This dissertation is an experimental study of laser-generated, atmospheric pressure, transient toroidal helium plasmas.The formation mechanism of these toroidal plasmas is identified and an... Show moreThis dissertation is an experimental study of laser-generated, atmospheric pressure, transient toroidal helium plasmas.The formation mechanism of these toroidal plasmas is identified and an estimate of their main plasma parameters is obtained. Furthermore, preliminary experiments are presented, aimed at heating these plasmas by absorption of microwave radiation, in order to counteract their transient nature.Through a tomographic reconstruction, cross-sectional images of the toroidal plasma are obtained, visualising the fluid flow responsible for the generation of the toroidal structure. The origin of the flow is traced back to the structure of the plasma kernel. The shocks generated by this kernel interact akin to a Mach reflection and generate a low pressure region whose replenishment transforms the plasma into a toroid. Schlieren imaging, complemented with a novel scanning-probe technique, and thermodynamic modelling, as well as deliberately breaking the flow symmetry, confirm the formation mechanism. A high-power, sub-microsecond rise time, pulsed magnetron source has been designed for the microwave heating experiments. Its detailed design and the effect of the microwave pulse on the plasma are discussed.This work is part of a larger study on self-organising knotted magnetic structures in plasma, which may find their application in nuclear fusion and astrophysical research. Show less
A plasma is an ionized gas with very low electrical resistivity. As such, magnetic field lines are 'frozen in' and move with the fluid. Magnetic field lines that are linked, knotted and... Show moreA plasma is an ionized gas with very low electrical resistivity. As such, magnetic field lines are 'frozen in' and move with the fluid. Magnetic field lines that are linked, knotted and tangled, cannot be undone by the fluid motions. In this thesis we investigate how this linking and knottedness influences the plasma dynamics through numerical simulations. One of the main results is the identification of a novel, self-organizing equilibrium, where every field line is linked with every other one. In such a structure all the field lines lie on toroidal magnetic surfaces, and the entire structure resembles the famous topological structure of the Hopf fibration. This magnetic equilibrium is localized, and kept in balance by a finite external pressure. Through resistive effects the structure slowly expands while the magnetic energy is dissipated. This research, and the novel structures identified have implications for nuclear fusion research and the study of astrophysical plasma phenomena. Show less
