LIMITER BIAS EXPERIMENTS IN DIII-D

In DIII-D, H-alpha emission was reduced by a factor of 2-3 both at the limiter and at the inner wall by negative limiter biasing (V(lim) almost-equal-to -300 V). In these discharges, the line-average electron densities were approximately the same with and without limiter bias, which suggests that tau-p was improved by negative bias. Edge bremsstrahlung emission signals were reduced by a factor of 2. Repetitive bursts were observed in H-alpha signals and edge bremsstrahlung signals. Impurity levels increased with limiter bias, which induced major disruptions. No improvement of tau-E was observed. These experimental data suggest that particle confinement characteristics of limiter biased discharges are close to those with divertor H-mode.