Edge and internal transport barrier formations in CHS

Full text: Edge transport barrier (ETB) formation is found in a small low-aspect-ratio stellarator CHS (R = 1 m, B = 0.95 T) with the neutral beam injection (NBI) heating. A sharp drop (∼1 msec) of H alpha emission is observed at the transition and the clear back transition appears when the heating power is decreased. The local density measurements show the increase of edge density at the transition although the edge temperature does not increase much. The heating power threshold exists (0.7 MW for ne = 2.7x1019m-3), which is about two times larger than the tokamak H-mode scaling. When the heating power is increased, the internal transport barrier (ITB) is also created at the transition of ETB formation with NBI heating. The electron temperature in the plasma core region increases (without ECH) with the temperature gradient foot point at (r/a) ≅ = 0.5. The line averaged density for this ITB formation is more than 5 times higher compared to the previous ITB experiments in CHS with the combination heating of NBI and ECH. (author)