Transport in turbulent plasmas at the interface between different levels of description

• Main Author: Carbajal-Gomez, Leopoldo
• Published: University of Warwick 2015
• Subjects: 530; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.668937

Energetic ion dynamics play an important role in magnetic confinement fusion (MCF) plasmas, as well as in the solar wind. In the former case, energetic ions such as neutral beam injection (NBI) ions and fusion-born alpha-particles, can interact with global modes in tokamak plasmas leading to instabilities that might result in loss of confinement and energy. In the latter case, ion dynamics must be taken into account in order to explain in situ and remote observations of heating of the solar wind, which show the occurrence of anisotropic heating of ions, as well as magnetohydrodynamics turbulence and intermittency all at the same time. In this thesis we address two scenarios in plasma physics where ion dynamics play a key role modifying the mass and energy transport in the plasma, specifically, ion cyclotron emission (ICE) in MCF plasmas, and preferential ion heating due to intermittent magnetic fields in the solar wind. ICE results from a radiative instability, probably the magnetoacoustic cyclotron instability (MCI), driven by energetic ions in MCF plasmas. Understanding the underlying physics of ICE is important for the exploitation of ICE as a non-perturbative diagnostic for confined and lost alpha-particles in deuterium-tritium (D-T) plasmas in future thermonuclear fusion reactors [McClements et al., Nucl. Fusion, 55, 043013 (2015); Dendy and McClements, Plasma Phys. Controlled Fusion, 57, 044002 (2015)]. On the other hand, preferential ion heating in the solar wind, observed as the occurrence of an ion beam which drifts along the background magnetic field with a velocity close to the local Alfven speed, is still an open problem. Despite the large amount of studies conducted in this issue, none of them included intermittency self-consistently. Therefore, the relationships between preferential ion heating and intermittency have remained unknown, until now. We study in detail the previously mentioned scenarios through numerical simulations using the hybrid approximation for the plasma, which treat ions as kinetic particles and electrons as a neutralizing massless fluid. Our hybrid simulations of the MCI confirm predictions of the analytical theory of the MCI, and recover some features of ICE as observed in D-T plasmas in JET. Furthermore, by going deep into the nonlinear stage of the MCI, we recover additional features of ICE which are not predicted by the linear theory of the MCI but are present in the measured ICE signal, resulting in a good match between our simulation results and the measured ICE intensity in JET. On the other hand, we present the first study of preferential ion heating in the fast solar wind including intermittent electromagnetic fields in a self-consistent way. We find that the temporal and spatial dynamics of the mechanisms driving preferential ion heating in our simulations (gyrobunching and ion trapping by the electric field), the ion temperature anisotropy T=T (perpendicular temperature/parallel temperature), and the degree of correlation between velocity and magnetic field fluctuations, show strong dependence on the level of intermittency in the electromagnetic fields.