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

TL;DR

This study combines diagnostic imaging and simulations to analyze runaway electron behavior during plasma disruptions in a tokamak, revealing the dominance of high-energy seed populations in synchrotron emission and their radial redistribution.

Contribution

It introduces a coupled diagnostic and simulation approach to characterize runaway electron distribution evolution and develops an analytic model for seed population dynamics.

Findings

High-energy seed populations dominate synchrotron emission.

Radial redistribution causes change in synchrotron pattern shape.

Runaway distribution evolves from hot-tail seed to avalanche tail.

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.