Results of counterneutral beam injection on the DIII-D tokamak

Limiter, single-null, and double-null divertor plasmas have been heated by hydrogen neutral beam injection in the direction opposite to that of the plasma current. The H-mode power threshold is reduced by approximately 25% for counterinjection. Sawtooth suppression was possible only for high neutral beam power and for high values of the safety factor. No confinement improvement over similar coinjection discharges was observed in H-mode, L-mode, or limiter discharges. In plasmas without sawteeth, metallic impurity buildup in the central region of the plasma eventually leads to a reduction in confinement. The parametric dependence of energy confinement for both L-and H-mode plasmas was found to behave in the same manner during similar coinjection experiments. H-mode confinement increased linearly with increasing plasma current (65 msec/MA at 9 MW). Both L-mode and H-mode energy confinement decreased with increasing neutral beam power. At 1.25 MA, the L-mode plasmas had a confinement time that was less than that predicted by Kaye–Goldston scaling.