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 GP11: Poster Session III: Basic Plasma Physics: General; Space and Astrophysical Plasmas; ICF Measurement and Computational Techniques, Direct and Indirect Drive; MIF Science and Technology (9:30am-12:30pm) |
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Room: OCC Exhibit Hall A1&A |
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GP11.00001: PHAse Space MeAsurements (PHASMA) Experiment Earl E Scime A new experiment, called the PHAse Space MeAsurements (PHASMA), features laser induced fluorescence diagnostics for ion measurements, Thomson scattering diagnostics for electron velocity distribution function measurements, and a microwave scattering system for turbulence measurements. PHASMA is designed to enable the direct measurement of ion and electron vdfs in space-relevant plasma phenomena including reconnection, shocks, and turbulence. To create the conditions necessary for different experimental regimes, PHASMA will employ a 5 kW, steady-state helicon source capable of generating variable-density background hydrogen, helium, and argon plasmas with controllable plasma pressure (relative to the magnetic pressure), collisionality, and azimuthal flow shear. Reconnecting flux ropes will be created through the merging of discharges from two pulsed plasma guns. Measurement objectives include fully 3D ivdf measurements in a 3D volume with spatial resolution less than 0.2 cm (the expected electron skin depth in PHASMA) and similarly resolved evdf measurements, in all three laboratory coordinate axes. We will present design specifications and initial construction results of PHASMA. |
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GP11.00002: Ion velocity distribution dependence on λ/ρi and ψ in a weakly magnetized plasma boundary Derek S Thompson, Earl E Scime, Rinat Khaziev, Davide Curreli We report velocity distribution functions measured in the E × B direction and perpendicular to a plasma-facing surface in a boundary with a weak magnetic field. In these scenarios, the mean free path and the ion gyroradius are comparable (λ ∼ ρi) and the Debye sheath dimension is small (λD « ρi). Ion flows in the E × B direction, which are predicted to attain significant fractions of the sound speed, have recently been experimentally observed. We report measurements of ion velocity distributions in this direction for a range of collisionality λ/ρi and magnetic angles ψ. Experiments are conducted in the vicinity of a boundary region created in the HELIX helicon source, which produces plasma densities of 1016-1018 m-3 and magnetic fields up to 1 kG. Electrostatic probe measurements of electron energy distributions and plasma potential are presented as well. Measurements are compared to distribution functions calculated using the fully-kinetic full-orbit Particle-in-Cell code hPIC including Monte-Carlo collisions. |
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GP11.00003: Validation simulations of the Particle-in-Cell code hPIC with the Laser-Induced Fluorescence measurements at the HELIX facility Rinat Khaziev, Derek Thompson, Davide Curreli, Earl Scime We report clear evidence of the three-dimensional structure of the ion flow in a magnetic presheath formed at the plasma-wall transition, using both Particle-In-Cell simulations using the hPIC code, and experimental measurements of the ion flow conducted at the HELIX facility at West Virginia University using 3D Laser Induced Fluorescence. The PIC-MCC simulations are performed using the fully kinetic, full-orbit PIC code hPIC with heavy ions (argon) and adiabatic electrons. The experiments are run in conditions of intermediate magnetization with magnetized electrons and weakly unmagnetized ions ($\omega \tau \approx 1$). Based on the results of the simulations and experiments, we demonstrate how the structure of the plasma sheath can significantly be altered by the drifts of the background gas and/or the ambient electric fields. Finally, we highlight the relevance of inclusion of the collisional processes for achieving a quantitative agreement of PIC simulations with the experiments. |
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GP11.00004: Electron Effects on the Formation of Ion Beams in Expanding Plasmas Evan M Aguirre, Timothy N Good, Rikard Bodin, Neng Yin, Derek S Thompson, Earl E Scime We report measurements of the electron energy probability function (EEPF) in an argon helicon plasma using the Druyvesteyn method. At low pressure, the EEPFs transition from a single Maxwellian energy distribution in the plasma core to a plasma with a fast electron component near the edge. This electron effect is studied as a function of pressure. Simultaneous measurements of the ion velocity distribution function (IVDF) using laser induced fluorescence (LIF) are also reported. Previous work determined that the rf power is strongly deposited at the skin depth resulting in a sharp decrease in the plasma potential and the formation of an ion hole. The effect of rf power on both the electrons (EEPFs) and ions (IVDFs) is presented. The spatial region surrounding the ion hole is studied in more detail. Current theories of ion beam and ion hole formation in expanding plasmas are incomplete and these measurements provide critical information regarding the spontaneous formation of these structures. |
