Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session GO9: Strongly Coupled and Other Basic Plasmas |
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Chair: Rich Petrasso, Massachusetts Institute of Technology Room: 211 CD |
Tuesday, November 1, 2016 9:30AM - 9:42AM |
GO9.00001: Laser-cooling calcium ions in an ultracold neutral plasma Stephen Rupper, Scott Bergeson The temperature in ultracold neutral plasmas is typically limited by nearest-neighbor interactions during the initial formation stage. The heating occurs because the ions are formed from resonantly-ionized laser-cooled atoms. These atoms have a completely flat pair correlation function before ionization occurs. After ionization, a deep hole in the pair correlation function develops at small radius. This increases the ion temperature by two orders of magnitude (10 mK to 3 K) in less than 1 $\mu$s. Overcoming this source of heating remains a major priority for this field. It would allow careful measurements of the transport and kinetic properties of the strongly coupled plasma over a wide range in the Coulomb coupling parameter, from 1 to about 30. We have built an experiment to laser-cool the ions in an ultracold neutral calcium plasma. The parameters in which the laser cooling is expected to occur are presented. Cooling is complicated because the Coulomb forces are typically orders of magnitude larger than the optical forces. Results from our initial work in laser cooling the plasma will be presented. [Preview Abstract] |
Tuesday, November 1, 2016 9:42AM - 9:54AM |
GO9.00002: Progress towards energy relaxation studies in an ultracold dual-species Yb/Ca plasma Scott Bergeson, Michaela Kleinert Ultracold neutral plasmas provide a unique laboratory system for studying dynamics of strongly coupled Coulomb systems. The precision spectroscopy and imaging tools of atomic physics are brought to bear on these systems of resonantly-ionized laser-cooled atoms. We have simultaneously laser-cooled and trapped Yb and Ca atoms at densities of $10^{10}~\mbox{cm}^{-3}$. The Yb and Ca atoms differ by a factor of 4 in mass. Using resonant laser excitation, we selectively ionize the two different species for the purpose of studying energy relaxation in a strongly coupled Coulomb system. The strong coupling parameter and ion mass ratio are expected to be relevant to equilibration studies in warm dense matter experiments. Sequential ionization of the two species allows the later-ionized system to abruptly perturb the first one. Adjusting the stoichiometry of the plasma allows us to carefully determine the amount of additional heat deposited into the plasma. Molecular dynamics simulations suggest that in some regimes, the energy relaxation is nearly chaotic. This talk will summarize our progress towards ultracold plasma work in this dual-species system. [Preview Abstract] |
Tuesday, November 1, 2016 9:54AM - 10:06AM |
GO9.00003: Species Diffusion in Plasma Mixtures Robert Rudd, Tomorr Haxhimali, William Cabot, Frank Graziani Diffusion of ions in a plasma is a non-equilibrium process that occurs in response to concentration gradients and other drivers. In weakly coupled plasmas the rate of diffusion is largely governed by binary collisions, and the diffusivity can be calculated reliably using Chapman-Enskog theory. As typical of this kind of calculation, the well-known result includes a Coulomb logarithm. When the plasma is more strongly coupled, the binary scattering approximation fails and the Coulomb logarithm becomes problematic. Here we report the result of molecular dynamics simulations of ion diffusion in a D-Ar plasma mixture over a range of temperatures 100 eV to 10 keV. These results extend our prior work which focused on 100 eV [Haxhimali et al., Phys. Rev. E 90, 023104 (2014)]. In the process we clarify how the strongly coupled liquid-like diffusion transitions smoothly to the weakly coupled gas-like diffusion at higher temperatures, getting good agreement with kinetic theory calculations at the higher temperatures [Paquette et al., Astrophys. J. Suppl. 61, 177 (1986)]. [Preview Abstract] |
Tuesday, November 1, 2016 10:06AM - 10:18AM |
