Edge plasma phenomena of the low density plasma to high density transition in the HL-1 tokamak

The transition of a low density plasma to high density in ohmic discharge was demonstrated in the HL-1 tokamak with metal wall and limiter configuration. The line averaged density n(e)BAR jumps up from 3-4 X 10(19) m-3 to 7-10 X 10(19) m-3 within 50-80 ms by switching off additional He pulse gas puffing into the D+ plasma and from 2.5-3 X 10(19) m-3 to 5-7 X 10(19) m-3 within 50-70 ms by turning off additional D2 pulse gas puffing. The density transition characterized by the improvement of confinement is associated with a change of the edge plasma parameters in terms of temperature, density and their profiles in SOL as well as neutral particle pressure and D(alpha) emission. Our investigations concentrated on the edge plasma phenomena in pure deuterium plasma and mixed deuterium and helium plasma before and after the density transition. Central impurity (OVI, CIII) line emission intensities and total radiation losses were also observed. The comparison of the experimental results of the edge plasma in low density with high density ones can be very informative for understanding the whole mechanism of confinement improvement.