Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session BO5: Strongly Coupled Plasmas |
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Chair: Scott Baalrud, University of Iowa Room: 200 |
Monday, November 16, 2015 9:30AM - 9:42AM |
BO5.00001: Electron-ion collision rate measurement in an ultracold neutral plasma Wei-Ting Chen, Craig Witte, Jacob Roberts By applying sharp electric field pulses to an ultracold plasma (UCP), it is possible to induce an electron center-of-mass oscillation around the ion center-of-mass that subsequently damps at a measurable rate. By tuning the experimental parameters of the UCP, the main contribution to the damping rate can be made to be electron-ion collisions. Thus, the electron-ion collision rate can be studied in these systems. We have measured the electron-ion collision rate at multiple temperatures and compared them to theory predictions and will report the results of these measurements. We are able to explore parameters where strong-coupling effects are predicted to be relevant. In addition to presenting our recent results, we describe our plans to extend these measurements further towards a more strongly-coupled regime. [Preview Abstract] |
Monday, November 16, 2015 9:42AM - 9:54AM |
BO5.00002: Modeling Evaporative Electron Cooling in an Ultracold Neutral Plasma Craig Witte, Jacob Roberts Ultracold plasmas (UCPs) are formed by photoionizing a collection of laser cooled atoms. Once formed, these plasmas expand, cooling over the course of their expansion. In theory, further cooling should be obtainable by forcibly inducing electron evaporation by applying DC electric fields to extract electrons. However, this cooling is difficult to quantify experimentally. Any attempt to obtain such a measurement requires a firm knowledge of evaporation dynamics in the system. For UCPs, electron mean free paths are smaller than the width of the plasma, resulting in significant transport effects that are not included in standard evaporation treatments. We have developed a simple Monte Carlo model that incorporates these effects. This talk will discuss this model, and how it can be utilized to measure evaporation and cooling in UCPs. [Preview Abstract] |
Monday, November 16, 2015 9:54AM - 10:06AM |
BO5.00003: Non-Markovian Dynamics and Self-Diffusion in Strongly Coupled Plasmas Trevor Strickler, Thomas Langin, Patrick McQuillen, Jerome Daligault, Nikola Maksimovich, Thomas Killian In weakly coupled plasmas, collisions are dominated by long range, small angle scattering, and each collision is an uncorrelated binary event. In contrast, collisions in strongly coupled plasmas (coupling parameter $\Gamma > 1$) are dominated by short range, large angle scattering in which the collisions may be correlated and non-independent in time, \textit{i.e.}, non-Markovian. In this work, we present experimental results indicative of non-Markovian processes in a strongly coupled ultracold neutral plasma (UCNP) created by photoionizing strontium atoms in a magneto-optical trap. We use optical pumping to create spin ``tagged'' subpopulations of ions having non-zero average velocity ${\langle}v{\rangle}$, and use laser induced fluorescence (LIF) imaging to measure the relaxation of ${\langle}v(t){\rangle}$ back to equilibrium. We observe clear non-exponential decay in ${\langle}v(t){\rangle}$, which indicates non-Markovian dynamics. We further demonstrate there is a theoretical basis to consider ${\langle}v(t){\rangle}$ as an approximation to the ion velocity autocorrelation function (VAF). We then calculate diffusion coefficients from our data, demonstrating experimental measurement of self-diffusion coefficients for $0.3 < \Gamma < 3.5$. [Preview Abstract] |
Monday, November 16, 2015 10:06AM - 10:18AM |
BO5.00004: Universality in the Equilibration of Quenched Yukawa One Component Plasmas Thomas Langin, Patrick McQuillen, Trevor Strickler, Nikola Maksimovic, Thomas Pohl, Thomas Killian We study the equilibration of a Yukawa One Component Plasma (OCP) after a rapid change in the screening parameter from $\kappa_{0}=\infty$ to $\kappa_{f}(n,T_{e})$, which is realized by photoionizing a laser cooled ($T\sim 10$\,mK), uncorrelated gas of $^{88}$Sr atoms with density $n$ between $10^{14}$\,m$^{-3}$ and $3\times10^{16}$\,m$^{-3}$ using a two photon process in which the energy of one of the photons is adjustable. The excess photon energy above the ionization threshold sets the electron temperature, $T_{e}$, and thus gives us control of $\kappa_{f}$. The resultant plasma is a classical plasma with strongly coupled ions, and is therefore described by the Yukawa OCP model with the electrons treated as a screening background. After photoionization, the ions develop spatial correlations to minimize their interaction energy, thus heating the ions. Since the dynamics of a Yukawa OCP depend solely on $\kappa$, we expect the heating process to be uniquely determined by $\kappa_{f}$. We verify this behavior by measuring the ion heating curve and comparing it to molecular dynamics simulations. We also report on how this behavior can be used to accurately measure $n$ given a measured equilibration curve at a known $T_{e}$. [Preview Abstract] |
Monday, November 16, 2015 10:18AM - 10:30AM |