GO9.00004: Effective Potential Theory for Diffusion in Binary Ionic Mixtures Nathaniel R. Shaffer, Scott D. Baalrud, Jerome Daligault We present theoretical predictions of diffusion coefficients for classical binary ionic mixtures spanning weak to strong coupling. Strongly coupled, classical ionic mixtures are realized in non-neutral plasmas, and they serve as a useful reference system for ultracold plasmas and warm dense matter. We model many-body correlation effects on transport by treating binary interactions via the potential of mean force and by treating the Coulomb hole around each ion with an effective exclusion radius. This approach is known to agree closely with molecular dynamics results for the transport properties of single-component plasmas - including warm dense matter - up to the onset of liquid-like correlations, and we find a comparable range of agreement for the interdiffusion coefficient of binary ionic mixtures. We also present the self-diffusion coefficients of the two ion species in a mixture, in light of recent measurements in ultracold neutral plasmas\footnote{Strickler, T.~S., Langin, T.~K., McQuillen, P., Daligault, J., \& Killian, T.~C.\ 2016, Phys. Rev. X, 6, 021021 }. An outlook for applying the theory to electron-ion transport in the strong coupling regime is also considered. [Preview Abstract] |
Tuesday, November 1, 2016 10:18AM - 10:30AM |
GO9.00005: Diffusive Mixing in Strongly Coupled Plasmas Abdourahmane DIAW, Michael Murillo A multispecies hydrodynamic model based on moments of the Born-Bogolyubov-Green-Kirkwood-Yvon (BBGKY) hierarchy [1] is developed for physical conditions relevant to astrophysical plasmas. The modified transport equations incorporate strong correlations through a density functional theory closure, while fluctuations enters through a mixture BGK operator. This model extends the usual Burgers equations for a dilute gas to strongly coupled and isothermal plasmas mixtures. The diffusive currents for these strongly coupled plasmas is self-consistently derived. The settling of impurities and its impact on cooling of white dwarfs and neutron stars [2] can be greatly affected by strong Coulomb coupling, which we show can be quantified using the direct-correlation function. [1] A. Diaw and M. S. Murillo, Phys. Rev. E, 92, 013107 (2015) [2] A. Diaw and M. S. Murillo (to appear in Astrophysical Journal, 2016). [Preview Abstract] |
Tuesday, November 1, 2016 10:30AM - 10:42AM |
GO9.00006: Molecular dynamics studies of electron-ion temperature equilibration in the coupled-mode regime Lorin X. Benedict, Michael P. Surh, Christian R. Scullard, Liam G. Stanton, Alfredo A. Correa, John I. Castor, Frank R. Graziani, Lee A. Collins, Joel D. Kress We use classical molecular dynamics (MD) to study electron-ion temperature equilibration in two-component plasmas in regimes in which the presence of coupled collective modes substantively impacts the equilibration rate. Guided by previous kinetic theory work in which predictions were made of both the regimes and the sizes of this effect, we examine hydrogen plasmas at a density of $n = 10^26$ 1/cm$^3$, $T_i = 10^5$ K, and $10^7$ K $< T_e < 10^9$ K. The non-equilibrium classical MD simulations are performed with inter-particle interactions modeled by quantum statistical potentials (QSPs). We compare our MD results to theoretical predictions using a Lenard-Belescu scheme, together with an accounting of time-varying potential energy (in the sense of a non-ideal, two-temperature equation of state), and discuss the extent to which our findings are expected to reflect those of true quantum hydrogen, as determined from quantum molecular dynamics simulations. This work is performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and by Los Alamos National Laboratory under Contract DE-AC52-06NA25396. [Preview Abstract] |
Tuesday, November 1, 2016 10:42AM - 10:54AM |
GO9.00007: Particle-Particle-Particle-Mesh (PPPM) method generalized to arbitrary linear screening of Coulomb potential for large-scale simulation of strongly coupled plasmas G. Dharuman, L. Stanton, J. Glosli, M. S. Murillo Molecular Dynamics (MD) simulation is a powerful method for studying large-scale correlated plasmas like warm dense matter, high-energy density plasmas and dusty plasmas. The bottleneck in MD is force calculation which naively scales as O(N\textasciicircum 2) where N is number of particles which limits the length and time scales achievable. Depending on the nature of interaction between particles, forces can span short-intermediate-long range and efficient methods like Linked-Cell-list (LCL) [1] algorithm for short range forces scales as O(N) and PPPM [1] algorithm for long range forces scales as O(N log N). In some plasmas of interest, forces can be of intermediate range depending on screening, raising a question of which of the two algorithms needs to be used. We generalized PPPM to systems described by an arbitrary dielectric response function [2]. From this generalization we optimized to find a boundary in the space of simulation size and screening that demarcates PPPM from LCL in terms of computational efficiency. We examine the implications of different choices of screening function on the cost of computing dynamic structure factor that provides insight into plasma dynamics spanning small to large wavelengths and frequencies. [1] Computer simulation using particles, CRC press 1988 [2] Phys. Rev. E 91, 033104 (2015) [Preview Abstract] |
