2006 Volume 1 Pages 046
A drop in divertor plasma density was observed during the transition to the ELM free H-mode phase as well as the drop in Dα emission intensity in JT-60U. The time scale for the drop in divertor plasma density ranged from a few ms to several tens of ms. 2-D fluid divertor code simulations indicated that the time scale for the divertor plasma density drop was ranged from several tens of ms to a hundred ms, when thermal and particle diffusivities were instantly reduced in the main plasma edge with a constant recycling coefficient. When it was assumed that the recycling coefficient decreased with a decrease in heat flux and increased with a decrease in particle flux, the time scale was reduced to a few ms in the simulations accompanied by a reduction of edge thermal diffusivity. These results indicate that the sharp drop of the divertor plasma density concomitant with the sharp drop of the Dα emission intensity on a time scale of a few ms can be ascribed to the change of plasma-wall interaction depending on the heat flux to the divertor plates.