Runaway Electron Behaviour with the ITER-like Wall in JET

Runaway electrons (RE) with energies of several megaelectronvolts have been observed during disruptions in JET and other large tokamaks. These intense electron beams are the result of the toroidal electric field induced by the sudden cooling in connection with disruptions and may cause severe damage to the plasma facing components. Therefore REs are considered to be a potential threat to the operation of tokamaks with high currents, such as ITER. Reliable runaway electron (RE) mitigation after disruptions is one of the most important challenges for safe ITER operation. A proper understanding of the generation and loss of REs is therefore essential. This paper investigates the effect of the ITER-like wall (ILW) on RE generation in JET. The ILW comprises solid beryllium limiters and a combination of bulk tungsten and tungsten-coated carbon fibre composite divertor tiles. Similar JET disruptions in Lmode limiter discharges are compared with different wall materials: carbon (Carbon Fibre Composite, CFC) and beryllium (for ILW). In both cases the disruption was induced by slow argon injection. Typically with the ILW, a significantly lower fraction of energy is radiated during the disruption process, yielding higher plasma temperatures after the thermal quench and thus longer current quench times. As a consequence, in the carbon wall case, a runaway electron plateau is observed, while with the ILW the runaway current is negligibly small. Runaway electron dynamics is a complicated process, therefore numerical modelling is necessary to provide a deeper insight into the physics. Detailed modelling was carried out to study which factors affected the RE formation in these two cases.
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