Transport analysis of transient phenomena in JET

Abstract

A transport analysis of the different kinds of heat pulses in JET L and H mode discharges has been made. These discharges include L-H transitions, sawtooth crashes, cold pulses initiated by the impurity ablation and giant edge localized modes (ELMs). Analysis of experiments shows that all cold pulses initiated near the separatrix propagate inward on a time-scale that is much shorter than the characteristic energy confinement time. On the other hand, outward propagation of the sawtooth heat pulse, if it is not influenced by the short lived ballistic effect, can be easily described by the usual diffusive model. Two mechanisms 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 across the magnetic field. In this case, the information about a sudden change of the turbulence induced transport coefficients propagates along the radius with the group velocity of the unstable oscillations. The second mechanism is based on the idea of critical marginality, which implies the existence of a finite threshold in T* for the excitation of the turbulence. If the thermal conductivity is sufficiently large above the threshold, the temperature profile will adjust in such a way as to be close to marginal stability almost everywhere. Numerical analysis of the above mentioned heat pulses leads to the conclusion that although the critical marginality models can reproduce some of the features of the global and local transport they, unlike the global model, fail to reproduce the asymmetry in the time-scales observed during transient phenomena