suggesting that the fishbone instability in DIII-D is the ion diamagnetic branch of the internal kink.

Experiments to explore the long-time evolution of noninductive, high /3 p plasmas in the 0111-0 159], 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 qo ( > 2) and moderate Ij( -1.3). It is reached by slow relaxation of the current profile, on the resistive time scale. As the profiles relax, qo rises and Ii falls. When qo goes above 2 (approximately), magnetohydrodynamic (MHO) 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 (;>1 X 10 19 m-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.

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