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IEEE International Conference on Plasma Science (ICOPS)

Temperature Relaxation in Strongly-Coupled Binary Ionic Mixtures

Luciano G. Silvestri, Michael S. Murillo, R. Tucker Sprenkle, Scott Bergeson

Published Sep 12, 2021• 2 citations
Publisher

Abstract

Relaxation in non-ideal plasmas is a complex process that involves expansions/contractions, thermal conduction, thermalization and temperature equilibration. Unfortunately, very little is known experimentally about these processes because of the challenges associated with isolating them individually. As such, we rely heavily on unvalidated theoretical models. This knowledge gap is particularly large for the very challenging case of ionic relaxation. Current improvements in experimental techniques now allow for the creation of ultracold binary mixtures for the investigation of equilibrium relaxation phenomena dominated by ion-ion collisions. These experiments, when complemented by large-scale molecular dynamics (MD) simulations, promise to provide detailed data on these processes. We have modeled a binary relaxing ultracold neutral plasma with MD, motivated by experiments at Brigham Young University, and compared the results to the most common theoretical models. None of the temperature relaxation rates from current theoretical models is within an order of magnitude of the MD simulations, suggesting serious gaps in our knowledge of ionic relaxation. We argue that such disagreement is due to dynamical correlations neglected by the currently available theoretical models 1 , 2 .

