| 1. |
- Holmberg, Mika, 1982-
(författare)
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A study of the structure and dynamics of Saturn's inner plasma disk
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a study of the inner plasma disk of Saturn. The results are derived from measurements by the instruments on board the Cassini spacecraft, mainly the Cassini Langmuir probe (LP), which has been in orbit around Saturn since 2004. One of the great discoveries of the Cassini spacecraft is that the Saturnian moon Enceladus, located at 3.95 Saturn radii (1 RS = 60,268 km), constantly expels water vapor and condensed water from ridges and troughs located in its south polar region. Impact ionization and photoionization of the water molecules, and subsequent transport, creates a plasma disk around the orbit of Enceladus. The plasma disk ion components are mainly hydrogen ions H+ and water group ions W+ (O+, OH+, H2O+, and H3O+). The Cassini LP is used to measure the properties of the plasma. A new method to derive ion density and ion velocity from Langmuir probe measurements has been developed. The estimated LP statistics are used to derive the extension of the plasma disk, which show plasma densities above ~20 cm-3 in between 2.7 and 8.8 RS. The densities also show a very variable plasma disk, varying with one order of magnitude at the inner part of the disk. We show that the density variation could partly be explained by a dayside/nightside asymmetry in both equatorial ion densities and azimuthal ion velocities. The asymmetry is suggested to be due to the particle orbits being shifted towards the Sun that in turn would cause the whole plasma disk to be shifted. We also investigate the ion loss processes of the inner plasma disk and conclude that loss by transport dominates loss by recombination in the entire region. However, loss by recombination is still important in the region closest to Enceladus (~±0.5 RS) where it differs with only a factor of two from ion transport loss.
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| 2. |
- Castello Lucco, Federico
(författare)
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Bridge functions in strongly coupled plasmas : theory, simulations and applications
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Strongly coupled or non-ideal plasmas are multi-component charged systems in which at least one species possesses an average interaction energy that is comparable or larger than its thermal energy. Non-ideal plasmas are naturally occurring in dense astrophysical objects (e.g. giant planet interiors) but also engineered in the laboratory (e.g. plasma discharges seeded with solid particulates). They are typically encountered in the liquid state, whose theoretical description is particularly challenging due to the lack of small parameters. This thesis is focused on the development of a novel theoretical approach for the accurate calculation of the structural and thermodynamic properties of plasma liquids. Apart from their inherent significance, these properties also constitute necessary input to advanced theories of dynamical correlations, collective excitations and transport coefficients. The theoretical approach is based on the integral equation theory framework, whose central quantity is the bridge function; an abstract object of diagrammatic analysis that is impossible to calculate or even approximate through virial-type expansions. Here the bridge function is accurately determined by combining elements of the isomorph theory of R-simple liquids with indirect extractions from computer simulations. The unprecedented level of accuracy in both the structural and thermodynamic properties and the very low computational cost, render the approach the most efficient alternative to computer simulations of classical and quantum plasma liquids. Applications to collective modes and metastable properties are also discussed.
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| 3. |
- Huo, Chunqing
(författare)
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Modeling High Power Impulse Magnetron Sputtering Discharges
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- HiPIMS, high power impulse magnetron sputtering, is a promising technology that has attracted a lot of attention ever since its appearance. A time-dependent plasma discharge model has been developed for the ionization region in HiPIMS discharges. As a flexible modeling tool, it can be used to explore the temporal variations of the ionized fractions of the working gas and the sputtered vapor, the electron density and temperature, and the gas rarefaction and refill processes. The model development has proceeded in steps. A basic version IRM I is fitted to the experimental data from a HiPIMS discharge with 100 μs long pulses and an aluminum target. A close fit to the experimental current waveform, and values of density, temperature, gas rarefaction, as well as the degree of ionization shows the validity of the model. Then an improved version is first used for an investigation of reasons for deposition rate loss, and then fitted for another HiPIMS discharge with 400 μs long pulses and an aluminum target to investigate gas rarefaction, degree of ionization, degree of self sputtering, and the loss in deposition rate, respectively. Through these results from the model, we could analyse further the potential distribution and its evolution as well as the possibility of a high deposition rate window to optimize the HiPIMS discharge. Besides this modeling, measurements of HiPIMS discharges with 100 μs long pulses and a copper target are made and analyzed. A description, based on three different types of current systems during the ignition, transition and steady phase, is used to describe the evolution of the current density distribution in the pulsed plasma. The internal current density ratio is a key transport parameter. It is reported how it varies with space and time, governing the cross-B resistivity and the energy of the charged particles. From the current ratio the electron cross-B transport can be obtained and used as essential input when modeling the axial electric field, governing the back-attraction of ions.
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| 4. |
- Olson, Jonas
(författare)
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Object-plasma interaction in the vicinity of Enceladus
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Cassini spacecraft orbits Saturn since 2004, carrying a multitude of instruments for studies of the plasma environment around the planet as well as the constituents of the ring system. Of particular interest to the present thesis is the large E ring, which consists mainly of water ice grains, smaller than a few micrometres, referred to as dust. The work presented here is concerned with the interaction between, on the one hand, the plasma and, on the other hand, the dust, the spacecraft and the Langmuir probe carried by it. In Paper I, dust densities along the trajectory of Cassini, as it passes through the ring, are inferred from measured electron and ion densities. In Paper II, the situation where a Langmuir probe is located in the potential well of a spacecraft is considered. The importance of knowing the potential structure around the spacecraft and probe is emphasised and its effect on the probe’s current-voltage characteristic is illustrated with a simple analytical model. In Paper III, particle-in-cell simulations are employed to study the potential and density profiles around the Cassini as it travels through the plasma at the orbit of the moon Enceladus.
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| 5. |
- Thorén, Emil
(författare)
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Modelling of macroscopic melt motion in fusion devices
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic confinement fusion is one of the most well developed methods envisioned to achieve thermonuclear fusion energy in the future. A central obstacle that remains in the way of safe and sustainable reactor operation is the interaction that occurs between the plasma and vessel wall components. Lengthy or intense plasma exposures will lead to surface erosion or plasma pollution. Metal plasma-facing components can melt, in which case the liquid is subsequently displaced by various accelerating forces resulting to macroscopic surface deformation, which will ultimately decrease the functionality and lifetime of the armour. Experiments have been performed in numerous contemporary tokamaks in order to elucidate the various processes behind wall heating, metal melting, and surface deformation. Combined with numerical tools, these provide the framework for predictive studies and conclusions for the armour effectiveness in future tokamaks ITER and DEMO.This thesis is focused on one such numerical tool: MEMOS-U, a heat transfer and fluid motion code that was developed specifically to model macroscopic surface deformation in magnetic confinement devices. The code employs the shallow water approximation of the Navier-Stokes equations, which drastically reduces the computational cost and enables multi-timescale simulations over large exposed areas. A detailed overview of the theoretical framework and numerical implementation of the code is provided, followed by results from benchmarking activities with various melt experiments as well as predictive studies for ITER. Model limitations are also discussed.
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