Runaway-electron model development and validation in tokamaks
Doctoral thesis, 2021
Magnetic confinement fusion (MCF), in which a hot plasma at more than 100 million kelvin is confined using magnetic fields, is the most successful fusion energy concept developed to date. After decades of research, MCF devices designed to demonstrate a positive net energy output are being constructed, completing a crucial milestone on the path to making fusion a commercially viable energy source. Several hurdles remain on this path, however, and one of the most pressing issues concerns the sudden and rapid loss of confinement of the fusion plasma, known as a disruption. An undesirable consequence of disruptions is the acceleration of a fraction of the plasma electrons to relativistic energies which---if the electrons were to strike the device wall---could deposit a significant portion of the plasma energy on a small area, causing severe and potentially irreparable damage.
The aim of this thesis is to develop a robust simulation tool capable of accurately predicting the number of runaway electrons produced in different disruption scenarios. Since the evolution of the runaway electrons affects the background plasma, it is important to also allow quantities such as electron temperature, ion density, and electric field to evolve self-consistently in the simulation. This leads to a tightly coupled system of non-linear equations, and to solve it we have developed the numerical tool DREAM.
The complexity of the models used to simulate runaway electrons demands that the validity of the models is carefully evaluated by comparing predictions with existing experimental data. One of the most informative techniques for studying the dynamics of runaway electrons in MCF experiments utilises synchrotron radiation, and to facilitate direct comparison of runaway electron simulations with experiments we have developed the synthetic diagnostic framework SOFT. Using SOFT, we study runaway electrons in the ASDEX Upgrade and TCV fusion devices, and develop powerful techniques for accurately extracting information about the location and momentum of runaway electrons.
The aim of this thesis is to develop a robust simulation tool capable of accurately predicting the number of runaway electrons produced in different disruption scenarios. Since the evolution of the runaway electrons affects the background plasma, it is important to also allow quantities such as electron temperature, ion density, and electric field to evolve self-consistently in the simulation. This leads to a tightly coupled system of non-linear equations, and to solve it we have developed the numerical tool DREAM.
The complexity of the models used to simulate runaway electrons demands that the validity of the models is carefully evaluated by comparing predictions with existing experimental data. One of the most informative techniques for studying the dynamics of runaway electrons in MCF experiments utilises synchrotron radiation, and to facilitate direct comparison of runaway electron simulations with experiments we have developed the synthetic diagnostic framework SOFT. Using SOFT, we study runaway electrons in the ASDEX Upgrade and TCV fusion devices, and develop powerful techniques for accurately extracting information about the location and momentum of runaway electrons.
magnetic confinement fusion
Fokker--Planck equation
synchrotron radiation
tokamaks
runaway-electrons
plasma physics
PJ
Opponent: Dr. Philipp Lauber, Max Planck Institute for Plasma Physics, Garching, Germany
Author
Mathias Hoppe
Subatomic, High Energy and Plasma Physics PP
SOFT: A synthetic synchrotron diagnostic for runaway electrons
Nuclear Fusion,; Vol. 58(2018)p. 026032-
Journal article
Interpretation of runaway electron synchrotron and bremsstrahlung images
Nuclear Fusion,; Vol. 58(2018)
Journal article
Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod
Nuclear Fusion,; Vol. 59(2019)
Journal article
Runaway electron synchrotron radiation in a vertically translated plasma
Nuclear Fusion,; Vol. 60(2020)
Journal article
Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption
Journal of Plasma Physics,; Vol. 87(2021)
Journal article
Hot-Tail Runaway Seed Landscape during the Thermal Quench in Tokamaks
Physical Review Letters,; Vol. 127(2021)
Journal article
DREAM: A fluid-kinetic framework for tokamak disruption runaway electron simulations
Computer Physics Communications,; Vol. 268(2021)
Journal article
Ett av de stora hinder som kvarstår innan fusion kan förse våra elnät med energi är plötslig uppkomst av ultrasnabba elektroner, kallade "skenande elektroner". Dessa partiklar, som rör sig vid nära ljusets hastighet, kan smälta delar av fusionsreaktorns vägg om de lyckas bryta sig loss från den magnetiska bur som håller dem instängda, vilket skulle kräva dyra och långvariga reparationer som gör fusion ekonomiskt ohållbar.
