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 UI3: Quantum & HED PlasmasInvited
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Chair: Alec Thomas, University of Michigan Room: Oglethorpe Auditorium |
Thursday, November 19, 2015 2:00PM - 2:30PM |
UI3.00001: Analyzing non-LTE Kr plasmas produced in high energy density experiments: from the Z machine to the National Ignition Facility Invited Speaker: Arati Dasgupta Designing high fluence photon sources above 10 keV are a challenge for High Energy Density plasmas. This has motivated radiation source development investigations of Kr with K-shell energies around 13 keV. Recent pulsed power driven gas-puff experiments on the refurbished Z machine at Sandia have produced intense X-rays in the multi-keV photon energy range. K-shell radiative yields and efficiencies are very high for Ar, but rapidly decrease for higher atomic number (Z$_{\mathrm{A}})$ elements such as Kr. It has been suggested that an optimum exists corresponding to a trade-off between the increase of photon energy for higher Z$_{\mathrm{A}}$ elements and the corresponding fall off in radiative power. However the conversion efficiency on NIF, where the drive, energy deposition process, and target dynamics are different, does not fall off with higher Z$_{\mathrm{A}}$ as rapidly as on Z. We have developed detailed atomic structure and collisional data for the full K-, L- and partial M-shell of Kr using the Flexible Atomic Code (FAC). Our non-LTE atomic model includes all collisional and recombination processes, including state-specific dielectronic recombination (DR), that significantly affect ionization balance and spectra of Kr plasmas at the temperatures and densities of concern. The model couples ionization physics, radiation production and transport, and magnetohydrodynamics. In this talk, I will give a detailed description of the model and discuss 1D Kr simulations employing a multifrequency radiation transport scheme. Synthetic K- and L-shell spectra will be compared with available experimental data. This talk will analyze experimental data indicative of the differences between Z and NIF experimental data and discuss how they affect the K-shell radiative output of Kr plasma. [Preview Abstract] |
Thursday, November 19, 2015 2:30PM - 3:00PM |
UI3.00002: Using 1D theory to understand 3D stagnation of a wire-array Z pinch in the absence of radiation Invited Speaker: Edmund Yu Many high-energy-density systems implode towards the axis of symmetry, where it collides on itself, forming a hot plasma. However, experiments show these imploding plasmas develop three-dimensional (3D) structures. As a result, the plasma cannot completely dissipate its kinetic energy at stagnation, instead retaining significant 3D flow. A useful tool for understanding the effects of this residual flow is 3D simulation, but the amount and complexity of information can be daunting. To address this problem, we explore the connection between 3D simulation and one-dimensional (1D) theory. Such a connection, if it exists, is mutually beneficial: 1D theory can provide a clear picture of the underlying dynamics of 3D stagnation. On the other hand, deviations between theory and simulation suggest how 1D theory must be modified to account for 3D effects. In this work [1], we focus on a 3D, magnetohydrodynamic simulation of a compact wire-array Z pinch. To provide a simpler background against which to test our ideas, we artificially turn off radiation during the stagnation phase. Examination of the initial accumulation of mass on axis reveals oblique collision between jets, shock accretion, and vortex formation. Despite evidence for shock-dominated stagnation, a 1D shockless stagnation solution is more appropriate for describing the global dynamics, in that it reproduces the increase of on-axis density with time. However, the 1D solution must be modified to account for 3D effects: the flows suggest enhanced thermal transport as well as centrifugal force. Upon reaching peak compression, the stagnation transitions to a second phase, in which the high-pressure core on axis expands outward into the remaining imploding plasma. During this phase, a 1D shock solution describes the growth of the shock accretion region, as well as the decrease of on-axis density with time. However, the effect of 3D flows is still present: the on-axis temperature does not cool during expansion, which appears due to ``channels'' of plasma carrying heat to the core center.\\[4pt] [1] E.P. Yu, A.L. Velikovich, and Y. Maron, Phys. Plasmas 21, 082703 (2014) [Preview Abstract] |
Thursday, November 19, 2015 3:00PM - 3:30PM |
UI3.00003: Towards an effective nonlinear Quantum Mechanics for High Energy-density (HED) Matter Invited Speaker: Swadesh Mahajan A relativistic quantum framework is presented for dealing with high energy density matter, in particular, an assembly of particles in the field of an electromagnetic (EM) wave of arbitrary magnitude. Two different approaches are presented: 1) A Statistical Mechanical model for the HED matter is developed - Principal steps involve solving the eigenvalue problem for a quantum relativistic particle in the presence of arbitrary strength EM field. The resulting energy eigenvalue (dependent on the magnitudes A, $\omega $ and k) defines the appropriate Boltzmann factor to construct expressions for physical variables for a weakly interacting system of these field-dressed particles. The fluid equations are the conservation laws, 2) Second, an equivalent nonlinear quantum mechanics is constructed to represent a hot fluid with and without internal degrees of freedom (like spin). Representative initial results are displayed and discussed: 1) fundamental changes in the particle energy momentum relationship 2) The EM wave induces anisotropy in the energy momentum tensor, 3) the EM wave splits the spin-degenerate states, 4) the propagation characteristics of the EM wave are modified by thermal and field effects causing differential self-induced transparency, 5) Particle trapping and ``pushing'' by the high amplitude EM wave. Attempts will be made to highlight testable predictions. [Preview Abstract] |
