Experimental Measurement of Self-Diffusion in a Strongly Coupled Plasma

Abstract

We present a study of the collisional relaxation of ion velocities in a strongly coupled, ultracold neutral plasma on short time scales compared to the inverse collision rate. The measured average velocity of a tagged population of ions is shown to be equivalent to the ion-velocity autocorrelation function. We thus gain access to fundamental aspects of the single-particle dynamics in strongly coupled plasmas and to the ion self-diffusion constant under conditions where experimental measurements have been lacking. Nonexponential decay towards equilibrium of the average velocity heralds non-Markovian dynamics that are not predicted by traditional descriptions of weakly coupled plasmas. This demonstrates the utility of ultracold neutral plasmas for studying the effects of strong coupling on collisional processes, which is of interest for dense laboratory and astrophysical plasmas.

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Plain Language Summary

In strongly coupled plasmas, the Coulomb interaction energy between neighboring particles exceeds the thermal kinetic energy. Dense laboratory and astrophysical plasmas can be strongly coupled, such as in inertial confinement fusion experiments, white dwarf stars, and gas giant planet interiors. These strong interactions limit our ability to model and understand these systems because they lead to a breakdown of fundamental assumptions underlying the standard theoretical description of collision rates and transport coefficients. Here, we describe the first experimental study of short-time-scale, collisional dynamics in a bulk, strongly coupled plasma, including a new method to probe plasma dynamics and a measurement of self-diffusion.

We conduct our experiments with neutral plasmas that are among the coldest in existence, with temperatures barely 1 degree above absolute zero. These plasmas are created by ionizing an ensemble of laser-cooled Sr88 atoms. Strong coupling is accordingly obtained at relatively low density, which leads to slow dynamics and makes short-time-scale processes (compared with the inverse collision rate) experimentally accessible. An optical pump-probe technique that tracks the plasma ions isolates the effects of strong coupling on collisional processes. This physics has traditionally been studied using large-scale computer simulations, but direct comparisons of results with experiment have been lacking until now. We study the ion velocity distributions, and we show that our findings are consistent with molecular dynamics simulations.

The combination of atomic and plasma physics opens up a new direction in an area of plasma physics that has traditionally been the playground of astrophysics and large national facilities.