LOSS OF CONFINEMENT DUE TO REDUCTION OF THE EDGE PEDESTAL IN JET

There are various conditions in JET under which the energy confinement of a plasma is reduced from H mode towards L mode levels. Examples ar e reported here for which the loss of confinement is associated with a reduction of the electron pressure pedestal. This loss is brought abo ut by strong gas puffing that increases the frequency of edge localize d modes (ELMs). The effect of ELMs on confinement is modelled, and use d to explain the observed degradation of confinement. The model is gen eralized for application to other machines.