Study of edge electric field and edge microturbulence at the L-H transition in DIII-D

Experimental studies of the L-H Transition in the DIII-D tokamak clearly show that the edge poloidal rotation velocity and the shear in the edge electric field increase dramatically at the L-H transition, and that the edge density fluctuations are simultaneously suppressed in a narrow region just within the separatrix. The greatest changes in the poloidal rotation and electric field are within 3 cm of the separatrix and overlap with the the region of suppressed microturbulence. Furthermore, this zone of sheared electric field and suppressed fluctuations also marks the location of the transport barrier, as indicated by the increased gradients of Ti, ne, Te and the carbon density after the transition. The shear in the edge electric field is maintained later into the H-mode with the sign of the electric field remaining negative as a result of the pressure gradient contribution to the total electric field becoming more significant than the rotational term, which becomes more positive with time into the H-mode.(author) 17 refs., 5 figs