Tuesday, November 1, 2016 10:54AM - 11:06AM |
GO9.00008: Thermodynamic State Variables In An Ultracold Plasma Sanat Kumar Tiwari, Nathaniel R. Shaffer, Scott D. Baalrud Pressure and internal energy are evaluated for a quasi-equilibrium ultracold plasma medium using classical molecular dynamics simulation. A two-component plasma model is employed in which Coulomb collapse is avoided by adding a repulsive core to the attractive Coulomb potential. As the recombining plasma is composed of free and bound electrons and ions, a method is proposed to separate the contributions of both species while evaluating thermodynamic state variables. The partial pressure and partial internal energy associated with the free charges are found to be independent of repulsive core scale length when it is sufficiently small. The partial pressure due to free charges closely follows the one-component plasma model, reaching negative values at strong coupling, but the total system pressure is always positive. Apart from the quasi-equilibrium studies, we also employ a two-temperature plasma model that is more realistic representation of ultracold plasma. Effects of mass and temperature ratio on thermodynamic state variables will be presented. [Preview Abstract] |
Tuesday, November 1, 2016 11:06AM - 11:18AM |
GO9.00009: Continuum considerations for Rydberg atom formation in low-density ultracold neutral plasmas Wei-Ting Chen, Jacob Roberts Rydberg atoms are formed in ultracold neutral plasmas primarily through three-body recombination for typical experimental conditions. At low densities, the relative importance of electron-Rydberg state-changing collisions in the dynamical evolution of the Rydberg atom state populations increases, leading to temperature scalings different from the usual $T^{-9/2}$ scaling associated with the three-body recombination rate. We report our measurement of Rydberg atom formation rates in low-density ultracold neutral plasmas. We also discuss continuum considerations in the calculation of the three-body recombination rate and its relation to our observations. [Preview Abstract] |
Tuesday, November 1, 2016 11:18AM - 11:30AM |
GO9.00010: Thermalization via collisional and non-collisional mechanisms in ultracold neutral plasmas Craig Witte, Jacob Roberts Many ultracold neutral plasmas (UCPs) are formed with non-uniform electron and ion densities. They are also formed in a way that the initial electron velocity distribution is not in thermal equilibrium. We present the results of a numerical simulation that compares the electron velocity distribution evolution after UCP formation between uniform and non-uniform density UCPs. We find three distinct thermalization time periods for the electron velocity: a rapid thermalization on the order of the electron plasma frequency timescale where position variations lead to velocity randomization; a slower second phase where non-collisional effects play a role in thermalization as evidenced by differences between thermalization rates in uniform density and non-uniform density plasmas; and an even slower third phase where the highest velocity portion of the electron thermal distribution equilibrates primarily via collisional mechanisms. These mechanisms are relevant for understanding the establishment of equilibrium in the electron component of UCPs in experimentally relevant conditions. [Preview Abstract] |
Tuesday, November 1, 2016 11:30AM - 11:42AM |
GO9.00011: Flow past an obstacle immersed in a Yukawa liquid: An atomistic study Harish Charan, Rajaraman Ganesh Understanding vortex dynamics in the flow past an obstacle in conventional fluids is a fundamental problem which exhibits a Universal relation between dimensionless vortex generation frequency described by Strouhal number (St) and dimensionless viscosity represented by Reynolds number (Ry). Considering Yukawa liquid as a prototype for strongly coupled fluids, characterized by coupling strength ($\Gamma$, ratio of average potential to kinetic energy per particle) and screening parameter ($\kappa$, ratio of mean inter-particle distance to shielding length), we address the fundamental problem of flow past an obstacle immersed in a Yukawa liquid using first principles based classical molecular dynamics simulations. The flow past an obstacle is seen to indeed develop patterns at the wake and vortices which are seen to dynamically shed the obstacle. We investigate the phenomena for the range of values of subsonic flow speeds, Ry (2-35), $\Gamma$ (10-100) and $\kappa$ (0.25-1.5). We demonstrate the existence of a new Universal St-Ry relation for Yukawa liquids and growth rate dependency on Ry. Universality of the St-Ry relationship is shown to be valid at very low Ry numbers considered ($2<$Ry$<35$). Explicit growth rate analysis has been done for the first time at the atomic scale. [Preview Abstract] |
