EVOLUTION OF EDGE ELECTRIC-FIELD AT THE L TO H TRANSITION IN JET

Measurements of edge impurity ion parameters have been made in JET neu tral beam heated H-modes, using charge exchange spectroscopy. In gener al there is no evidence of any change in the impurity poloidal velocit y at the L to H transition (to within 5 km s(-1) in the best measureme nts). The upper limit of the changes in E(r) at transition due to chan ges in poloidal how is significantly smaller than those seen in smalle r devices. Evidence is seen of the rapid formation of a particle trans port barrier at the plasma edge and this leads to a modification of th e density profile (and also therefore the E(r) profile) at the separat rix. In established H-modes an extended region of step density gradien t and a gradient in toroidal rotation give a negative well-like struct ure to E(r).