ELM Processes and Properties in 2T 2MA JET Plasmas

During July 2012, 150 almost identical H-mode plasmas were consecutively created in the Joint European Torus (JET), providing a combined total of approximately 8 minutes of steady-state plasma with 15,000 Edge Localised Modes (ELMs). In principle, each of those 15,000 ELMs are statistically equivalent. Here the changes in edge density and plasma energy associated with those ELMs are explored, using the spikes in Beryllium II (527 nm) radiation as an indicator for the onset of an ELM. Two timescales are found during the ELM process. The first timescale is associated with the density drop, and the second (longer) timescale is associated with a reduction in MHD energy that is consistent with a resistive relaxation of the plasma's edge. The statistical properties of the energy and density losses due to the ELMs are explored. Surprisingly the commonly reported link between ELM energy (E) and ELM frequency (f), of E proportional to 1/f, is not found. Instead beyond the first 0.02 seconds of waiting time between ELMs, the energy losses due to the ELMs are found to be statistically the same. Surprisingly no correlation is found between the energies of consecutive ELMs either. A weak link is found between the density drop and the ELM waiting time. Consequences of these results for ELM control and modelling are discussed.