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Title: Modelling of kinetic effects in parallel transport in the tokamak scrape-off layer
Author: Mijin, Stefan
ISNI:       0000 0004 9357 0335
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2020
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Heat flow onto divertor targets in the tokamak represents one of the main design problems in current fusion research. Excessive heat loads, especially during violent transient events, can significantly damage the plasma-facing components. In order to mitigate the heat load it is beneficial to run the tokamak in a detached regime. This involves formation of a neutral cloud in front of the divertor targets that protects the surface material. Thus interaction of the plasma with the detached neutrals becomes important. Energy, on its way to the divertor targets, travels through the boundary region of the tokamak, the Scrape-Off Layer. The main transport direction is parallel to the open magnetic field lines of the Scrape-Off Layer. Fluid models are routinely used to simulate particle and energy transport in the boundary region, but these fail to capture kinetic effects, which have been linked to discrepancies between said fluid model predictions and experimental data[1]. Kinetic/non-local effects have been observed in simulations of parallel transport, including heat flux suppression and enhancement[2], as well as effects on ionisation rates[3]. The plasma sheath at the divertor target is also affected by non-local phenomena[4,5]. Exploring how kinetic effects change facets of parallel transport, and especially how they cause departure from widely used fluid models, is important in furthering our understanding of heat load issues, which are critical to future machine design. In this thesis, a novel framework for studying electron kinetic effects in parallel transport is developed, with special focus on the proper treatment of the sheath boundary condition and inelastic collisions, and the capability of self-consistent comparison between a kinetic and fluid model. This framework is implemented in the new electron kinetic code SOL-KiT, and details on the model and numerics, as well as benchmarking, are presented. With the developed code, a systematic power scan study of non-local effects in both equilibria and transients has been conducted. Kinetic effects are found to be especially prominent in the interaction of transients with detachment, where heating due to fast particles, heat flux suppression, as well as modification of ionisation rates have been observed.
Supervisor: Kingham, Robert Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral