The edge barrier collapse induced by spontaneous and pellet triggered ELMs has been characterized by means of a new technique based on the reflectometry group delay. The analysis of the behaviour of the density layers at the plasma edge revealed small differences between the two types of ELMs. With this technique we are able to detect profile steepening and fuelling effects due to pellet injection, mainly at the high field side. The presence of asymmetries between high and low field sides in the ELM dynamics has also been studied by evaluating the ELM onset time on both sides. The ELM onset on the HFS is observed to be only slightly delayed compared with the LFS onset—the barrier collapses are nearly coincident within the diagnostic time resolution. In our new findings, which apply to both spontaneous and triggered ELMs, there is no clear evidence of long HFS/LFS delays as previously observed for spontaneous ELMs.
Tokamak edge localized mode onset prediction with deep neural network and pedestal turbulence