Investigating the Dynamics of Canonical Flux Tubes
von der Linden, Jens
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Observations indicate that the dynamics of plasmas in our cosmos, the heliosphere, and terrestrial experiments can involve conversions between magnetic and kinetic energies over a wide range of plasma scales, such as in reconnection and dynamos. Canonical flux tubes present the distinct advantage of reconciling all plasma regimes, e.g. single particle, kinetic, two-fluid, and magnetohydrodynamics (MHD), with the topological concept of helicity: twists, writhes, and linkages. This thesis presents the first application of the theory of canonical helicity transport to design a laboratory canonical flux tube experiment, develop methods to measure a large volumetric dataset of magnetic field and ion flow, reconstruct the 3D dynamics of canonical flux tubes, and determine the canonical helicity evolution. Newcomb’s variational ideal MHD stability analysis is used to identify the possibility of a lengthening current-carrying magnetic flux tube undergoing a sausage-kink instability cascade. The instability cascade may couple to smaller scales at which conversions between species helicities are expected to occur. Experiment control and high-throughput field-programmable gate array (FPGA) based digitizers provide the means to measure the large datasets. Ion and electron canonical flux tubes are visualized from a dataset of Mach, triple, and Ḃ probe measurements at over 10,000 spatial locations of a gyrating kinked plasma column. The flux tubes co-gyrate with the peak density and electron temperature in and out of a measurement subvolume. The electron and ion flux tubes twist with opposite handedness and the ion canonical flux tube writhes around the electron canonical flux tube. Videos of the canonical flux tube reconstructions are available as supplemental material in ProQuest Dissertations & Theses Global. The cross helicity between the magnetic and ion flow vorticity flux tubes dominates the ion canonical helicity and is anticorrelated with the magnetic helicity. The methods developed in this thesis can be applied to other laboratory experiments to improve the understanding of canonical helicity transport; which could help identify how destabilizing magnetic twist is converted to stabilizing shear flows in astrophysical jets and could aid in developing methods for driving shear flow transport barriers in fusion devices.