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GP11.00005: Measurements of the neutral and ion density under the antenna in a helicon source Miguel Francisco Henriquez, Derek S Thompson, Earl E Scime Using confocal laser induced fluorescence (LIF), argon neutral and ion velocity distribution functions (N/IVDFs) are measured in inductively coupled and helicon mode plasmas. Velocity distributions in these plasmas provide insight into the ionization processes in the helicon antenna region and a measure of the total ionization efficiency of the helicon source. LIF has been used to measure these processes in past studies. However, the use of confocal optics to measure the relative metastable state densities directly under the helicon source antenna demonstrates the advantage of employing confocal optical systems in sources with limited optical access. |
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GP11.00006: Investigation of the preplasma characteristics produced by the interaction of a laser pulse and a thin foil/film target. Vanessa Ling Jen Phung, Minseok Kim, Jinju Kim, Han Sup Uhm, Hyyong Suk With the current state-of-the art of laser technology, the existence of amplified spontaneous emission (ASE) and prepulse can be limited but still inevitable. As the laser intensity increases, the ASE and prepulse effects become significant in interaction of the intense laser pulse and a thin foil target before the main pulse arrives. When the ASE level exceeds the ionization and damage thresholds of the target materials, formation of the preplasma on the target surface due to the ASE will lead to destruction of the thin foil target. This early interaction may be very important in ion acceleration using a high power laser and a thin foil. So far, there have not been much studies on the formation and dynamics of the preplasma. In this work, we used a small mJ-scale laser system where the laser pulse was focused on the surface of a thin foil/film (aluminium and mylar) to generate a preplasma intentionally and its characteristics was investigated by using the pump and probe method. Some detailed results will be shown in this presentation. |
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GP11.00007: Numerical studies of the momentum transfer in inverse Bremsstrahlung absorption Vadim Munirov, Nathaniel J Fisch The laser momentum absorption due to inverse Bremsstrahlung is studied numerically using test particle Monte Carlo and molecular dynamics simulations. The difference between numerical simulations that treat momentum absorption correctly and the ones that treat only energy absorption correctly is emphasized. The results are compared with the analytical predictions, specifically that the average momentum absorption rate of the electrons is higher than the naive estimates suggest, due to the recoil momentum from the ions. |
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GP11.00008: A comparison of Lagrangian and Dirac constraints for the ideal incompressible fluid and magnetofluid Philip J. Morrison, Tommaso Andreussi, Francesco Pegoraro The imposition of the incompressibility constraint in fluid flow was imposed by Lagrange in the so-called Lagrangian variable description using his method of multipliers in a variational formulation [1]. An alternative approach is the imposition of this constraint in the Eulerian variable description by a generalization of Dirac's constraint method [2,3] using noncanonical [1] J. L. Lagrange, Me'canique Analytique (Paris, 1788). |
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GP11.00009: Properties of the Magnetic Monopole Plasma Abraham Loeb, Mikhail Medvedev Magnetic monopoles are putative particles having a pure magnetic charge are inevitable in most grand unification theories, though the existing theoretical and observational constraints on the abundance of magnetic monopoles are limited. Here we demonstrate that an ensemble of monopoles forms a plasma whose properties are well determined and whose collective effects cannot be ignored. Coexistence of the tentative monopole plasma and normal plasmas (i.e., ionized gas) in the universe sets the nearly complete equivalence of the monopole plasma and the textbook/classical non-magnetized electron-positron pair plasmas on terms of their basic properties, though spatial and temporal scales are obviously vastly different. We discuss how our findings constrain the abundance of the monopoles in the universe. Certain observational predictions are made. |
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GP11.00010: Thermodynamics properties of spinning plasma Vasily Geyko, Nathaniel Fisch An ideal magnetized and non-magnetized spinning plasma is studied in thermodynamic equilibrium in cylindrical geometry. Under the assumptions of smooth wall boundary condition, energy and canonical angular momentum conservation, the plasma thermodynamics potentials and all other thermodynamic properties are derived. Plasma heat capacity is shown to be increased compared to ideal gas case due to electrostatic force of charge separation. Axial and radial adiabatic compression properties are strongly dependent on system parameters, such as density, temperature, magnetic field and rotation rate. This dependence can be utilized for optimization of energy deposition and compression of Z-pinches. |
