The effect of ELMs on energy confinement in JET has been analysed. ELMs are characterized using emission which is decomposed into two components, a baseline level with superimposed pulses due to the ELMs. The analysis of the experimental data shows that the baseline, which reflects the neutral pressure at the plasma edge, is an important parameter in determining the energy confinement deterioration. The origin of the baseline is either from the neutralization of plasma particles which are expelled by the ELM pulses, or from external gas puffing. An ELM severity parameter, taking into account both the baseline and the ELM pulses, is defined. The energy confinement time normalized to the energy confinement time of ELM free phase, , decreases linearly as the ELM severity increases. The results are independent of divertor configurations.
Introduction to the ELMs special issue