BO5.00005: Generalized Yukawa PPPM for molecular dynamics simulation of strongly coupled plasmas Gautham Dharuman, Liam Stanton, James Glosli, John Verboncoeur, Andrew Christlieb, Michael Murillo The Particle-Particle-Particle-Mesh (PPPM) method is an efficient way of treating the Ewald sum for long range interactions in a periodic system [1]. It makes use of the Fast-Fourier-Transform algorithm that scales as O(N logN). In this work we have applied the PPPM method to long range interactions in the weak screening limit of generalized Yukawa interaction to identify the range of screening over which PPPM is computationally more efficient than the minimum image method [2] which is usually usedfor the well-known Yukawa interactions [3]. Generalized Yukawa interaction is obtained by including arbitrary linear dielectric screening in the Yukawa model [4]. In the PPPM method the Fourier space part of the Ewald sum is treated by assigning charges to a mesh and computing the potential using an optimized Green's function that minimizes the discretization errors introduced in the forces [1]. [1] H.A. Stern and K.G. Calkins, J. Chem. Phys. 128, 214106 (2008). [2] M.P. Allen and D.J. Tildesley, Computer Simulation of Liquids (Clarendon, Oxford, 1987). [3] S. Hamaguchi, R.T. Farouki and D.H.E. Dubin, J. Chem. Phys. 105, 7641 (1996). [4] G. Dharuman, L. Stanton, J. Glosli, J. Verboncoeur, A. Christlieb and M.S. Murillo, {\-}?J. Comp. Phys. (submitted). [Preview Abstract] |
Monday, November 16, 2015 10:30AM - 10:42AM |
BO5.00006: ABSTRACT WITHDRAWN |
Monday, November 16, 2015 10:42AM - 10:54AM |
BO5.00007: A Generalized Hydrodynamics Model for Strongly Coupled Plasmas Abdourahmane Diaw, Michael Sean Murillo Starting with the equations of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy, we obtain the density, momentum and stress tensor-moment equations. The closure proceeds in two steps. The first that guarantees an equilibrium state is given by density functional theory. It ensures self consistency in the equation-of-state properties of the plasma. The second involves modifying the two-body distribution function to include collisions in the relaxation of the stress tensor. The resulting generalized hydrodynamics thus includes all impacts of Coulomb coupling, viscous damping, and the high-frequency response. We compare our results with those of several known models, including generalized hydrodynamic theory and models obtained using the Singwi-Tosi-Land-Sjolander approximation and the quasi-localized charge approximation. We find that the viscoelastic response, including both the high-frequency elastic generalization and viscous wave damping, is important for correctly describing ion-acoustic waves. We illustrate this result by considering three very different systems: ultracold plasmas, dusty plasmas, and dense plasmas. The new model is validated by comparing its results with those obtained from molecular-dynamics simulations of Yukawa plasmas, and the agreement is excellent. [Preview Abstract] |
Monday, November 16, 2015 10:54AM - 11:06AM |
BO5.00008: Temperature evolution of strongly coupled electron-ion plasmas Sanat Kumar Tiwari, Nathaniel Shaffer, Scott D. Baalrud Molecular dynamics simulations of electron-ion plasmas have been carried out, focusing on the classical strongly coupled regime relevant to ultracold neutral plasmas. The interaction of oppositely charged species is modeled using a pseudopotential with a repulsive core at a specified distance $\epsilon$ in units of average interparticle spacing. This parameter distinguishes classical from quantum statistical regimes. Simulations are initiated with an equilibration phase in which ions and electrons are held to fixed independent temperatures using a thermostat. Subsequently, the thermostats are removed and the system is allowed to evolve. Two effects are observed: (1) For sufficiently small values of $\epsilon$, the plasma rapidly heats, (2) electrons and ions equilibrate on a longer time scale. The critical $\epsilon$ value for the onset of heating and the temperature equilibration rate are compared with existing theory. Excess pressure is calculated in each case based on the equilibrium radial distribution functions obtained during the equilibration phase. The $\Gamma-\epsilon$ phase space is explored, revealing qualitative differences in the temperature evolution due to electron-ion interactions in the classical and quantum regimes. [Preview Abstract] |
Monday, November 16, 2015 11:06AM - 11:18AM |
BO5.00009: Experimental evidence for phase separation in hydrogen-helium mixtures at Jovian planet conditions G.W. Collins, S. Brygoo, M. Millot, J.R. Rygg, P.M. Celliers, J. Eggert, T.R. Boehly, R. Jeanloz, P. Loubeyre Whether or not H-He mixtures phase separate in Jovian planets is important to our understanding of the structure and evolution of Jupiter and Saturn. Also integral to such planet models, as well as mechanisms for H-He phase separation, are the insulating-to-conducting and the molecular-to-atomic-hydrogen transitions in the H-He mixture. Coupling static and dynamic compression techniques has allowed us to make the first thermodynamic and transport measurements of H-He mixtures at deep Jovian planet conditions. These data provide evidence that the H-He fluid demixes at the high pressures and temperatures expected to exist deep inside Saturn and Jupiter. This phase separation may result in the differentiation of heavier helium clusters, leading to helium rain in the deep interior of Saturn and perhaps even in a significant outer layer of Jupiter. [Preview Abstract] |
Monday, November 16, 2015 11:18AM - 11:30AM |