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GP11.00011: High Resolution Ion Velocity Distribution Map Downstream of Expanding Magnetic Field Cuyler B Beatty, Thomas Steinberger, Risa Beatty, Regis John, Evan M Aguirre, Earl E Scime High spatial resolution measurements, both perpendicular and parallel to the system axis, were carried out to investigate the ion velocity distribution function (IVDF) in an expanding argon helicon plasma. Several hundred 2 mm resolution measurements of the IVDF, electron temperature, and electron density were performed over 5 cm along the system axis and from -5 cm to -10 cm along the radial axis. Previous laser induced fluorescence (LIF) measurements of IVDFs revealed surprisingly broad perpendicular IVDFs (consistent with perpendicular ion temperatures in excess of 1.5 eV). With typical parallel temperatures of approximately 0.4 eV, the thermal anisotropy of the ion velocity distribution greatly exceeds unity. These new high spatial resolution measurements display significant structure at sub-ion-gyroradius scales are consistent with the previous measurements and demonstrate that two-dimensional or possibly three-dimensional models are needed to describe the evolution of the IVDF in expanding plasmas. |
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GP11.00012: Effects of background gas composition on stability and properties of free space microwave-driven plasmas Erin Thornton, Adrian Lopez, Remington Reid The Air Force Research Laboratory is investigating the characteristics of a free space plasma sustained by a focused high power microwave beam. A multi-kW, 4.7 GHz microwave system is used to generate quasi-stable and stable plasmas with background gas of various compositions of argon, oxygen, and nitrogen at pressures ranging from 100 to 200 mTorr. The composition of the background gas has been previously shown to have a major impact on the stability of the plasma, however there is limited data with regards to how the gas composition affects plasma parameters of the various discharge modes. Using various plasma diagnostic techniques, the background gas’ effect on the plasma density and temperature are quantified and their relation to the stability properties of the plasma are presented. |
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GP11.00013: Doppler velocimetry of moving ionization fronts in free space plasmas generated by continuous high power microwave beams. Remignton Reid, Adrian Lopez, Erin A Thornton The Air Force Research Laboratory (AFRL) is studying the stability of plasmas generated and sustained indefinitely in high power microwave beams. The plasmas exhibit two typical discharge modes: fully stable (typically at lower power) and quasi-stable (typical at higher power). The quasi-stable mode consists of a series of highly repetitive ionization fronts propagating from near the beams focus towards the broadcast antenna. This phenomenon is closely related to similar behavior observed in pulsed experiments and in descending ionization layers overserved by the HAARP facility. The motion of the reflective ionization front illuminated by the drive beam provides a natural opportunity for the use of Doppler velocimetry to measure the speed of the ionization front. This velocity can be related to key plasma parameters such as the electron temperature and diffusion rates. Plasma parameters estimated using this technique are compared to measurements taken with conventional diagnostics to quantify velocimetry’s utility in circumstances where direct diagnostic access may not be practical. DISTRIBUTION A. Approved for public release: distribution unlimited. PR number AFMC-2018-0104. |
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GP11.00014: Development of Spectroscopic Diagnostics for Assessing the Role Neutral Fueling Plays in the Production of High Density Helicon Plasmas Jonathan Green, Patrick J Leonard, Nicholas I Arnold, Stuart David Loch, Gregory D Severn, Oliver Schmitz Helicon plasmas capable of producing electron densities in the range of 1020 m-3 are being considered for use in next generation plasma wakefield particle accelerators. The ability to maintain the appropriate density with an axial uniformity on the order of <1% is a demanding requirement. The MARIA device at UW-Madison has been developed to study the role of neutral particle fueling in meeting these density requirements using non-invasive spectroscopic techniques. An improved algorithm based on collisional radiative modeling has been developed to assess the atomic populations of neutral argon as well as fundamental plasma parameters. Measurements of the electron temperature and the density ratio of the various metastable states is currently possible, while the ability to measure electron densities is under way. Additionally, laser induced fluorescence has been used to map the distribution of ion outflow and neutral inflow to the plasma. By combining these measurements, a picture of how a high-powered helicon plasma is fueled begins to emerge. |
