TRANSPORT ANALYSIS OF TRANSIENT PHENOMENA IN JET

A transport analysis of the different kinds of heat pulses in JET L an d H mode discharges has been made. These discharges include L-H transi tions, sawtooth crashes, cold pulses initiated by the impurity ablatio n and giant edge localized modes (ELMs). Analysis of experiments shows that all cold pulses initiated near the separatrix propagate inward o n a time-scale that is much shorter than the characteristic energy con finement time. On the other hand, outward propagation of the sawtooth heat pulse, if it is not influenced by the short lived ballistic effec t, can be easily described by the usual diffusive model. Two mechanism s of very fast propagation of the heat pulses have been tested. One is based on the assumption that plasma turbulence due to either toroidal or non-linear effects generates very long correlated structures acros s the magnetic field. In this case, the information about a sudden cha nge of the turbulence induced transport coefficients propagates along the radius with the group velocity of the unstable oscillations. The s econd mechanism is based on the idea of critical marginality, which im plies the existence of a finite threshold in T- for the excitation of the turbulence. If the thermal conductivity is sufficiently large abo ve the threshold, the temperature profile will adjust in such a way as to be close to marginal stability almost everywhere. Numerical analys is of the above mentioned heat pulses leads to the conclusion that alt hough the critical marginality models can reproduce some of the featur es of the global and local transport they, unlike the global model, fa il to reproduce the asymmetry in the time-scales observed during trans ient phenomena.