Investigation of Turbulence Dynamics in the Presence of Flow Shear, Electrode Biasing, Magnetic Shear and X-Points

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Turbulence and turbulent transport of heat, momentum, and particles in the presence of spatially varying (sheared) flows are ubiquitous in fusion, space, and laboratory plasmas. It has been observed that when the shear in the flow -- either "equilibrium" ExB flows, or turbulent Reynolds stress self-driven (zonal) flows -- is sufficiently large, turbulence and/or turbulent transport can be reduced or suppressed. The high confinement mode (H-mode) widely observed in toroidal fusion devices is one example of such flow shear suppression. There has been a significant amount of work, including experimental, computational, and theoretical, on understanding the interactions between fully developed turbulence (and its associated transport) and flow shear. However, since the observed transitions to suppressed or reduced turbulence states typically occur on very fast time scales, little has been done experimentally to elucidate the detailed nonlinear dynamics of this transition. Because of the difficulty in observing the dynamics, a theoretical understanding of the transition is also lacking, since few detailed tests of models have been possible.The goal of this research is to improve the understanding of the detailed dynamics of transitions between states of unstable drift fluctuations (including broadband turbulence) and fluctuation-suppressed states in a controlled laboratory environment. Experiments, which are being conducted in the dual-source HelCat (Helicon-Cathode) device at the University of New Mexico (UNM), are investigating the effects of controlled magnetic shear and magnetic X-points on these dynamics, as well as the detailed effects of electrode biasing. These experiments are being complemented by direct comparisons with a fully nonlinear global Braginskii code, as well as other computational tools. The primary numerical code, a fully 3D global (full plasma equilibrium and fluctuations), is being used to model and understand both large-scale equilibrium flows and fluctuations. This code is complemented by a linear stability code, which will be used to interpret the nature of the fluctuations, and a 1D, 3V particle in cell (PIC) code to understand the details of the plasma potential profile as electrodes are biased to arbitrary voltages to affect flows. Additionally, measurements and simulations from the linear HelCat device are being compared with those from the toroidal TORPEX basic plasma device at the Centre de Recherche en Physique des Plasmas, École Polytechnique Fédérale de Lausanne (CRPP-EPFL) in Lausanne, Switzerland.The international research team includes researchers from UNM, the University of Alaska, Fairbanks, and CRPP-EPFL, Lausanne, Switzerland (who are participating at no cost to NSF). The team is composed of full-time faculty, graduate and undergraduate student researchers, including female and minority students, and thus has a strong STEM educational component. This work will increase the fundamental knowledge of plasma turbulence/flow shear dynamics, and is expected to have an impact on astrophysical, space, laboratory, and fusion plasmas, as well as on neutral fluid systems.