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GP11.00015: Diffraction effect on spectral line shape of optical vortex Doppler absorption spectroscopy Mitsutoshi Aramaki, Masaki Yamamoto, Hirokazu Kobayashi, Shinji Yoshimura, Kenichiro Terasaka, Tomohiro Morisaki Since the optical vortex has twisted wavefront, the motion in the wave field causes the Doppler shift in all the three-dimensional directions. We are developing the optical vortex Doppler absorption spectroscopy that is sensitive to the movement in the direction across the optical path. Since the Doppler shift in the optical vortex field depends on the position with respect to the beam center, which is the phase singularity, the observation of local absorption is required for the optical vortex Doppler absorption spectroscopy. However, as the laser propagates from the plasma to the imaging device, the small spatial structure is deformed due to the diffraction. In order to avoid the diffraction effect, a 4f imaging system was constructed and the absorbed beam was transferred to the imaging device using the lens system. In this presentation, we discuss the influence of the diffraction on the spectral line shape and the imaging system to avoid it. |
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GP11.00016: Laser-induced fluorescence measurements using optical vortex beams Shinji Yoshimura, Kenichiro Terasaka, Mitsutoshi Aramaki Flow velocities of ions and neutrals are important parameters for characterizing plasmas. We have been developing novel methods for flow-velocity measurement based on optical vortex laser spectroscopy. In addition to usual longitudinal Doppler effect, an atom moving in an optical vortex beam experiences the azimuthal Doppler effect, which arises from azimuthal phase gradient around its singularity point. Therefore, it is expected that the Doppler spectrum contain information on the flow perpendicular to the beam path. We have so far studied optical-vortex laser absorption spectroscopy to measure neutral flow. Because metastable excited neutrals are abundant in weakly ionized laboratory plasmas, the absorption spectrum can be obtained by single path absorption. However, it is not applicable to flow measurement of ions due to very slight absorption. Here we consider laser-induced fluorescence (LIF) measurements using optical vortex beams to expand the capability of optical vortex plasma diagnostics. Comparison of the shapes of LIF spectra taken with optical vortex beams and with conventional plain-wave like beams will be given and discussed. |
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GP11.00017: Measuring near-DC conductivity of laser generated warm dense aluminum using single-shot terahertz spectroscopy Benjamin Ofori-Okai, Zhijiang Chen, Anthea Weinmann, Lars Seipp, Siegfried Glenzer We present experimental measurements of the near-DC electrical conductivity of laser generated warm dense matter (WDM) measured by single-shot terahertz (THz) time-domain spectroscopy. WDM refers to a state where the temperature is between ~1-100 eV and with density is 0.1-10 times that of a solid [1]. This makes the properties of WDM difficult to model using conventional plasma or condensed matter theories. The DC electrical conductivity of WDM is of particular importance for its applications to fusion science and planterary astrophysics. A promising method to determine the DC conductivity is to use THz spectroscopy. THz pulses can probe properties close to DC because the fields evolve on significantly slower timescales compared to the electron-electron and electron-ion interactions. Recently, we have developed an experimental platform for studying laser heated WDM using single-shot THz time domain spectroscopy [2-3]. Here we present experimental results using single-shot THz transmission of nm-thick aluminum films. 1) A. Ng, et al., Laser Part. Beams 23(4), 527–537 (2005). 2) B. Russell et al., Opt. Expr. 25, 16140 (2017) 3) B. K. Ofori-Okai et al., Accepted in Rev. Sci. Instr. |
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GP11.00018: An electron plasma wave absorption diagnostic Frederick N Skiff, Ryan T Hood, Lucas P Beving We test a prototype electron plasma wave absorption diagnostic in a low density Argon DC gas discharge plasma with multi-dipole confinement. A primary difficulty with microwave probe diagnostics near the electron plasma frequency is the fact that the probe fields typically sample principally the spatial region dominated (and perturbed) by the material probe. In addition, the probe impedance is normally dominated by the evanescent surface-wave component which produces a significant capacitance with a dielectric that changes sign at the plasma frequency - and thus passes through zero. We attempt to avoid these difficulties by using a two-part probe that is designed to cancel the surface wave component and permit the excitation and detection of electron plasma waves across a volume of plasma well outside of the plasma sheath. In principle, the electron plasma wave absorption signal can provide information on the electron density, temperature, and flow. We present preliminary results of transmission signals in an unmagnetized plasma with density near 10^9 cm^-3. |
