Session GM9: Mini-conference: Nonequilibrium Transport, Interfaces, and Mixing in Plasmas I

Three Dimensional Magnetic Field Reconnection in Flux Rope Experiments

Magnetic flux ropes are bundles of twisted magnetic fields and their associated currents. One or more flux ropes are routinely generated in the Large Plasma Device at UCLA. When the current in a flux rope is large it is subject to a ``kink'' instability. If two side by side ropes kink they can collide and fully 3D magnetic reconnection occurs. In a reconnection process magnetic energy is destroyed. The energy is transformed to heat, energized particles and waves. In the UCLA experiment the time dependent magnetic fields, plasma flows, electron temperature, plasma density, and the space charge and inductive electric fields were measured at over 42,000 spatial positions throughout the plasma volume over several million rope collision experiments. Magnetic field lines are followed and used to derive quasi-seperatrix layers (QSL), extended surfaces within which reconnection occurs. Pinpointing the reconnection location(s), however, is non-trivial. We use the topological concepts of the winding number, a measure of how all field lines wind around every individual field line, as well as the twist and writhe to shed light on where reconnection occurs where there are no nulls in the magnetic field.   

Elastic Turbulence in Flatland: Interfaces, Blobs and Transport

In this paper, I discuss transport and mixing in 2D MHD and 2D Cahn--Hilliard--Navier--Stokes turbulence. These are closely related systems of fundamental importance. In each case, we see that: i) a system of blobs and barriers (interfaces) form in the turbulent state, ii) the blobs form by an inverse cascade-like aggregation process, and iii) the barriers at interfaces regulate transport. We see that barrier location and packing fraction are central to mixing and transport. Special attention is focused on how interfaces encode memory in these systems. We discuss implications for the classic problem of turbulent resistivity.   

Unusual dynamics of convection in the Sun

Turbulent convection in the Sun, which is the dominant mode of heat transport outwards of 70 percent of its radius, differs in nearly all its aspects from convection processes known on Earth, certainly those under controlled laboratory conditions, thus seriously challenging existing physical models of convective turbulence and boundary conditions. Solar convection is a multiscale-multiphysics phenomenon including the transport of mass, momentum and heat in the presence of rotation, dynamo action, radiation fluxes and partial changes in chemical composition. Standard variables of state such as pressure, mass density, or temperature vary over several orders of magnitude thus generating a highly stratified flow. Yet, it is useful to attempt to shed some light on these results with a view to gaining a deeper understanding of dynamical aspects of solar convection.~In particular, we discuss characteristic scales and dimensionless parameters from the perspective of turbulent convection in laboratory conditions, a research field that has rapidly progressed in the last few decades.~Our estimates and calculations are mostly based on the standard solar model.~   

Intermittent turbulence in mixed-ion plasmas in LAPD

Intermittent turbulence is observed in the edge region of a wide range of magnetic confinement devices. The intermittency is explained by the generation of field-aligned filamentary structures, called "blobs," within the turbulent edge region. These blobs are observed to propagate outward and are responsible for a large fraction of particle transport in the edge region of magnetic confinement devices. Experiments on the Large Plasma Device at UCLA have shown that blobs have a typical cross-field size scale of $\sim 10 \rho_s$ and a typical cross-field velocity of $\sim C_s/10$\footnote{T.A. Carter, PoP 13, 010701 (2006)}. Recent experiments have focused on the properties of blobs in mixed-ion plasmas, in particular D-H plasmas with varying concentration. Such data is relevant to fusion plasmas which will ultimately be mixes of D and T. Results on the scaling of blob size, velocity, transport and other statistics (waiting time) with ion mix will be presented.   

Multiscale modeling and nested simulations of three-dimensional ionospheric plasmas: Rayleigh–Taylor turbulence and non-equilibrium layer dynamics at fine scales

Multiscale modeling and high resolution three-dimensional simulations of non-equilibrium ionospheric dynamics are major frontiers in the field of space sciences. The latest developments in fast computational algorithms and novel numerical methods have advanced reliable forecasting of ionospheric environments at fine scales. These new capabilities include improved physics-based predictive modeling, nesting and implicit relaxation techniques that are designed to integrate models of disparate scales. A range of scales, from mesoscale to ionospheric microscale, is included in a 3D modeling framework. Analyses and simulations of primary and secondary Rayleigh–Taylor instabilities in the equatorial spread F (ESF), the response of the plasma density to the neutral turbulent dynamics, and wave breaking in the lower region of the ionosphere and non-equilibrium layer dynamics at fi ne scales are presented for coupled systems (ions, electrons and neutral winds), thus enabling studies of mesoscale/microscale dynamics for a range of altitudes that encompass the ionospheric E and F layers. We examine the organizing mixing patterns for plasma flows, which occur due to polarized gravity wave excitations in the neutral field, using Lagrangian coherent structures (LCS). LCS objectively depict the   

