Session TM12: Mini-Conference on Transport in Non-Ideal, Multi-Species Plasmas I

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The solar atmosphere is composed of many species which are populated at different ionization and excitation levels. The upper chromosphere, transition region and corona are nearly collision less. Consequently, slippage between, for instance, ions and neutral particles, or interactions between separate species, may play important roles. We have developed a 3D multi-fluid and multi-species numerical code (Ebysus) to investigate such effects. Ebysus is capable of treating species (e.g., hydrogen, helium etc) and fluids (neutrals, excited and ionized elements) separately, including non-equilibrium ionization, momentum exchange, radiation, thermal conduction, and other complex processes in the solar atmosphere. Treating different species as different fluids leads to drifts between different ions and an electric filed that couple these motions. The coupling for two ionized fluids can lead to an anti-phase rotational motion between them. Different ionized species and momentum exchange can dissipate this velocity drift, i.e., convert wave kinetic energy into thermal energy. High frequency Alfven waves and reconnection, both thought to occur in the solar atmosphere, can drive such multi-ion velocity drifts.   

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This talk presents a theoretical study of semi-classical turbulence in a dusty plasma monolayer, where the energy cascade from large to smaller discretized vortices is guided by both spatial defects and non-local interactions. The range of vortex scales available within the dusty plasma system is defined through the participation ratio of particles in each vortex, with a dissipation scale defined by the fluctuations of individual dust grains. Spatial defects due to dust charging and non-local effects due to dust-plasma interactions result in anomalous dust particle diffusion. This introduces statistical anisotropies (preferential spatial directions) within the turbulent dynamics and deviations from Kolmogorov-like energy cascade. These processes are examined analytically using a recently developed fractional Laplacian spectral technique, which identifies the active energy channels and dissipation scales as a function of disorder and nonlocality. The predictions from the theoretical analysis are compared against results from many-body simulations of dusty plasma monolayers.   

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First Light Fusion is exploring a unique route toward controlled inertial confinement fusion, a core part of which is the use of a uniaxial, projectile-based driver. An important example of a fusion target aligned with this configuration is that considered by Derentowicz et al. [1]. The fuel energetics in this target are influenced by numerous transport processes: thermal conduction, material interface transport, temperature relaxation, radiation transport, self-generated fields, kinetic reactivity reduction, strong shear flows and viscous shock damping. Preliminary results show that heat flow between the fuel and anvil is the strongest handle on the predicted yield. This is exacerbated by the prediction of warm, dense states of matter, wherein thermophysical properties remain broadly uncertain. This submission will present sensitivity study results from integrated simulations focusing on the influence of salient transport phenomena. The impact of widely-used, simplistic models on the predicted yield is examined in contrast to more detailed approaches. The way in which admixtures of high-Z dopants, and more generally multi-component materials, should be handled is also considered. References [1] H. Derentowicz, et al., Bull. L'Acad. Polon. Sci. Ser. Sci. Techn. 25, 135 (1977)   

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Collisions often play a significant or even dominant role in transport phenomena, including collisions between the electron and ion components of a plasma. The near-absolute-zero electron temperatures and low densities of ultracold neutral plasmas make it possible to produce extreme electron magnetization with only moderate laboratory magnetic fields. We have developed a technique to measure electron-ion collision rates in such magnetized plasmas through determining the electron heating rate caused by applying radiofrequency (RF) fields. We have measured unexpectedly low rates and unexpected variation as a function of magnetic field as compared to theory predictions. These prior measurements were limited in RF frequency range and electron temperature, though. A new extension to our technique allows measurements to be conducted with time-varying RF amplitudes that enable measurements over a greater span of electron temperature and RF frequency range, enabling studies where perturbation theory is more applicable to other studies that are sensitive to effects from strong coupling. Through scaling relations, these experimental measurements can be related to comparable systems in high-energy density plasmas and astrophysical plasmas with GigaGauss or greater magnetic fields.   

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We report measurements of ion temperature evolution in a dual-species ultracold neutral plasma. This plasma is created by photo-ionizing laser-cooled Yb and Ca atoms to create a plasma just inside the strong-coupling regime. As this binary mixture expands, we measure the time-evolving velocity, density, and ion temperature using laser-induced fluorescence. We determine the spatially-resolved density and temperature in a two-dimensional slice of the plasma distribution. The 4:1 mass ratio of the Yb and Ca ions gives rise to a classic two-temperature plasma near the center of the system. We measure the rate at which the central Yb and Ca ion temperatures relax over a range of different initial conditions. We find that the measured relaxation rate depends more weakly on density than theoretical models predict. This may be explained by spatial gradients in the plasma density and temperature profiles.   

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Relaxation in non-ideal plasmas is a complex process that involves expansions/contractions, thermal conduction, thermalization and temperature equilibration. Unfortunately, very little is known experimentally about these processes because of the challenges associated with isolating them individually. As such, we rely heavily on unvalidated theoretical models. This knowledge gap is particularly large for the very challenging case of ionic relaxation. Current improvements in experimental techniques now allow for the creation of ultracold binary mixtures for the investigation of equilibrium relaxation phenomena dominated by ion-ion collisions. These experiments, when complemented by large-scale molecular dynamics (MD) simulations, promise to provide detailed data on these processes. We have modeled a binary relaxing ultracold neutral plasma with MD, motivated by experiments at BYU, and compared the results to a wide variety of theoretical models. None of the temperature relaxation rates from current theoretical models is within an order of magnitude of the MD simulations, suggesting serious gaps in our knowledge of ionic relaxation. This disagreement may be due to the strong coupling effects that are ignored or underestimated by current theoretical models.   

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Density functional theory asserts that all the thermodynamic and linear transport properties are a functional of the one-body electron density $n(r)$ of an electron-ion system. If we consider a warm-dense matter (WDM) system it usually consists of an electron system and an ion system in equilibrium at some temperature $T$. We may consider any single representative ion in the WDM together with its neutralizing electron density distribution $n(r)$ around it as well as the ion distribution $\rho(r)$. The latter is the crystal structure if the system is a solid. In a fluid $\rho(r)$ is spherically symmetric and simple. Such a neutral object is known as a neutral pseudoatom. The $n(r)$ can be reconstructed to give the free electron pile up $\Delta n_f(r)$ at the ion in a uniform electron gas of the appropriate density. We have proposed (since the 1980s) the construction of simple linear local (i.e., $s$-wave) electron-ion potentials $U_{ei}(q)$ as well as second-order ion-ion pair potentials $V_{ij}(q)$ derived from $\Delta n_f(q)$ as an appropriate and usually accurate method for obtaining thermodynamic and transport properties of WDMs. We illustrate this approach by applications to Al, Li, C and Silicon WDM fluids as well as mixtures. [1]PHYSICAL REVIEW E 94, 053211 (2016)