Thursday, November 19, 2015 3:30PM - 4:00PM |
UI3.00004: Thomson scattering measurements of ion interpenetration in cylindrically converging, supersonic magnetized plasma flows Invited Speaker: George Swadling Ion interpenetration driven by high velocity plasma collisions is an important phenomenon in high energy density environments such as the interiors of ICF vacuum hohlraums and fast z-pinches. The presence of magnetic fields frozen into these colliding flows further complicates the interaction dynamics. This talk focuses on an experimental investigation of ion interpenetration in collisions between cylindrically convergent, supersonic, magnetized flows (M$\approx $10, V$_{\mathrm{flow}}\approx $100km/s, n$_{\mathrm{i}}\approx $10$^{17}$cm$^{-3})$. The flows used in this study were plasma ablation streams produced by tungsten wire array z-pinches, driven by the 1.4MA, 240ns Magpie facility at Imperial College, and diagnosed using a combination of optical Thomson scattering, Faraday rotation and interferometry. Optical Thomson scattering (TS) provides time-resolved measurements of local flow velocity and plasma temperature across multiple (7 to 14) spatial positions. TS spectra are recorded simultaneously from multiple directions with respect to the probing beam, resulting in separate measurements of the rates of transverse diffusion and slowing-down of the ion velocity distribution. The measurements demonstrate flow interpenetration through the array axis at early time, and also show an axial deflection of the ions towards the anode. This deflection is induced by a toroidal magnetic field ($\sim$ 10T), frozen into the plasma that accumulates near the axis. Measurements obtained later in time show a change in the dynamics of the stream interactions, transitioning towards a collisional, shock-like interaction of the streams, and rapid radial collapse of the magnetized plasma column. The quantitative nature of the spatial profiles of the density, flow velocities and ion temperatures measured in these experiments will allow detailed verification of MHD and PIC codes used by the HEDP community. [Preview Abstract] |
Thursday, November 19, 2015 4:00PM - 4:30PM |
UI3.00005: Particle-In-Cell Modeling for MegaJoule Dense Plasma Focus Invited Speaker: Anthony Link Megajoule scale dense plasma focus (DPF) Z-pinches with deuterium gas fill are compact devices capable of producing 10$^{12}$ neutrons per shot but past predictive models of large-scale DPF have not included kinetic effects such as ion beam formation or anomalous resistivity. We report on progress of developing a predictive DPF model by extending our 2D axisymmetric collisional kinetic particle-in-cell (PIC) simulations from the 4 kJ, 200 kA LLNL DPF to 1 MJ, 2 MA Gemini DPF using the PIC code LSP. These new simulations are by far the most detailed and computationally intensive DPF simulations run to date. They incorporate electrodes, an external pulsed-power driver circuit, and model the plasma from insulator lift-off through the pinch phase. To accommodate the vast range of relevant spatial and temporal scales involved in the Gemini DPF within the available computational resources, the simulations were performed using a new hybrid fluid-to-kinetic model. This new approach allows single simulations to begin in an electron/ion fluid mode from insulator lift-off through the 5-6 $\mu$s run-down of the 50$+$ cm anode, then transition to a fully kinetic PIC description during the run-in phase, when the current sheath is 2-3 mm from the central axis of the anode. Simulations are advanced through the final pinch phase using an adaptive variable time-step to capture the fs and sub-mm scales of the kinetic instabilities involved in the ion beam formation and neutron production. An anode shape scan as well as a scan in stored energy/charging voltage has been performed. A comparison of MJ performance for different drivers will be presented. Validation assessments are being performed, comparing against experimental measurements of neutron yield, neutron anisotropy and plasma density. [Preview Abstract] |
Thursday, November 19, 2015 4:30PM - 5:00PM |
UI3.00006: QED-PIC Simulations: from the Breakdown of Classical Physics to the Schwinger Limit Invited Speaker: Thomas Grismayer We intend to show three types of QED effects that can be simulated in OSIRIS-QED: \begin{itemize} \item [-] QED-Radiation Reaction: we have incorporate a module which allows photons emission from nonlinear Compton scattering. The typical electron energy sufficient to diagnose weak QED effects is around few GeV and such beams can be generated from an all-optical source. Simulations can show the influence of quantum emission on the energy spread of an electron beam colliding head-on with an intense laser. \item [-] Pair cascades in counter-propagating lasers: in this configuration, the stimulated pair production is due to the Breit-Wheeler process. We have developed an algorithm that allows particle merging (while conserving particle distributions) to avoid memory overflow. 2D/3D simulations and analytical predictions for the growth rates will be presented. \item [-] Vacuum polarisation: the process of photon-photon scattering leads to a set of corrected Maxwell's equations, effectively creating a non linear polarization and magnetization of the vacuum. To study this interaction, we incorporated a robust solver in the OSIRIS. Our work also shows vacuum birefringence and high harmonics generation. \end{itemize} [Preview Abstract] |
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