ELM power deposition in JET and ASDEX-U: Theory and experiment

The reference regime for a next step device like ITER is an H-mode associated with the occurrence of Type-I ELMs. The major limitation of divertor lifetime for such a device like ITER is the transient power load by ELMs. The power load is estimated by two basic parameters, the amount of lost energy and the deposition time of the energy on the target. The energy deposited within one ELM is in the order of some percent of the stored energy for all larger tokamaks (ASDEX-U, JET, DIII-D, JT60U) and may be described by the so called 'plug-in' effect. In this model one dominant parameter is the parallel transport time. Furthermore new measurements by IR-thermography at JET and ASDEX-Upgrade show a clear correlation between the parallel transport time and the time of the energy deposition itself. This supports the importance of the energy transport in the SOL from the origin to the target, complementing the physical picture of the MHD-instability causing the sudden expulsion of energy and particles. These new findings are compared to the theoretical picture of Type-I ELMs. In particular the possibility of a magnetic reconnection process between the H-mode pedestal and the SOL is discussed and compared to the experimental findings, e.g. temporal evolution of power profiles during ELMs. The expected values for the transient power load during Type-I ELMs is extrapolated to ITER-relevant discharge conditions. (orig.)