Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session NO7: Turbulence and Transport and Strongly Coupled Plasmas |
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Chair: Scott Baalrud, University of Iowa Room: OCC B117-119 |
Wednesday, November 7, 2018 9:30AM - 9:42AM |
NO7.00001: Optical-field-ionized plasma: a new platform for testing the kinetic theory of plasma instabilities Chaojie Zhang, Chen-Kang Huang, Kenneth A Marsh, Christopher E Clayton, Warren B Mori, Chan Joshi With the capability of precisely initializing anisotropic velocity distributions, an optical-field-ionized plasma may be used as a new platform for testing the kinetic theory of plasma instabilities. Here we show that a high-density helium plasma ionized by a circularly polarized laser consists of two pairs of radially counter-streaming beams due to the different ionization potential of the two helium electrons, therefore is unstable to streaming instability and filamentation instability. These instabilities grow on a sub-picosecond (ps) timescale and saturate after ~1 ps as a result of the transverse (perpendicular to the laser propagation direction) phase space diffusion which ultimately terminates the counter streaming of different electron species. The resultant distribution still has a large temperature anisotropy because the plasma remains much colder in the longitudinal direction, which in turn drives a Weibel instability on a ps timescale to further isotropize the plasma. The oscillation frequency and growth rate of these instabilities are measured using Thomson scattering of a probe beam. Particle-in-cell simulations and kinetic theory show good agreement with experiments. |
Wednesday, November 7, 2018 9:42AM - 9:54AM |
NO7.00002: Demonstration of momentum-space diffusive particle acceleration in kinetic simulations of relativistic plasma turbulence Kai Wong, Vladimir V Zhdankin, Dmitri A Uzdensky, Gregory R Werner, Mitchell C Begelman We present a statistical study of nonthermal particle acceleration (NTPA) in 3D particle-in-cell (PIC) simulations of driven turbulence in relativistic pair plasmas. NTPA is an important topic for astrophysical systems such as pulsar wind nebulae and black hole accretion flows. However, the diffusive nature of particle energy evolution assumed by Fokker-Planck models of NTPA has not yet been rigorously tested by self-consistent simulations. We develop a novel methodology to measure energy diffusion using the time evolution of tracked particle samples in simulations of relativistic plasma turbulence. We find that the standard deviation of energies for particles with the same initial energy indeed grows in time close to t1/2, i.e., diffusively. The energy diffusion and advection coefficients (Dε and Aε) for the Fokker-Planck equation are measured as functions of particle energy ε. We find Dε to be proportional to ε2 in the nonthermal power-law tail, in line with theoretical expectations. However, we observe a much shallower scaling at lower energies. We also investigate the dependence of Dε and Aε on the magnetization. These results support models of NTPA in which particles stochastically gain energy from scattering by turbulent fluctuations. |
Wednesday, November 7, 2018 9:54AM - 10:06AM |
NO7.00003: Kinetic turbulence in relativistic electron-proton plasmas Vladimir Zhdankin, Dmitri A Uzdensky, Gregory R Werner, Mitchell C Begelman We use particle-in-cell simulations to investigate driven turbulence in collisionless, relativistic electron-proton plasmas. We perform a parameter scan across initial temperature that covers the relativistic ($T_p \gg m_p c^2$) and sub-relativistic ($T_p \ll m_p c^2$) proton regimes, while electrons are always relativistic ($T_e \gg m_e c^2$). After turbulence fully develops, we find that the ratio of electron to proton dissipated energies varies from $1$ in the ultra-relativistic regime (where the plasma behaves as a pair plasma) to $\sim 0.1$ in the sub-relativistic regime. We propose an empirical formula to describe the dependence of this electron-proton energy partition on the instantaneous ratio of electron to proton characteristic gyroradii (which is a function of initial temperature and plasma beta). We also find that, whereas both particle species are efficiently accelerated in the relativistic regime, protons exhibit a more substantial nonthermal population than electrons in the sub-relativistic regime. Our results have important implications for the establishment of two-temperature plasmas ($T_p \gg T_e$) in black-hole accretion flows and jets. |
Wednesday, November 7, 2018 10:06AM - 10:18AM |
