Turbulence in fluids and plasmas is a complex phenomena which couples structures at different scales. Turbulent flows possess spectral cascades, as well as coherent structures. When a flowing system can be considered as two-dimensional, the coupling through nonlinear interaction generates large-scale structures which extend to the system size in an inverse energy cascade. Plasmas confined by the dipole magnetic configuration in the Collisionless Terrella Experiment (CTX) display two-dimensional, intense interchange-mode dynamics. The plasma fluctuations are driven by gas injection and microwave heating, which produces a plasma maintained near marginal stability. The turbulence in CTX is investigated with respect to both local and global measurements. When viewed locally, the intense fluctuations exhibit characteristics of fully developed turbulence, with a broad power-law spectrum and finite correlation length. When viewed globally, the dynamics are found to be describable by the chaotic temporal variation of a limited number of simple spatial modes. The fluctuation energy spectrum is calculated to be consistent with the power-law trends for the inverse energy cascade. Using analysis techniques for determining spectral energy flow, it is found that three-wave interaction transfers energy to low wavenumbers, as predicted for two-dimensional turbulence. A fully parallelized, self-consistent simulation including a conserving source and sink is used to test the model equations for interchange mode dynamics in a dipolar magnetic field. The model reproduces the driven fluctuations observed in CTX, producing the rotating, radially broad, large-scale structures.
|Adviser||Michael E. Mauel|
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