Evolution of edge electric field at the L to H transition in JET

Measurements of edge impurity ion parameters have been made in JET neutral beam heated H-modes, using charge exchange spectroscopy. In general there is no evidence of any change in the impurity poloidal velocity at the L to H transition (to within 5 km in the best measurements). The upper limit of the changes in at transition due to changes in poloidal flow is significantly smaller than those seen in smaller devices. Evidence is seen of the rapid formation of a particle transport barrier at the plasma edge and this leads to a modification of the density profile (and also therefore the profile) at the separatrix. In established H-modes an extended region of step density gradient and a gradient in toroidal rotation give a negative well-like structure to .