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• W3048295625 abstract "Author(s): Poulos, Matthew Joseph | Advisor(s): Morales, George J. | Abstract: The operation of an emissive cathode embedded in a large magnetized plasma column is examined, illuminating its role in a basic heat transport experiment. The application of a bias voltage between the emissive cathode and a distant anode mesh leads to the formation of a global current system in the quasi-neutral plasma accompanied by a self-consistent electric field. The E�B drift due to the perpendicular electric field leads to global sheared rotation of the plasma column, while heating due to Ohmic currents forms a region of elevated plasma pressure. Drift-Alfv en fluctuations excited by perpendicular gradients in pressure and azimuthal flow grow from initial noise levels and cause highly nonlinear dynamical evolution of contemporary interest including avalanche-like behavior, spontaneous profile collapse, and sudden transitions to turbulent regimes.A new analytical model for the plasma potential formed in a homogenous magnetized plasma column is presented and compared with experiments performed in the Large Plasma Device (LAPD). The model self-consistently incorporates emissive sheath boundaries, al- lowing the total current to be expressed analytically in terms of the bias applied between cathode and anode. A dimensionless parameter determining the global shape of the current system is identified and shown to be equal to the square-root of the conductivity ratio timesthe effective aspect ratio. Two regimes of heating, Ohmic and primary, are found to exist depending on the partition of the applied bias between the bulk of the plasma and the emis- sive sheath region. With the derived model, the effectiveness of a biased emissive cathode as a means for controlling internal parameters of the plasma is quantitatively assessed.The diffusive evolution of density and temperature due to the presence of an emissive cathode is studied in the context of a drift-reduced diffusive transport model based on Bra- ginskii’s transport equations. The model includes diffusive effects due to both Coulomb collisions and ion-neutral collisions and is numerically matched with emissive sheath bound- ary conditions. The linear stability of the self-consistent pressure and flow profiles to drift- Alfv en fluctuations is assessed in the context of a collisional drift-kinetic formalism, where it is shown that both an increase or a decrease in the magnitude of sheared azimuthal flow can cause the stabilization or destabilization of different modes depending on the parameter values.The nonlinear evolution of the self-consistent profiles in the presence of drift-instabilities is analyzed in the context of an axially-averaged transport model, whose primary element is nonlinear E�B convection. The model leads to nontrivial time evolution in the presence of a source term; for varying locations in parameter space, the model demonstrates inter- mittent avalanching behavior and sudden transitions from quiescent to turbulent regimes. Time traces of ion-saturation current measured in the LAPD are compared with those from the model and are found to qualitatively agree, providing simple insight to the physical mechanisms at work during these complex dynamical regimes." @default.
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• W3048295625 date "2020-01-01" @default.
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• W3048295625 title "Emissive cathode operation in a large magnetized plasma and its role in a basic heat transport experiment" @default.
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