Ion friction at small values of the Coulomb logarithm

Abstract

Transport properties of high-energy-density plasmas are influenced by the ion collision rate. Traditionally, this rate involves the Coulomb logarithm, ln Λ. Typical values of ln Λ are ≈ 10 to 20 in kinetic theories where transport properties are dominated by weak-scattering events caused by long-range forces. The validity of these theories breaks down for strongly-coupled plasmas, when ln Λ is of order one. We present measurements and simulations of collision data in strongly-coupled plasmas when ln Λ is small. Experiments are carried out in the first dual-species ultracold neutral plasma (UNP), using Ca + and Yb + ions. We find strong collisional coupling between the different ion species in the bulk of the plasma. We simulate the plasma using a two-species fluid code that includes Coulomb logarithms derived from either a screened Coulomb potential or a the potential of mean force. We find generally good agreement between the experimental measurements and the simulations. With some improvements, the mixed Ca + and Yb + dual-species UNP will be a promising platform for testing theoretical expressions for ln Λ and collision cross-sections from kinetic theories through measurements of energy relaxation, stopping power, two-stream instabilities, and the evolution of sculpted distribution functions in an idealized environment in which the initial temperatures, densities, and charge states are accurately known.

Temperature relaxation in strongly-coupled binary ionic mixtures

“…This is surprising given the previously demonstrated accuracy of this model in reproducing experimental data and MD simulations of momentum transfer, joule heating, diffusion, viscosity, thermal conductivity, etc. (11,18,46).…”

“…Recent laboratory experiments have shown ultracold neutral plasmas (UNPs) to be effective HEDP simulators over a limited range of parameters (11,(43)(44)(45)(46)(47)(48)(49)(50). In these stronglycoupled, non-degenerate, quasi-homogeneous, quasi-steady-state plasmas the charge state is well-known, the initial electron temperature, independent of the ion temperature, is chosen with sub-percent accuracy, and the time-evolving temperatures and densities of each ion species are readily determined.…”

“…In this paper we report the first measurements of the ion-ion temperature relaxation rate using a strongly-coupled dual-species UNP (46). This system allows precise control of the ion mass ratio and plasma stoichiometry.…”

“…(C22-C24) in Appendix C of Ref (18). with n = m = 1.Experimental detailsIn the experiment, up to 20 million Ca and Yb atoms are trapped in a dual-species magnetooptical trap (MOT)(11,46). The spatial density profile of the trapped neutral atoms is approximately spherically symmetric and Gaussian, n = n 0 exp[−r 2 /(2σ 2 )] with an rms size σ ≈ 300 µm and peak density n 0 4 × 10 10 cm −3 .…”

Sculpted ultracold neutral plasmas

“…Because of this level of control of the ionization process, UNPs have a well-defined ionization of unity, and doubly ionized UNPs can also be created [28]. Moreover, UNP mixtures can be created [29]. Importantly for this paper, it is possible to modulate the ionization laser spatially by using a mask to create a UNP with a desired initial density distribution [30,31]; we will refer to such UNPs as sculpted ultracold neutral plasmas (SUNPs).…”