Spatio-temporal analysis of runaway electrons in a JET disruption with material injection

Abstract

One of the outstanding issues of the tokamak, the magnetic confinement device on the forefront of fusion energy research, is that of relativistic electrons (so called runaway electrons) generated during disruption events. These energetic particles may cause significant damage to plasma-facing components if they escape confinement on a short timescale, and pose an even bigger threat to the next-generation tokamaks with larger toroidal plasma currents (such as ITER), since the runaway electron generation is exponentially sensitive to the plasma current in the tokamak. The runaway electron dynamics depend on a large variety of parameters, but current machines only have access to a small region of that parameter space. Numerical modeling of disruptions allows us to bridge the gap between what we have learned from present day machines and how their successors will behave. Such numerical models must be validated against available experimental data. In this thesis, an argon gas injection induced disruption in the Joint European Torus (JET) was modeled with the kinetic equation numerical solver Dream. With a thermal quench model where the temperature decay was assumed to be instantaneous and a runaway seed was prescribed, Dream was able to reproduce the plasma current evolution through the disruption. Dream allowed for simulations with a conducting wall, enabling us to reproduce the net increase in total plasma current seen after the current quench in the experimental data. Coupling the output from Dream to the synthetic synchrotron diagnostic tool Soft gave synthetic synchrotron signals from the modeled discharge. To investigate the effect of the radial distribution of the runaways on the plasma current dynamics and resulting synchrotron images, the disruption was modeled two times in Dream: once with a runaway density profile which was peaked around the magnetic axis, and once with a ‘hollow’ density profile with its maximum close to the plasma edge. Comparisons of the experimental and simulated diagnostic signals allowed us to conclude that different qualitative features of the experimental synchrotron images could be reproduced with the two respective runaway seeds, indicating that in the experiment, the runaway population was radially redistributed as the disruption progressed.