NO7.00004: Turbulent Dissipation in Simple Vlasov Simulations James Juno, Jason M TenBarge, Ammar Hakim, William D Dorland In this talk, we step back from the grander challenge of elucidating the entire dynamics and thermodynamics of a magnetized, weakly collisional plasma and instead focus on a few simple systems where the solution of how dissipation occurs has an analytic result in certain limits. Our goal is to 1) demonstrate that said analytic result does indeed hold upon simulation, and 2) extend the result marginally beyond the analytic solution with the use of simulations. In particular, we consider a driven Vlasov system of Langmuir turbulence, as well as the turbulence downstream of a transverse electromagnetic shock. With the use of the Gkeyll framework, which contains both a Vlasov-Maxwell solver as well as a simplified Fokker-Planck operator, the Lenard-Bernstein operator, for self-collisions, we can model the fully driven system with both energy injection via the fields, and energy dissipation via the collision operator. The inclusion of the collision operator is in fact key for not just a philosophical, but also a physical understanding of the processes at play, as a collision operator allows one to balance the power input and dissipated to form a phase space cascade in real and velocity space. |
Wednesday, November 7, 2018 10:18AM - 10:30AM |
NO7.00005: Results from Experiments on Multiple Interacting Magnetized Electron Temperature Filaments Richard Sydora, Scott Karbashewski, Bart Van Compernolle, Matthew Poulos, Jarred Loughran, Aparajit Gnanasekaran, Samraat Thakur Steep thermal gradients in a magnetized plasma can induce a variety of spontaneous low frequency excitations such as thermal waves, drift-Alfven waves, and convective cells. We present results from experiments with multiple heat sources in close proximity. The setup consists of three biased probe-mounted CeB6 crystal cathodes that inject low energy electrons along a strong magnetic field into a pre-existing cold afterglow plasma forming three electron temperature filaments. A triangular spatial pattern is chosen for the thermal sources and multiple axial and transverse probe measurements allow for determination of the cross-field mode patterns and axial filament length. When the three sources are activated and placed within a few collisionless electron skin depths a complex wave pattern emerges due to interference of the various wave modes on each filament, thus leading to enhanced cross-field transport from chaotic mixing (E×B). Detailed eigenmode analysis of the configuration and comparison with nonlinear 2-fluid and gyrokinetic simulations will be reported. |
Wednesday, November 7, 2018 10:30AM - 10:42AM |
NO7.00006: Exploring Role of Reynolds and Maxwell Stress towards shear layer formation in ETG turbulence dominated Large Laboratory Plasma Prabhakar Srivastav, Rameswar Singh, Lalit Mohan Awasthi, Amulya Kumar Sanyasi, Pankaj Kumar Srivastav, Ritesh Sugandhi, Raghvendra Singh Shear flows are well known for transport reduction by decorrelating the turbulent eddies and hence study of mechanisms of spontaneous generation of shear flow is important. Electron Temperature Gradient (ETG) turbulence is successfully excited in finite beta, (β∼0.01-0.4 ) plasma of Large Volume Plasma Device (LVPD) having frequency and wavelength ordering, fci<f(2-15kHz)≪fce, k⊥ ρe≤1 and k⊥ ρi>1, where fci and fce are cyclotron frequencies and ρe, ρi are larmor radii of ion and electrons respectively. An E×B shear layer is observed at mid radius in LVPD. The steady state flow profile results from balance of divergence of net momentum flux with flow dissipation where the net momentum flux is made of sum of Reynolds and Maxwell stresses. It is often conjectured that Maxwell stress opposes Reynolds stress leading to weakening of zonal flows in electromagnetic turbulence. We aim to study the competing roles of Reynolds and Maxwell stresses towards shear layer formation in LVPD. Details of the novel findings will be discussed in the meeting. |
Wednesday, November 7, 2018 10:42AM - 10:54AM |
NO7.00007: Dynamic structure factor of strongly coupled Yukawa plasmas with dissipation Hanno Kaehlert The dynamic structure factor (DSF) contains a wealth of information on the collective properties of strongly coupled plasmas. In the context of warm dense matter, it has been shown that dissipative processes can lead to the emergence of a strong peak in the ionic DSF around zero frequency [1]. In complex plasmas, dissipation caused by friction with the neutral gas has been found to affect the current fluctuation spectra in a two-dimensional layer of dust particles [2]. Here, the DSF of a 3D Yukawa one-component plasma is investigated in detail using Langevin dynamics simulations. Simulations are performed for a wide range of parameters, covering weakly and strongly damped systems. The numerical results are compared with a generalized hydrodynamics model that accounts for the dissipation mechanism [2,3]. |
Wednesday, November 7, 2018 10:54AM - 11:06AM |
NO7.00008: Effects of Coupling on Stopping Power and Particle Slowing in the Classical One-Component Plasma David J Bernstein, Scott D Baalrud, Jerome Daligault We conducted molecular dynamics simulations of the slowing of charged projectile particles in one-component plasmas over a wide range of background Coulomb coupling strengths, and projectile masses and speeds. |
Wednesday, November 7, 2018 11:06AM - 11:18AM |
