The thin region of plasma at the edge of a tokamak, as the boundary condition to the confined plasma in the core, plays an oversized role in the performance of such devices. As such, the topic of exhaust physics is central to the ongoing effort to make magnetically confined fusion a viable approach to clean energy generation. A defining feature of this edge plasma, called the scrape-off layer (SOL), is a large temperature gradient in the direction parallel to the magnetic field. Large temperature drops are probably crucial to avoid excessive heat loads to the solid components which make up the walls of the device. However, their presence means that classical transport models, which assume the plasma is at or close to local thermodynamic equilibrium (LTE) and which are used widely in SOL modelling, can lose their predictive power [1-3]. The aim of this thesis is to investigate the extent of this effect in detail by performing kinetic simulations of parallel transport in SOL plasmas, with a focus on the electrons. There is an emphasis on quantifying the modelling uncertainties that exist in classical ('fluid') approaches to SOL simulations by performing self-consistent comparisons between kinetic and fluid models.
To do this, the one-dimensional SOL kinetic code SOL-KiT has been used [4]. By extending the capabilities of this code, reducing its computational expense, and developing a standalone atomic physics code (all of which are described), it has been possible to study electron kinetics in a range of conditions relevant to current and future tokamaks. A number of distinct investigations have been performed. Firstly, it has been shown that fluid models are in fact very good at capturing the transfer of energy between ions and electrons in SOL plasmas. Secondly, it is demonstrated that a kinetic treatment leads to significant differences in parallel conductive heat transport and behaviour at the wall boundary, both of which contribute to modified temperature profiles. A set of simple scaling laws for these effects has been proposed. Finally, the effect of non-LTE electrons on plasma-atomic physics has been investigated. Here, strongly enhanced reaction rates due to the form of the electron velocity distribution have been observed, but this effect is largely reversed when considered alongside the modified temperature profiles.Open Acces
