Magnetized Plasma Pressure Filaments: Thermal Diffusion Waves and Multi-Filament Dynamics
Creators: Karbashewski, Scott G
Publisher: name: University of Alberta Library
Year: 2022

<p>One of the prominent areas of interest in current plasma science is focused on understanding the mechanisms of heat transport beyond classical Coulomb collisions. The topic has applications in many areas of plasma physics, such as the solar corona and nuclear fusion devices. In strongly magnetized plasmas the thermal conductivity parallel to the magnetic field is numerically quite large and orders of magnitude larger than the transverse conductivity; thus, the effective study of heat transport in these conditions requires a large system that is much longer along the magnetic axis than across. In this thesis, results are presented from basic heat transport experiments using magnetized plasma pressure filaments. The experiments are performed in the Large Plasma Device (LAPD) at the Basic Plasma Science Facility (BaPSF) at the University of California, Los Angeles (UCLA). A cerium hexaboride crystal cathode injects low-energy electrons along a magnetic field into the centre of a preexisting, cold, quiescent plasma forming a hot electron filament embedded in a colder plasma. Previous experiments observed gradient-driven drift-Alfvén waves that lead to enhanced cross-field transport; and a spontaneous thermal diffusion wave that was speculated to meet a quarter-wave resonance of the filament length.</p>

<p>In the first set of experiments, a low amplitude sinusoidal perturbation is added to the cathode discharge bias that creates an oscillating heat source capable of driving large amplitude electron temperature oscillations. Langmuir probes are used to measure the amplitude and phase of the thermal wave field over a wide range of driver frequencies. The results are used to verify the excitation of thermal waves in magnetized plasma and confirm the presence of thermal resonances of the filament. The diagnostic potential of thermal waves is demonstrated through measurement of the parallel thermal diffusivity and distinguishing between classical transport, drift-Alfvén growth, and turbulent transport regimes using the cross-field structure. Two models for the thermal wave field are investigated and used to describe the experiment results; a two-dimensional homogeneous model based on a Green function approach and a one-dimensional inhomogeneous model based on a classical mechanics approach using the Hamilton-Jacobi equation.</p>

<p>The second set of experiments have used three crystal cathodes in a triangular configuration to investigate the interactions between multiple pressure filaments with varying separation distances. The results are used to establish the scale length of interaction between the filaments. Within an electron skin depth, enhanced cross-field transport from chaotic E × B mixing rapidly relaxes the gradients in the inner triangular region of the filaments and leads to the growth of global nonlinear drift-Alfvén modes that are driven by the thermal gradient in the outer region of the filament bundle. Linear stability analysis of the temperature, density, and transverse flow profiles is used to accurately predict the observed wave modes. Coupling between the global modes leads to large intermittent transport events that are characterized by exponential frequency spectra and Lorentzian-shaped pulses that are signatures of chaotic dynamics. Mode decomposition and conditional averaging are used to reconstruct the pulse events and demonstrate that they are driven by nonlinear interactions between drift-Alfvén wave modes. The time series analysis tools of the complexity-entropy plane and Hurst exponent are used to investigate the chaotic nature and memory of the intermittent fluctuations. Last, the addition of a sinusoidal driver to one of the filaments in the configuration facilitates driving of the drift-Alfvén modes. The driver is shown to synchronize with several eigenmodes at once due to the asymmetric perturbation.</p>