Spatio-temporal analysis of runaway electrons in a JET disruption with material injection
Typ
Examensarbete för masterexamen
Program
Publicerad
2021
Författare
Brandström, Boel
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
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.
Beskrivning
Ämne/nyckelord
nuclear fusion , runaway electrons , numerical modeling , tokamak , Joint European Torus , disruption , synchrotron radiation