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.

We present measurements of the hyperfine splitting in the Yb-173 6s6p 1 P o 1 (F = 3/2, 7/2) states that disagree significantly with those measured previously by Das and Natarajan [Phys. Rev. A 76, 062505 (2007)]. We point out inconsistencies in their measurements and suggest that their error is due to optical pumping and improper determination of the atomic line center. Our measurements are made using an optical frequency comb. We use an optical pumping scheme to improve the signal-to-background ratio for the F = 3/2 component.