I denna avhandling utvecklar vi datorprogrammet DREAM som tillåter fysiker att testa olika tekniker för att förhindra uppkomsten av eller eliminera skenande elektroner i fusionsmaskiner. DREAM representerar en viktig del i fusions-pusslet eftersom problemet med skenande elektroner kommer vara många gånger värre i framtidens reaktorer, vilket gör det omöjligt att enbart utvärdera föreslagna lösningar på problemet i dagens fusionsexperiment.
För att säkerställa att DREAM och andra liknande verktyg väl beskriver fysiken som utspelar sig i en fusionsmaskin så måste dess förutsägelser jämföras mot experiment på dagens fusionsmaskiner. Vi utvecklar därför även datorprogrammet SOFT, som kan simulera en kamera i en fusionsmaskin och den strålning från skenande elektroner den ser, för att tillåta direkta jämförelser mellan teori och experiment. Genom att kombinera DREAM och SOFT kan teoretiker därför göra förutsägelser om skenande elektroner och simulera de kamerabilder de ger upphov till, vilket kan användas av experimentalister för att direkt jämföra förutsägelserna mot sina experiment.
I denna avhandling utvecklar vi datorprogrammet DREAM som tillåter fysiker att testa olika tekniker för att förhindra uppkomsten av eller eliminera skenande elektroner i fusionsmaskiner. DREAM representerar en viktig del i fusions-pusslet eftersom problemet med skenande elektroner kommer vara många gånger värre i framtidens reaktorer, vilket gör det omöjligt att enbart utvärdera föreslagna lösningar på problemet i dagens fusionsexperiment.
För att säkerställa att DREAM och andra liknande verktyg väl beskriver fysiken som utspelar sig i en fusionsmaskin så måste dess förutsägelser jämföras mot experiment på dagens fusionsmaskiner. Vi utvecklar därför även datorprogrammet SOFT, som kan simulera en kamera i en fusionsmaskin och den strålning från skenande elektroner den ser, för att tillåta direkta jämförelser mellan teori och experiment. Genom att kombinera DREAM och SOFT kan teoretiker därför göra förutsägelser om skenande elektroner och simulera de kamerabilder de ger upphov till, vilket kan användas av experimentalister för att direkt jämföra förutsägelserna mot sina experiment.
One of the big obstacles remaining before nuclear fusion can provide energy to our electrical grids is the sudden occurrence in reactors of ultrafast electrons, called ``runaway electrons''. These particles, which move at close to the speed of light, can melt parts of the wall of the fusion reactor if they manage to escape the magnetic cage keeping them confined, leading to lengthy and costly repairs which would make fusion commercially unviable.
In this thesis we develop the computer program DREAM which allows physicists to test different techniques for preventing or reversing the generation of runaway electrons in fusion devices. DREAM provides an important piece to the puzzle of fusion since the runaway electron problem will be many times more severe in future reactors, making it impossible to fully test any given prevention technique in present experiments before we build the reactor.
To ensure that DREAM, and other tools like it, accurately describe the physics of a fusion device, it must be compared with those experiments that can be conducted on the fusion devices of today. Therefore, to allow theory and experiment to be compared directly, we also develop the computer program SOFT, which can simulate a camera inside a fusion device, and the radiation it sees from runaway electrons. Combining DREAM and SOFT, theoreticians can therefore make predictions about runaway electrons and simulate the radiation camera images they lead to, which experimentalists can directly compare to their measurements.
In this thesis we develop the computer program DREAM which allows physicists to test different techniques for preventing or reversing the generation of runaway electrons in fusion devices. DREAM provides an important piece to the puzzle of fusion since the runaway electron problem will be many times more severe in future reactors, making it impossible to fully test any given prevention technique in present experiments before we build the reactor.
To ensure that DREAM, and other tools like it, accurately describe the physics of a fusion device, it must be compared with those experiments that can be conducted on the fusion devices of today. Therefore, to allow theory and experiment to be compared directly, we also develop the computer program SOFT, which can simulate a camera inside a fusion device, and the radiation it sees from runaway electrons. Combining DREAM and SOFT, theoreticians can therefore make predictions about runaway electrons and simulate the radiation camera images they lead to, which experimentalists can directly compare to their measurements.
Runaway electrons in fusion plasmas
Swedish Research Council (VR) (2018-03911), 2018-12-01 -- 2021-12-31.
Driving Forces
Sustainable development
Areas of Advance
Energy
Roots
Basic sciences
Subject Categories
Fusion, Plasma and Space Physics
ISBN
978-91-7905-598-1
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5065
Publisher
Chalmers