Tuesday, November 1, 2016 11:42AM - 11:54AM |
GO9.00012: Measurements of the temporal onset of mega-Gauss magnetic fields in a laser-driven solenoid Clement Goyon, B. B. Polllock, D. T. Turnbull, A. Hazi, J. S. Ross, D. A. Mariscal, S. Patankar, G. J. Williams, W. A. Farmer, J. D. Moody, S. Fujioka, K. F. F. Law We report on experimental results obtained at Omega EP showing a nearly linear increase of the B-field up to about 2 mega-Gauss in 0.75 ns in a \textasciitilde 1 mm3 region. The field is generated using 1 TW of 351 nm laser power (\textasciitilde 8*10\textasciicircum 15 W/cm2) incident on a laser-driven solenoid target. The coil target converts about 1{\%} of the laser energy into the B-field measured both inside and outside the coil using proton deflectometry with a grid and Faraday rotation of probe beam through SiO2 glass. Proton data indicates a current rise up to hundreds of kA with a spatial distribution in the Au solenoid conductor evolving in time. These results give insight into the generating mechanism of the current between the plates and the time behavior of the field. These experiments are motivated by recent efforts to understand and utilize High Energy Density (HED) plasmas in the presence of external magnetic fields in areas of research from Astrophysics [1] to Inertial Confinement Fusion [2]. We will describe the experimental results and scale them to a NIF hohlraum size. This work was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344. [1] G. Fiksel et al., Phys. Rev. Lett. 113, 105003 (2014). [2] L. J. Perkins, et al., Phys. Plasmas 20, 072708 (2013). [Preview Abstract] |
Tuesday, November 1, 2016 11:54AM - 12:06PM |
GO9.00013: Measuring Energy Scaling of Laser Driven Magnetic Fields Jackson Williams, Clement Goyon, Derek Mariscal, Brad Pollock, Siddharth Patankar, John Moody Laser-driven magnetic fields are of interest in particle confinement, fast ignition, and ICF platforms as an alternative to pulsed power systems to achieve many times higher fields. A comprehensive model describing the mechanism responsible for creating and maintaining magnetic fields from laser-driven coils has not yet been established. Understanding the scaling of key experimental parameters such as spatial and temporal uniformity and duration are necessary to implement coil targets in practical applications yet these measurements prove difficult due to the highly transient nature of the fields. We report on direct voltage measurements of laser-driven coil targets in which the laser energy spans more than four orders of magnitude. Results suggest that at low energies, laser-driven coils can be modeled as an electric circuit; however, at higher energies plasma effects dominate and a simple circuit treatment is insufficient to describe all observed phenomenon. The favorable scaling with laser power and pulse duration, observed in the present study and others at kilojoule energies, has positive implications for sustained, large magnetic fields for applications on the NIF. [Preview Abstract] |
Tuesday, November 1, 2016 12:06PM - 12:18PM |
GO9.00014: Optimization of capacitor coil targets for generation of mega Gauss level magnetic fields using kJ-ns class lasers Deepak Kumar, Collaborative Team A controlled magnetic field is extremely useful in various laser plasma experiments, especially in the fields of fast ignition, laboratory astrophysics and charged particle beam lensing. MG level fields for such applications can be created by the interaction of a kJ-ns class laser with a capacitor-coil target. Previous experiments with such targets rely on the ablated plasma short circuiting the capacitor target, which causes a current to flow through the coil. In recently concluded experiments at the Prague Asterix Laser Facility we used the Iodine laser ($600$ J, $350$ ps, $I\lambda^2=10^{16}-10^{17}$ W/cm$^2$) with targets of varying capacitance designed so that the plasma did not short circuit the opposite plates. Such a design is also beneficial for future applications, where the magnetized targets are not affected by the ablated plasma. Spatial and temporal behavior of the magnetic field was inferred by measuring the Faraday rotation through a TGG crystal placed near the coil. A B-dot probe provided qualitative information on the timescale of evolution of current in the coil, and an electron spectrometer measured the distribution of the hot electrons. This talk will describe the experimental setup and the results of magnetic field measurement for various targets. [Preview Abstract] |
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