Kinetic modeling of runaway-electron dynamics in partially ionized plasmas
Doktorsavhandling, 2020

An essential result of kinetic plasma physics is the runaway phenomenon, whereby a fraction of an electron population can be accelerated to relativistic energies. Such runaway electrons are formed in astrophysical settings, but are also of great practical relevance to fusion research. In the most developed fusion device, known as the tokamak, runaway electrons have the potential to cause severe damage to the first wall. Runaway-electron mitigation is therefore one of the critical issues in the design of a fusion power plant.

In many situations, runaway electrons interact with partially ionized atoms. In particular, the currently envisaged mitigation method for tokamaks is to inject heavy atoms which collisionally dissipate the runaway beam before it can collide with the wall, or prevent it from forming at all. When the atoms are partially ionized, their bound electrons screen out a fraction of the atomic charge, which directly affects the collisional scattering rates. However, accurate expressions for these collisional scattering rates between energetic electrons and partially ionized atoms have not been available previously.

In this thesis, we explore kinetic aspects of runaway dynamics in partially ionized plasmas. We derive collisional scattering rates using a quantum-mechanical treatment, and study the interaction between fast electrons and partially ionized atoms. We then apply these results to calculate the threshold field for runaway generation, as well as the production rate of runaway electrons via the avalanche and Dreicer mechanisms. We find that even if material injection increases the dissipation rates, it also enhances avalanche generation which could potentially aggravate the runaway problem. These results contribute to more accurate runaway-electron modeling and can lead to more effective mitigation schemes in the longer term.

runaway electrons


plasma physics

magnetic confinement fusion

electron-ion collisions

Fokker-Planck equation

FB, Fysikgården 4
Opponent: Dr. Xianzhu Tang, Los Alamos National Laboratory, New Mexico, USA


Linnea Hesslow

Chalmers, Fysik, Subatomär, högenergi- och plasmafysik

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The runaway-electron threat to fusion: an action-packed story

Runaway electrons in fusion research have all the necessary ingredients of an action movie. The runaway electrons play the role of the antagonists. Moving close to the speed of light, they threaten to destroy the reactor wall, which could put the entire fusion program at risk. In order to eliminate these criminal electrons, the scientists have developed a gun with magic bullets made of frozen deuterium and heavy ions. But is it enough to stop the runaway electrons? Will the scientists fire before it is too late? Or will the magic bullets only make the situation worse by helping the runaway electrons to multiply and take over the plasma? To find out, watch "Kinetic modeling of runaway-electron dynamics in partially ionized plasmas", which will be broadcast on September 4.

This thrilling plot is closer to reality than it first appears. In a magnetic fusion device – a tokamak – runaway electrons can form if the plasma inside it suddenly terminates. Since a strong runway beam could melt the tokamak wall, runaway electrons must either be prevented or safely slowed down before wall strike. This is typically done by injecting shattered pellets to the plasma at a speed of several hundred meters per second. By developing a model for the interaction between runaway electrons and the ions inside the pellets, this thesis studies whether these really are magic bullets for runaway mitigation, taking us one step closer to achieving nuclear fusion.

Forskning för framtida fusionsreaktorer: använda eller undvika orenheter

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Skena och skina

Europeiska kommissionen (EU) (EC/H2020/647121), 2015-10-01 -- 2020-09-30.

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Skenande elektroner i fusionsplasmor

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Vetenskapsrådet (VR) (2018-03911), 2018-12-01 -- 2021-12-31.


Hållbar utveckling




Grundläggande vetenskaper


Fusion, plasma och rymdfysik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4710



FB, Fysikgården 4


Opponent: Dr. Xianzhu Tang, Los Alamos National Laboratory, New Mexico, USA

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