Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption

Abstract

Synchrotron radiation images from runaway electrons (REs) in an ASDEX Upgrade discharge disrupted by argon injection are analyzed using the synchrotron diagnostic tool SOFT and coupled fluid-kinetic simulations. We show that the evolution of the runaway distribution is well described by an initial hot-tail seed population, which is accelerated to energies between 25-50 MeV during the current quench, together with an avalanche runaway tail which has an exponentially decreasing energy spectrum. We find that, although the avalanche component carries the vast majority of the current, it is the high-energy seed remnant that dominates synchrotron emission. With insights from the fluid-kinetic simulations, an analytic model for the evolution of the runaway seed component is developed and used to reconstruct the radial density profile of the RE beam. The analysis shows that the observed change of the synchrotron pattern from circular to crescent shape is caused by a rapid redistribution of the radial profile of the runaway density.

arXiv comment25 pages, 12 figures

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• Runaway dynamics in disruptions with current relaxation
• Tomographic reconstruction of the runaway distribution function in TCV using multispectral synchrotron images
• Electron runaway in ASDEX Upgrade experiments of varying core temperature
• Fluid and kinetic studies of tokamak disruptions using Bayesian optimization
• Modelling of runaway electron dynamics during argon-induced disruptions in ASDEX Upgrade and JET
• Parametric scaling of power exhaust in EU-DEMO alternative divertor simulations
• Feasibility of the EDICAM camera for runaway electron detection in JT-60SA disruptions