Collisional transport in edge transport barriers and stellarators
Doctoral thesis, 2019

Nuclear fusion has the potential to generate abundant and clean energy. In magnetic confinement fusion, the temperatures needed to achieve fusion are obtained by confining a hot plasma with magnetic fields. To maintain these hot temperatures and realize the potential of fusion, an understanding of transport mechanisms of particles and energy in these plasmas is needed. This thesis theoretically investigates two aspects of collisional transport in magnetically confined fusion plasmas: the collisional transport in tokamak transport barriers and of highly-charged impurities in stellarators.

The tokamak and the stellarator are the two most developed solutions to magnetically confining a plasma. Tokamaks frequently operate in a regime (the \emph{H-mode}) with a transport barrier near the edge of the plasma, in which turbulence is spontaneously reduced. This leads to reduced energy and particle transport and sharp temperature and density gradients. These sharp gradients challenge the modeling capabilities based on the conventional theory of collisional transport, which relies on the assumption that the density, temperature, and electrostatic potential of the plasma do not vary strongly over a particle orbit. This thesis explores an extension of the conventional theory that accounts for these effects, by means of numerical simulations.

Another limit that challenges the conventional assumptions is when the density of an impurity varies along the magnetic field. This happens for heavy impurities, such as iron or tungsten, which can enter the plasma from interactions with the walls of the reactor. Due to their high charge, these impurities are sensitive to even slight variations in electrostatic potential in the plasma, which causes their density to vary along the magnetic field. This density variation can qualitatively affect how the impurities are transported. This is explored in the latter half of this thesis, with an eye towards how this effect could be used to prevent impurities from accumulating in the core of stellarators, where they are detrimental.

plasma physics


collisional transport





impurity transport

Opponent: Xavier Garbet, CEA, Frankrike


Stefan Buller

Chalmers, Physics, Subatomic and Plasma Physics

Global effects on neoclassical transport in the pedestal with impurities

Plasma Physics and Controlled Fusion,; Vol. 58(2016)p. 085001-

Journal article

Neoclassical flows in deuterium-helium plasma density pedestals

Plasma Physics and Controlled Fusion,; Vol. 59(2017)p. 055019-

Journal article

Isotope and density profile effects on pedestal neoclassical transport

Plasma Physics and Controlled Fusion,; Vol. 59(2017)p. 105003-

Journal article

Collisional transport of impurities with flux-surface varying density in stellarators

Journal of Plasma Physics,; Vol. 84(2018)

Journal article

Transport phenomena in plasma – the flow of particles and energy

Transport processes describe how particles, heat and momentum are transported within a medium, such as a liquid, gas, or a plasma. Transport occurs naturally whenever a hot and cold object are in contact with each other, which results in energy flowing from the hot to the cold object. This can be very undesirable, as many processes require that a specific temperature is maintained: for example, maintaining a body temperature of about 37 °C is essential for staying alive; many chemical and nuclear processes will only take place at a reasonable rate above certain temperatures.

The highest temperature requirements in any application today are those for energy extraction from nuclear fusion, where temperatures ten times those in the core of the sun need to be sustained. At these temperatures, matter is ionized, and exists in the form of a plasma, which can be confined by a magnetic field.

Apart from extreme temperatures, these plasmas also display extreme variations in plasma parameters and composition. The edge of these plasmas often feature regions with sharp temperature and density variation, where the temperature goes from being comparable to that in the core of the sun to near zero over a distance of a few centimeters. In addition, highly-charged impurities can enter the fusion plasma from the surrounding walls, and these impurities behave differently from the singly-charged hydrogen that make up the bulk of the plasma.

This thesis is concerned with calculating the transport of particles, heat and momentum in these magnetically confined fusion plasmas, by utilizing new simulation codes and mathematical formulations that allow the treatment of collisional transport in regions with sharp variations of plasma characteristics, and the treatment of highly-charged impurities. With our novel descriptions of these processes, we have been able to investigate how aspects such as plasma composition and magnetic field configuration affect the transport in the plasma.

Implementation of activities described in the Roadmap to Fusion during Horizon 2020 through a Joint programme of the members of the EUROfusion consortium (EUROfusion)

European Commission (FP7), 2014-01-01 -- 2019-01-01.

Driving Forces

Sustainable development

Areas of Advance



Basic sciences


C3SE (Chalmers Centre for Computational Science and Engineering)

Subject Categories

Fusion, Plasma and Space Physics



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4618


Chalmers University of Technology


Opponent: Xavier Garbet, CEA, Frankrike

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