Velocity Relaxation in a Strongly Coupled Neutral Plasma

The analytical expressions developed by Landau and Spitzer to describe Coulomb collisions are fundamental to plasma physics, applicable over wide range of plasma conditions. However, in the strongly-coupled plasma regime (Coulomb coupling parameter $>$ 1), the theory underlying the Landau-Spitzer formula breaks down, yielding nonsensical values for collision rates. Efforts to extend theory into the strongly coupled regime have not yet led to an agreed upon solution to the problem. Also, because of the short time scales involved, experiments to measure ultra-fast relaxation times in a strongly coupled neutral plasma have also remained a challenge. In this work, we present the first direct measurement of velocity relaxation rates in strongly coupled neutral plasmas. Exploiting the very low temperatures in ultra-cold plasmas, we achieve strong coupling at low densities that permit time-resolved optical diagnostics of velocity relaxation. Already, we have measured relaxation rates with time resolution on the order of 100 ns. Currently, efforts are underway to achieve time resolutions on the order of 10 ns to study non-Markovian relaxation dynamics.