NO7.00009: Modeling Species Transport across a V/CH Interface Kristian Beckwith, Patrick F Knapp, Michael Sean Murillo, Jeffrey Haack We are developing a platform on Z to measure species transport across a V/CH interface, using an x-ray driven hohlraum to drive the sample to ~150 eV over 5 ns. Preliminary results show a change in the edge profiles over the ~5 ns heating time. Efforts to model this experiment take advantage of recent theoretical developments in multi-species plasma kinetics1,2 that have enabled development of computational tools3 for studying transport at material interfaces in experimentally relevant conditions. Here, we describe the application of this method to V/CH interface experiments conducted at the Z-facility at Sandia National laboratories, highlighting the role played by the development of strong electric fields at material interfaces and the sensitivity of these fields to interface conditions. 1Stanton & Murillo (2016): 10.1103/PhysRevE.93.043203 2Haack et al. (2017a): 10.1007/s10955-017-1824-9 3Haack et al. (2017b): 10.1103/PhysRevE.96.063310
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Wednesday, November 7, 2018 11:18AM - 11:30AM |
NO7.00010: Transport Properties of Two-dimensional Magnetized Yukawa Monolayers Yan Feng, John Arlin Goree, Michael Sean Murillo Stochastic transport of a two-dimensional dusty plasma liquid with a perpendicular magnetic field is investigated using Brownian dynamical simulations. The mean-squared-displacement and intermediate scattering function both suggest that the motion of dust particles tends to be more superdiffusive when the magnetic field is strong. The vibrational density of states of particles has only one dominant peak frequency, which can be expressed as a function of the cyclotron and plasma frequencies, revealing that the cyclotron motion has been coupled with the thermal motion. It is also found that the statistics of dust motion with a strong magnetic field tend to deviate from the classical Maxwellian distribution. The shear viscosity and thermal conductivity have been estimated using the Green-Kubo relations with the simulation data. As the magnetic field increases, the shear viscosity increases when the Yukawa liquid is cold; however, when the Yukawa liquid is hot, the variation trend of the shear viscosity is reverse. When the magnetic field increases, the thermal conductivity tends to be suppressed due to the strong cyclotron motion. |
Wednesday, November 7, 2018 11:30AM - 11:42AM |
NO7.00011: Abstract Withdrawn
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Wednesday, November 7, 2018 11:42AM - 11:54AM |
NO7.00012: Energy relaxation in a Ca/Yb dual-species ultracold neutral plasma Robert T Sprenkle, Scott Bergeson We report relaxation measurements in a Ca+/Yb+ dual species ultracold neutral plasma. The nearly 4:1 mass ratio of the ion species in our plasma is similar to the alpha:proton mass ratio important for fusion-class systems. Our system provides a platform for using Ca+ and Yb+ ions to find the Coulomb logarithm for momentum transfer collisions in a strongly coupled plasma environment. The velocity distributions are determined using laser-induced fluorescence. Measurements are compared to PIC code calculations that include ion friction and ambipolar field acceleration to describe dual species plasma expansion. We determine the Coulomb logarithm by comparing experimental data with the PIC results, and find Λ=0.04. |
Wednesday, November 7, 2018 11:54AM - 12:06PM |
NO7.00013: Sarkas: A High-Performant Pure-Python Molecular Dynamics Code for Strongly Coupled Plasmas Gautham Dharuman, Yongjun Choi, Michael S Murillo Molecular dynamics (MD) simulations are a powerful technique to understand strongly coupled plasmas (SCPs) at the atomistic level, helping us understand the influence of micro-scale structural and temporal properties on the observed macroscopic properties of interest. Though there exists MD codes for such simulations, to our knowledge there isn't a pure-Python production-scale MD code with GPU implementation. This motivated us to develop Sarkas - a production-scale pure-Python open-source MD code for particles interacting through Coulomb and screened Coulomb potentials that are predominant particle interactions in SCPs. Sarkas is high-performant with execution speeds comparable to compiled languages (eg. C) due to the extensive use of Numpy arrays and a just-in-time compilation using Numba. Sarkas simulates 3D systems with the potential energy and forces computed using a highly efficient Particle-Particle Particle-Mesh algorithm. The systems of our interest are ultracold neutral plasmas, dusty plasmas, early stages of inertial confinement fusion and generic warm dense matter. The user-friendliness of Python combined with high performance enables Sarkas to serve as a useful design tool for experimentalists. (G. Dharuman’s present address is LLNL.) |
Wednesday, November 7, 2018 12:06PM - 12:18PM |
NO7.00014: Dynamical Properties of Strongly Coupled Plasmas Yongjun Choi, Gautham Dharuman, Michael S. Murillo Dynamical properties of strongly coupled Yukawa plasmas are obtained over the entire liquid phase using highly-converged molecular dynamics. We obtain the dynamic structure factor, the ion-acoustic wave dispersion relations, and the intermediate scattering functions; these are compared with many theoretical approaches. In particular, we examine high frequency tails and find that most current theoretical models are unable to properly capture this feature. We also examine the opposite (hydrodynamic) limit and find important nonlocal viscosity corrections missed by most current models. |
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