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GP11.00019: Gas density evolution after single nanosecond pulsed discharge: experimental and computational results Kristina M Lemmer, Hannah Watts, Jared Miles, Russell Brayfield, Steven Adams, Boyd A. Tolson The application of a nanosecond-pulsed discharge across a pin-to-pin air gap was investigated to determine the gas density evolution after a single pulse. Using Rayleigh scattering of a 532 nm pulsed laser, the gas density was compared to the ambient density beginning at 0.3 ms after pulse initiation and continuing to 820 ms after pulse initiation. The scattered Rayleigh signal was measured through a spectrometer with an intensified charge coupled device (ICCD) camera. The results show that after the discharge, the gas density initially decreases between the electrodes. As new air rushes in toward the center of the discharge region, the density begins to increase to ambient conditions between the electrodes, while the lower density air that surrounds the region is pushed outwards. Five hundred microseconds after the nanosecond-pulsed discharge was initiated, the gas in the 4-mm region surrounding the electrodes returns to pre-discharge conditions. The experimental results are compared with computational models from Ansys/Fluent to elucidate the fluid mechanical phenomena associated with the discharge. |
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GP11.00020: Simulation of positrons in a magnetic dipole trap S. Nissl, H. Saitoh, J. Horn-Stanja, E. V. Stenson, U. Hergenhahn, T. Sunn Pedersen, M. R. Stoneking, M. Singer, M. Dickmann, C. Hugenschmidt, L. Schweikhard, J. R. Danielson, C. M. Surko APEX (A Positron Electron eXperiment) aims to create an electron-positron pair plasma in a magnetic dipole trap. To achieve this goal, a highly efficient positron injection scheme is an essential prerequisite. The large parameter space (multiple electrodes and steering coils to manipulate the beam) and the limited diagnostic capabilities of the experiment demanded a numerical counterpart to further understand the processes occurring during injection as well as confinement. Using discrete electric fields, analytic formulas to calculate the magnetic fields and a variant of the leapfrog integrator as particle pusher, full trajectory simulations were conducted and were able to reproduce the experimental data. A simulation speed-up of two orders of magnitude was achieved, in comparison to the previously used SIMION trajectory simulator. Furthermore, the numerical energy loss of particles is negligible for the leapfrog method in contrast to the widely used 4th-order Runge-Kutta algorithm. This made it possible to simulate long confinement of particles in the trap and estimate the major loss mechanisms. Possible future applications include also tests for adiabaticity and optimizations for the upcoming next stage of APEX with a levitating superconducting dipole coil. |
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GP11.00021: Simultaneous injection of electrons and positrons into a magnetic dipole M. Singer, M. Dickmann, C. Hugenschmidt, U. Hergenhahn, J. Horn-Stanja, S. Nissl, H. Saitoh, E. V. Stenson, T. Sunn Pedersen, M. R. Stoneking, J. R. Danielson, C. M. Surko Magnetized pair plasma consisting of electrons and positrons represents a unique state of matter whose experimental demonstration is still pending. Toward this goal, positrons produced in the NEPOMUC facility at FRM-II near Munich have been drift-injected into the magnetic field of a supported permanent magnet, which is surrounded by a set of electrodes, using a pair of ExB plates. Extensive investigations of the large parameter space consisting of electrostatic and magnetic fields have resulted in injection efficiencies of 100% as well as confinement times exceeding 1 second. In the beam line just upstream of the trap, a compact, low-energy electron gun can be situated to inject electrons parallel to the positron beam, resulting in electron injection into the confinement region using the parameters originally optimized for efficient injection of positrons (as their availability lies many orders of magnitude lower than that of electrons). For diagnostics, both annihilation counts as well as current are measured. The APEX project aims to magnetically confine an electron-positron pair plasma; the mixture of both electrons and positrons is a milestone en route to confining a plasma with a levitated superconducting coil. |
