Abstract
We present a study of the collisional relaxation of ion velocities in a strongly coupled, ultracold neutral plasma on short timescales compared to the inverse collision rate. Non-exponential decay towards equilibrium for the average velocity of a tagged population of ions heralds non-Markovian dynamics and a breakdown of assumptions underlying standard kinetic theory. We prove the equivalence of the average-velocity curve to the velocity autocorrelation function, a fundamental statistical quantity that provides access to equilibrium transport coefficients and aspects of individual particle trajectories in a regime where experimental measurements have been lacking. From our data, we calculate the ion self-diffusion constant. This demonstrates the utility of ultracold neutral plasmas for isolating the effects of strong coupling on collisional processes, which is of interest for dense laboratory and astrophysical plasmas.
Highlights
In strongly coupled plasmas [1], the Coulomb interaction energy between neighboring particles exceeds the kinetic energy, leading to nonbinary collisions that display temporal correlations between past and future collision events
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
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
Summary
In strongly coupled plasmas [1], the Coulomb interaction energy between neighboring particles exceeds the kinetic energy, leading to nonbinary collisions that display temporal correlations between past and future collision events. Such non-Markovian dynamics invalidates traditional theory for collision rates [2,3,4] and transport coefficients [5,6] used for weakly coupled plasmas and frustrates the formulation of a tractable kinetic theory.
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