BO5.00010: Molecular dynamics simulation of temperature relaxation in dense hydrogen with electron force field method Qian Ma, Jiayu Dai, Jing Zhao, Zengxiu Zhao Temperature relaxation between ion and electron is a pivotal non-equilibrium process. Molecular Dynamics (MD) is considered as the most effective method to deal with this problem. But the conventional classical MD regards not only ions but also electrons as charged points, which may lose some important physical process, such as ionization, rebound. To deal these problems we use the electron force field (eFF) method. The eFF method was firstly used by Julius T. Su and William A. Goddard III and it includes quantum effects as well as classical electrostatic interactions. It assumes that the electron is represented by a floating Gaussian wave packet and the whole wave packets are combined with Hartree product. The eFF method is found to be valid to describe the property of warm dense hydrogen. Here we want to use this method to investigate the electron-ion temperature relaxation process which includes the excited electrons and bound electrons. At the same time, the Coulomb Catastrophe in classical molecular dynamics may be avoided. [Preview Abstract] |
Monday, November 16, 2015 11:30AM - 11:42AM |
BO5.00011: Nuclear quantum effects on structure and transport properties of dense liquid helium Dongdong Kang, Jiayu Dai, Jianmin Yuan Transport properties of dense liquid helium under the conditions of planet's core and cool atmosphere of white dwarfs are important for determining the structure and evolution of these astrophysical objects. We have investigated these properties of dense liquid helium by using the improved centroid path-integral simulations combined with density functional theory. The results show that with the inclusion of nuclear quantum effects (NQEs), the self-diffusion is largely higher while the shear viscosity is notably lower than the results of without the inclusion of NQEs due to the lower collision cross sections even when the NQEs have little effects on the static structures. The potential surface of helium atom along the simulation trajectory is quite different between MD and PIMD simulations. We have shown that the quantum nuclear character induces complex behaviors for ionic transport properties of dense liquid helium. NQEs bring more fluctuations of local electronic density of states than the classical treatment. Therefore, in order to construct more reasonable structure and evolution model for the planets and WDs, NQEs must be reconsidered when calculating the transport properties at certain temperature and density conditions. [Preview Abstract] |
Monday, November 16, 2015 11:42AM - 11:54AM |
BO5.00012: Double core-hole emissivity of transient aluminum plasmas produced in the interaction with ultra-intense x-ray laser pulse Cheng Gao, Jiaolong Zeng, Jianmin Yuan Emissivity of single core-hole (SCH) and double core-hole (DCH) states of aluminum plasmas produced in the interaction with ultra-intense x-ray laser pulse interaction are investigated systematically by solving the time-dependent rate equation implemented in the detailed level accounting approximation. We first demonstrated the plasma density effects on level populations and charge state distribution. Compared with recent experiments, it is shown that the plasma density effects play important roles in the evolution dynamics. Then we systematically investigated the emissivity of the transient aluminum plasmas produced by the x-ray laser pulses with a few photon energies above the threshold photon energy to create DCH states. For the laser photon energy where there are resonant absorptions (RA), 1s-np transitions with both full 1s and SCH 1s states play important roles in time evolution of the population and DCH emission spectroscopy. The significant RA effects are illustrated in detail for x-ray pulses, which creates the 1s-2p resonant absorption from the SCH states of Al VII. With the increase of the photon energy, the emissions from lower charge states become larger. [Preview Abstract] |
Monday, November 16, 2015 11:54AM - 12:06PM |
BO5.00013: On the selforganization of a kind of field and highly diluted matter in astrophysics Daniel Berdichevsky It is explored the self organization of matter and field in regions beyond our common reach on the surface of our planet and its atmospheric surroundings. This state of matter, which most basic property, the freezing in the magnetic field, see e.g., Chew et al, 1956, has proved to exist in the regions where robotic observations in the near and far space perform detailed observations of magnetic fields, and extreme dilute plasma (commonly about 1000 to 0.1 or less ionized particles per cubic cm). We present and discuss here simple hypotheses on the nature of what appears to be substantial amount of matter covering very large amount of space and capable of possibly self organization, with the help of very strong magnetic fields, in the ways we explored resently in Berdichevsky and Shefers, 2015s. This work is in many ways an extension of Alfven work on magnetized space plasmas, Alven, 1942. We, like at his moment Alfven himself, are confronted with the difficult limitations of not being able to do the study in a laboratory where ideally is possible to control the initial conditions of the problem. Chew, G.F., M.L., Goldberger, and F.E. Low, 1956, the Royal Soc. London, section Math {\&} Phys Sc., 236, pp. 112. Alfv\'{e}n, H (1942). ``Existence of electromagnetic-hydrodynamic waves.'' \textit{Nature} \textbf{150}: 405.. \underline {doi}:\underline {10.1038/150405d0} Berdichevsky, D.B., and K., Schefers, ApJ, 803, 70, 2015, doi: 10.1088/0004-637X/805/1/70 [Preview Abstract] |
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