Shear Viscosity of Asymmetric Strongly Coupled Dense Plasmas

Brooklyn Noble, University of Utah

Understanding transport processes in dense and strongly coupled plasmas is key to goals as deep and compelling as the attempt to create a limitless, carbon-free energy source through inertial confinement fusion and the construction of galactic chronometers through white dwarf thermal decay rates. These processes include the transport of species (diffusion), momentum (viscosity), charge (electrical conductivity) and heat (thermal conductivity). Here we focus on viscosity and momentum transport. Conventional plasma theory is based on the kinetic theory of gases, which has been adapted to provide a good description of hot and rarefied plasmas. However, for relatively cold and dense plasmas, the spatial correlations of the ions are not negligible and kinetic theory fails. This behavior of the plasma is typically classified according to the coupling. As the temperature of a plasma is lowered, the plasma coupling increases and correlations become more important. The viscosity also increases because ion-ion interactions resist flow.

We used classical molecular dynamics (MD) simulation techniques and the Yukawa (screened Coulomb) potential to study the shear viscosity of a binary asymmetric plasma mixture (D-Ar) at stronger coupling than previously explored (temperature less than 100 eV). Our goal was to test the validity of previously developed hybrid models and to find parameter limits where the theoretical models are valid. Viscosity was calculated using the Green-Kubo method and compared to the predictions of the Chapman-Enskog (CE) kinetic theory, the Yukawa Viscosity Model (YVM) mixing rule and the Kinetics-Molecular Dynamics (KMD) mixing rule. As expected, CE was found to underestimate the shear viscosity compared to the MD results with as much as 195 percent error at strong coupling. YVM underestimated the shear viscosity at high temperatures and overestimated the shear viscosity at low temperatures. KMD showed good agreement for a temperature greater than 5 eV but overestimated the shear viscosity at lower temperatures.

Abstract Author(s): Brooklyn A. Noble, Tomorr Haxhimali, Robert E. Rudd, Heather D. Whitley