Physics of L and H mode confinement in JET

The results of numerical simulation of JET L, H and VH modes are presented. it is shown that L-H transitions are accompanied by a fast and large modification of the transport coefficients not only near the plasma edge but also across a large fraction of the plasma cross-section. Both experiments and numerical simulations show that giant ELMs in JET also have a global character and their penetration length increases with the amplitude of the D-alpha signal. Transport coefficients are of the order of the L mode values during these ELMs. Analysis shows also that within experimental accuracy the transport coefficients are the same for VH and ELM-free H mode discharges. The variation in the global energy confinement is mainly due to the differences in the impurity radiation, power deposition profiles and plasma recycling and related convection.