Critical Issues Addressed to Runaway Electron Generation Expected at Major Disruptions in Reactor-Scale Tokamaks

Data on Runaway Electrons (REs) generated during major disruptions in JET has been updated. Recent studies were focused on determination of the RE current fraction during current quench and establishing generation trends toward ITER experimental parameters domain. A study of the experimental trend, which covers 1.5-6MA disruptions before and after divertor installation in JET, when the noticeable RE currents have been observed, has demonstrated the dependence of runaway process on the different character of plasma energy evolution (losses) in different type of disruptions. This is directly related to the time scale of the fast thermal quench, the theoretical basis for which is not well understood and there is no validated basis to predict the rate of plasma energy loss. Another critical point, which was considered, is associated with the influence of fast recreation of the nested magnetic surfaces soon after reconnection and the evolution of the magnetic configuration on the behaviour of initial runaway electron population created during thermal quench. Influence of size and spatial evolution of the current-carrying channel on the dynamics of RE generation process has been studied.
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