DOI
10.1109/icops36761.2021.9588497

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Relaxation in non-ideal plasmas is a complex process that involves expansions/contractions, thermal conduction, thermalization and temperature equilibration. Unfortunately, very little is known experimentally about these processes because of the challenges associated with isolating them individually. As such, we rely heavily on unvalidated theoretical models. This knowledge gap is particularly large for the very challenging case of ionic relaxation. Current improvements in experimental techniques now allow for the creation of ultracold binary mixtures for the investigation of equilibrium relaxation phenomena dominated by ion-ion collisions. These experiments, when complemented by large-scale molecular dynamics (MD) simulations, promise to provide detailed data on these processes. We have modeled a binary relaxing ultracold neutral plasma with MD, motivated by experiments at Brigham Young University, and compared the results to the most common theoretical models. None of the temperature relaxation rates from current theoretical models is within an order of magnitude of the MD simulations, suggesting serious gaps in our knowledge of ionic relaxation. We argue that such disagreement is due to dynamical correlations neglected by the currently available theoretical models 1 , 2 .
A kinetic approach for the evolution of ultracold neutral plasmas including interionic correlations and the treatment of ionization/excitation and recombination/deexcitation by rate equations is described in detail. To assess the reliability of the approximations inherent in the kinetic model, we have developed a hybrid molecular dynamics method. Comparison of the results reveals that the kinetic model describes the atomic and ionic observables of the ultracold plasma surprisingly well, confirming our earlier findings concerning the role of ion-ion correlations [Phys. Rev. A 68, 010703 (2003)]. In addition, the molecular dynamics approach allows one to study the relaxation of the ionic plasma component toward thermodynamical equilibrium.
Using high-resolution laser-induced fluorescence spectroscopy, ion dynamics in\nan expanding ultracold neutral plasma (UNP) have been studied in unprecedented\ndetail. The evolution of the ion temperature is of great interest because the ions in\nan ultracold neutral plasma are strongly coupled, meaning their Coulomb interaction\nenergy exceeds the thermal energy. This leads to novel plasma properties that are\nhard to describe theoretically. Understanding all the factors that contribute to ion\ntemperature evolution is critical for designing schemes to cool the ions and achieve\neven stronger Coulomb coupling.\nThis work has also observed several phenomena that have not previously been\nstudied in ultracold plasmas. Ion adiabatic cooling has been observed in a UNP for\nthe first time, resulting in ion temperatures as low as 100mK and up to a tripling of\nthe Coulomb coupling parameter. The importance of electron-ion energy transfer is\ndemonstrated with the first observations of electron-ion collisional heating. Inclusion\nof this effect into existing numerical models provides better agreement with experimental\nresults. Results of molecular dynamics simulations of equilibrating plasmas\nhave been compared to early time experimental data, which provides a new method\nof density determination that is much less sensitive to experimental systematics and\nis accurate to within 10%. This improved certainty in the density allows accurate\ncomparison between data and a theoretical model of the plasma dynamics. The\nagreement is excellent, but on a hydrodynamic time scale a small amount of extra\nheat (on order of 100mK), not ascribable to electron-ion collisions or other sources in\nthe model, is detected in the ion component. Potential sources are discussed within\nas well as proposals for further studies and improvements.
Ultracold plasmas are a promising candidate for the creation of strongly coupled Coulomb systems. Unfortunately, the values of the coupling parameter Γe actually achieved after photoionization of the neutral atoms remain relatively small because of the considerable intrinsic heating of the electrons. A conceivable way to get around this obstacle might be to utilize a spontaneous ionization of the ultracold Rydberg gas, where the initial kinetic energies could be much less. However, the spontaneous avalanche ionization will result in a very inhomogeneous distribution (clusterization) of the ions, which can change the efficiency of the electron relaxation in the vicinity of such clusters substantially. In the present work, this hypothesis is tested by an extensive set of numerical simulations. As a result, it is found that despite a less initial kinetic energy, the subsequent relaxation of the electron velocities in the clusterized plasmas proceeds much more violently than in the case of the statistically uniform ionic distribution. The electron temperature, first, experiences a sharp initial jump (presumably, caused by the “virialization” of energies of the charged particles) and, second, exhibits a gradual subsequent increase (presumably, associated with a multi-particle recombination of the electrons at the ionic clusters). As a possible tool to reduce the anomalous temperature increase, we also considered a two-step plasma formation, involving the blockaded Rydberg states. This leads to a suppression of the clusterization due to a quasi-regular distribution of ions. In such a case, according to the numerical simulations, the subsequent evolution of the electron temperature proceeds more gently, approximately with the same rate as in the statistically uniform ionic distribution.
We describe a hybrid molecular dynamics approach for the description of ultracold neutral plasmas, based on an adiabatic treatment of the electron gas and a full molecular dynamics simulation of the ions, which allows us to follow the long-time evolution of the plasma including the effect of the strongly coupled ion motion. The plasma shows a rather complex relaxation behavior, connected with temporal as well as spatial oscillations of the ion temperature. Furthermore, additional laser cooling of the ions during the plasma evolution drastically modifies the expansion dynamics, so that crystallization of the ion component can occur in this nonequilibrium system, leading to lattice-like structures or even long-range order resulting in concentric shells.
We describe a hybrid molecular dynamics approach for the description of ultracold neutral plasmas, based on an adiabatic treatment of the electron gas and a full molecular dynamics simulation of the ions, which allows us to follow the long-time evolution of the plasma including the effect of the strongly coupled ion motion. The plasma shows a rather complex relaxation behavior, connected with temporal as well as spatial oscillations of the ion temperature. Furthermore, additional laser cooling of the ions during the plasma evolution drastically modifies the expansion dynamics, so that crystallization of the ion component can occur in this nonequilibrium system, leading to lattice-like structures or even long-range order resulting in concentric shells.
We investigate the strongly correlated ion dynamics and the degree of coupling achievable in the evolution of freely expanding ultracold neutral plasmas. We demonstrate that the ionic Coulomb coupling parameter Gamma(i) increases considerably in later stages of the expansion, reaching the strongly coupled regime despite the well known initial drop of Gamma(i) to order unity due to disorder-induced heating. Furthermore, we formulate a suitable measure of correlation and show that Gamma(i) calculated from the ionic temperature and density reflects the degree of order in the system if it is sufficiently close to a quasisteady state. At later times, however, the expansion of the plasma cloud becomes faster than the relaxation of correlations, and the system does not reach thermodynamic equilibrium anymore.