Exploring the universe through Discovery Science on NIF

An overview of recent research on hydrodynamic instabilities and mixing done on the 2 MJ, 192 beam NIF laser facility at LLNL through the NIF Discovery Science program, and on the Omega and EP laser facilities at LLE will be presented. A selection of examples will be drawn from laboratory experiments and simulations of hydrodynamic instabilities and mixing at laser or x-ray driven ablation fronts, [1, 2, 3] classical embedded interfaces, [4] strength stabilized scenarios, [5, 6] radiative shock stabilized flows, [7, 8] and collisionless, low density high velocity, interpenetrating plasma flows. [9, 10] Examples of experiments in nonlinear hydrodynamic instabilities and interface mixing will be given. \\ 1. D.T. Casey PRE 90, 011102(R) (2014). \\ 2. V.A. Smalyuk PRL 112, 025002 (2014). \\ 3. A. Casner, PoP 22, 056302 (2015). \\ 4. S.R. Nagel, PoP 24, 072704 (2017). \\ 5. H.-S, Park PRL 104, 135504 (2010). \\ 6. A. Krygier, PRL, submitted (2019). \\ 7. C.C. Kuranz, Nature Commun. 9, 1564 (2018). \\ 8. C.M. Huntington PoP 25, 052118 (2018). \\ 9. J. S. Ross et al., PRL 118, 185003 (2017).\\ 10. C.M. Huntington Nat. Phys. 11, 173 (2015).   

Richtmyer-Meshkov instability in magnetized laser plasmas

Richtmyer-Meshkov instability (RMI) is categorized in the interfacial instabilities, which occurs when an incident shock strikes a corrugated contact discontinuity. One of the curious questions related to the RMI is the interaction with a magnetic field. We have performed laser experiments by using the GEKKO laser in Osaka University to evaluate the growth of RMI in magnetized plasmas. The instability is triggered by laser-driven shock wave. An interface forms a CH foil target with surface modulation and nitrogen gas in the chamber. The growth of the interface perturbation is observed by optical radiography and the growth velocity is measured from the time evolution of the mixing layer. We measured the signal of the amplified field by B-dot probe, which shows the clear difference between the RMI growing case and no turbulence case. It is known that the growth of RMI is suppressed significantly when the seed magnetic field is stronger than the critical value, which is typically of the order of 100T in laboratory laser plasmas. We also performed the RMI experiments with a stronger field and observed the reduction of the grow velocity by X-ray radiography. In this talk, we will present our experimental results as well as the theoretical interpretation by the help of MHD simulations.   

Mix and hydrodynamic instabilities in indirect-drive ICF implosions on NIF

Hydrodynamic instabilities are major factor in degradation of spherical implosions in inertial confinement fusion (ICF). Recent results and plans for measuring the seeds and growth of capsule hydrodynamic instabilities in all phases of indirect-drive ICF implosions will be presented. In the acceleration phase of implosions, instability growth of perturbations was measured by x-ray radiography. To understand the stability of the ablator-ice interface, mix between ablator and the fuel is being inferred using monochromatic Bragg crystal radiography. In the deceleration phase of implosions, the technique of using the self-emission from the capsule inner shell to ``visualize'' and quantify the perturbations and asymmetries near peak compression will be extended to layered DT implosions. In the recent experiments, the extent and evolution of perturbations seeded by capsule support ``tents'', fill tubes, and low-mode asymmetries have been measured. The new developments in the area of instability seed mitigation will also be presented.   

Interface dynamics: New mechanisms of stabilization and destabilization and structure of flow fields

Interfacial mixing and transport are nonequilibrium processes coupling kinetic to macroscopic scales. They occur in plasmas, fluids, and materials over celestial events to atoms. Grasping their fundamentals can advance a broad range of disciplines in science, mathematics, and engineering. This work focuses on the long-standing classic problem of stability of a phase boundary - a fluid interface that has a mass flow across it. We briefly review the recent advances and challenges in theoretical and experimental studies, develop the general theoretical framework directly linking the microscopic interfacial transport to the macroscopic flow fields, discover the new mechanisms of interface stabilization and destabilization for both inertial and accelerated dynamics, and chart perspectives for future research.