Large ELMs Triggered by MHD in JET Advanced Tokamak Plasmas: Impact on Plasmas Profiles, Plasmas Facing Components and Heating Systems

Large ELMs triggered by MHD were observed in high bN advanced Tokamak (AT) plasmas in JET. They cause a considerable loss of the plasma thermal and fast particle energy (~10%) and an intensive plasma wall interaction, both of which affect the operation of additional heating systems. The ELM is followed by; a reduction in the electron and ion temperatures and toroidal rotation of the plasma, a rise in Zeff; an anomalous decrease in the radiation temperature, and a sharp increase in the peripheral electron density. These features have been modelled to gain an understanding of the plasma behaviour associated with these ELM events. A large, toroidally asymmetric, increase in the neutral gas pressure outside the plasma was observed after such ELMs, which may cause arcs in the Lower Hybrid (LH) and Ion Cyclotron (IC) heating systems and increased reionisation in the Neutral Beam (NB) injectors. The heat load in the divertor was found to be small as a fraction of the plasma energy loss compared with regular type-I ELMs. Instead significant heating of the main chamber wall was observed. The implication of these events for the operation of AT plasma scenarios with high additional heating power will be discussed.
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