WITH LOCAL AND GLOBAL MODELS OF ANOMALOUS TRANSPORT OF THE L-H TRANSITION IN JET

A numerical study of the dynamics of the L-H transi- tion in JET was done using models of anomalous transport which depend either on local or on global plasma parameters. Comparison with experimental results shows that the best agreement is reached when using a global model for reducing ,ye, x, and D in the region where shear becomes large (e.g., outside the q = 1 surface). Thus the L-H transition manifests itself as a sudden drop of the transport coeffi- cients everywhere outside the q = 1 surface, not only in a narrow layer near the plasma edge. The slow improvement of plasma confine- ment which follows such a fast transition could then be attributed to the dependence on the local plasma parameters of the transport coeffi- cients in the plasma core.