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Center for Nonlinear Studies |
Most technologically interesting plasmas have steep gradients. These occur at interfaces between high- and low-density materials, in plasma/wall interactions, and in every laser-produced plasma experiment. Gradients can occur across a wide range of regimes from classical weakly coupled plasmas, warm dense matter, and to strongly coupled systems. Understanding how these gradients relax remains a fundamental challenge. Standard hydrodynamic models break down in the presence of steep gradients and kinetic theories make predictions that have never been directly tested. In this talk, I will present new phase-space measurements for ions in a strongly-coupled plasma as it evolves from a sharp initial density gradients to thermal equilibrium. We use laser-induced fluorescence and Doppler spectroscopy to track the complete ion distribution function f(x,v,t) for Ca+ ions in an ultracold neutral plasma which we compare to phase-space data from kinetic simulations. When we form a plasma with a gap in the middle, we observe counter-streaming plasma flow into the gap that thermalizes on time scales somewhat at variance with kinetic models. Our measurements reveal that the initial ion acceleration response scales linearly with electron temperature, and that the simulations underpredict the initial ion response. This experimental platform enables precision tests of kinetic theories and opens new possibilities for studying plasma stopping power and flow-induced instabilities in strongly-coupled systems.
Host: Jeffrey Haack (CCS-2)