Pedestal Dynamics in ELMy H-mode Plasmas in JET

The pedestal ELM dynamics has been studied on JET with improved diagnostic capability. The new High Resolution Thomson Scattering system enables detailed measurement of Te and ne pedestal profiles. A simple ELM coherent data selection technique is applied to select pre and post ELM profiles. Initial estimates using the radial profiles of ELM filaments occurring in a short time window immediately after an ELM indicate that the filaments can carry up to 30% of the ELM lost energy. On a longer time scale of 0.1-1ms after an ELM the electron density profile on JET is seen to evolve differently from the electron temperature. The temperature shows an inward collapse, whereas the density loss in the pedestal cause an increase in the SOL density which decays on a parallel transport time scale of Lc/cs~1ms. This density profile evolution is very similar for a large range of ELM sizes and plasma conditions. As a result the ELM convective losses are found to be a constant fraction of ~5-10% the pedestal stored energy (Wped). In baseline ELMy H-mode the conductive energy losses vary greatly with collisionality (ve*) from ~20% of Wped at ve*< 0.1 to ~5% of Wped at ve* > 0.5. In Advanced Tokamak (AT) scenario plasmas the collisionality is low at ve* ~ 0.1 but still a large variation of the conductive ELM losses from ~5% to ~20% of Wped is observed. The inward temperature collapse extends to r/a~0.8 in baseline plasmas, but varies greatly in AT plasmas from r/a~ 0.5 in unfuelled discharges to r/a ~0.8 in lightly fuelled plasmas.
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EFDC080519 2.63 Mb