Ultracold neutral plasmas, formed by photoionizing laser-cooled atoms near the ionization threshold, explore matter at the intersection of atomic, soft condensed matter, and plasma physics. Because of the low electron and ion temperatures (Te=1-1000K and Ti=1K), the Coulomb interaction energy per particle can exceed the thermal energy, which makes the system strongly coupled. Strong coupling is of interest in many areas of physics. It leads to spatial correlations and surprising equilibration dynamics, and it makes theoretical description much more difficult. Ultracold plasmas provide a valuable window into these phenomena because of the excellent control of initial conditions and diagnostics that are available. I will describe recent results that give the first measurement of equilibration rates [1] and diffusion coefficients in the strongly coupled regime [2], which are relevant for plasmas produced through short-pulse laser irradiation of solid targets, such as in inertial confinement fusion. The dynamics also shows non-Markovian or memory effects that are reminiscent of the behavior of metal liquids.

This work is supported by the National Science Foundation, Department of Energy, and the Air Force Office of Scientific Research.

[1] “Velocity Relaxation in a Strongly Coupled Plasma,” G. Bannasch, J. Castro, P. McQuillen, T. Pohl and T. C. Killian, Phys. Rev. Lett. 109, 185008 (2012).

[2] “Experimental Measurement of Self-Diffusion in a Strongly Coupled Plasma,” T. S. Strickler, T. K. Langin, P. McQuillen, J. Daligault, and T. C. Killian, Phys. Rev. X 6, 021021 (2016).