Gyrokinetic Simulation and Theory of Non-diffusive Transport and Chaotic Flows in Non-Equilibrium Magnetized Plasmas

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The turbulent transport of heat, particles and momentum has an impact on a wide range of plasma phenomena ranging from laboratory magnetic and inertial confinement systems for fusion energy applications, to space weather and plasma astrophysics. A large number of advances in understanding plasma turbulence and transport have been made using analytic theory and numerical simulation, such as nonlinear mode interaction, energy cascades, secondary instabilities, turbulence spreading and the role of sheared flows. Another feature of transport in these systems that are driven from equilibrium, are large fluctuations that exhibit complex spatio-temporal patterns and coherent structures which induce large relative displacements. It is these displacements, and their temporal dependence, that cause departures from the classical, diffusive fluxes commonly described by a local Fick’s law. Instead the non-equilibrated systems evolve according to spatially non-local processes that can exhibit temporal memory and partial coherency. It is phenomena of this type, so-called non-diffusive transport, that is addressed in this proposal within the context of magnetized plasmas containing cross-field pressure and/or current density gradients. Recent theoretical work has suggested that non-diffusive transport properties are connected with chaotic dynamics and that fractional diffusion models are an appropriate theoretical description. Experimental evidence for non-diffusive transport has been documented in controlled temperature gradient experiments in a large linear plasma device (Large Plasma Device (LAPD), Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)) and underlying microscopic structures have been found to have universal features related to deterministic chaos. New transport models based on these experimental findings have been formulated based on the fractional Fokker-Planck equation and chaotic advection. One of the main purposes of this investigation is to apply fully nonlinear kinetic simulations to resolve the major question as to whether the underlying microscopic dynamics is indeed chaotic or stochastic in nature and to compare with predictions based on fractional diffusion and chaotic advection. The approach taken is based on 3D gyrokinetic particle simulation in cylindrical geometry including electromagnetic effects applied to thermal transport in magnetized temperature filament experiments and related high current density channels that form magnetic structures know as flux ropes. These non-equilibrium plasma conditions (temperature filaments or current channels) can also form during antenna-launch of large amplitude low frequency plasma normal modes called shear Alfven waves. In all cases the nonlinear evolution of the self-consistent electromagnetic fields as well as the diffusion of charged particles and heat in the LAPD-type plasmas will be analyzed and compared with experiment and theoretical models. The global gyrokinetic simulations use realistic plasma parameters and can follow the evolution on transport time scales. Spatially localized synthetic probes will be developed to generate time series that can be compared to laboratory measurements and individual tagging of particles will allow us to explore microscopic diffusivity in detail. Numerical simulations such as these are key to development of a predictive capability for turbulent transport in a variety of plasma environments. The research experience for trainees, at the leading edge between theory/modeling and experiment, will provide valuable skills and preparation for work in both academic and industrial settings.