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GP11.00022: Positron beams and pulses for pair plasma experiments E. V. Stenson, J. Horn-Stanja, H. Saitoh, U. Hergenhahn, S. Nissl, T. Sunn Pedersen, M. Singer, M. Dickmann, C. Hugenschmidt, M. R. Stoneking, J. R. Danielson, C. M. Surko, S. Koenig, M. Singer, L. Schweikhard The generation of magnetically confined electron-positron pair plasmas in the laboratory will open up an exciting frontier in plasma physics that has been eagerly anticipated for four decades. This is the goal of the APEX (A Positron Electron eXperiment) project. The limiting ingredient for accomplishing this --- i.e., for achieving 10 Debye lengths for both species in the confinement device --- is the supply of antimatter. The required number of positrons depends strongly on the parameters of the positron source, as these determine what methods can be efficiently used to transfer positrons into the trap, as well as the resulting density and temperature. Therefore, experiments have been conducted to optimize the world-class, monoenergetic NEutron-induced POsitron source MUniCh for the unique needs of APEX. The higher-flux “primary beam” has been demonstrated at low energies (5–60 eV) for the first time, which enabled drift-injection and trapping of a record number of positrons in a prototype dipole device; in situ SiC remoderation of a 400-eV beam, followed by drift injection of the remoderated positrons, has been demonstrated in the same device. Work is now underway toward adding linear non-neutral plasma traps that will be used to furnish intense, tailorable positron pulses. |
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GP11.00023: Magnetic dipole traps for pair plasma experiments M. R. Stoneking, H. Saitoh, J. Horn-Stanja, E. V. Stenson, U. Hergenhahn, T. Sunn Pedersen, M. Singer, M. Dickmann, C. Hugenschmidt, J. R. Danielson, C. M. Surko, L. Schweikhard Magnetized pair plasmas (comprising positively and negatively charged particles with the same mass) have been the topic of theoretical and computational work for decades, and the goal of the APEX (A Positron Electron eXperiment) collaboration is to confine them in the laboratory. A levitated dipole trap is an ideal device for this purpose, for reasons including the facts that the confining field requires no plasma current and that dipolar fields are relevant to astrophysical systems (in which pair plasmas naturally arise). Compared to existing levitated dipole experiments, the APEX device will be smaller and lighter and have less heat load.In parallel with the design and construction of the levitated dipole, a number of key questions have been investigated in a prototype trap based on a supported permanent magnet; these include the demonstration of lossless injection (by means of the ExB drift), the manipulation of positrons once they are in the trap (by means of oscillating wall biases), and the observation of confinement times exceeding 1 s (despite system asymmetries). Efforts are now underway to upgrade the prototype trap first to a supported and then to a levitated high-temperature super conducting (HTSC) coil. |
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GP11.00024: Entropy modes in pair plasmas Barrett Rogers, Ben Zhu, Manaure Francisquez, Xueqiao Xu Unlike many instabilities that are absent in electron-positron plasmas due to their unique mass symmetry [1], we find that slab entropy modes -- an electromagnetic micro-instability driven by plasma inhomogeneity still persists in pair plasmas. In order to appropriately describe magnetized relatively low density electron-positron plasmas, our previous gyrokineitc analysis on the slab entropy modes [2] has been generalized to include electron finite Larmor radius (FLR) effects and Debye shielding effects. Both electron FLR and Debye shielding effects are stabilizing; in particularly, as Debye length $\lambda_D$ approaches electron/positron gyroradius $\rho$, entropy modes are largely suppressed. Analytical derivation of dispersion relation, stability analysis as well as nonlinear gyrokinetic simulations of this mode in pair plasmas will be presented.[1] Tsytovich and Wharton, Comments Plasma Phys. Controlled Fusion \textbf{4}, 91 (1978)[2] Rogers, Zhu and Francisquez, \textit{Phys. Plasmas} \textbf{25}, 052115 (2018) |
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GP11.00025: A general plasma-neutral fluid model with molecules Eric Meier, Iman Datta, Aria Johansen, Uri Shumlak The plasma-neutral model [Meier and Shumlak, PoP 19 (2012)] is extended to include atomic and molecular fluids. In magnetic confinement and electric propulsion, molecules are generated due to recycling of ions and atoms on walls. Accurately modeling these plasmas requires accounting for the energetics of dissociation, and redistribution of energy to the product atoms and ions. Dynamics of the molecular fluid, which couples weakly to the main ion species through elastic scattering, differs from atomic dynamics, which is dominated by resonant charge exchange. The model presented here captures electron-impact dissociation through both excited and ionized molecular states, and elastic scattering of molecules on ions. Derivation of a four-fluid model—with electrons, ions, atoms, and molecules—is presented, along with results from an implementation in WARPXM, a DG code developed at U. Washington. In a 1D plasma accelerator problem, a current sheet interacts with a slug of either molecular or atomic gas. The atomic gas is almost fully entrained in the passing current sheet, while a significant fraction of molecules is left in its wake. Application of the model to a tokamak-like SOL plasma is also under development and results will be presented. |
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GP11.00026: Truncated ionization injection with a chirp laser pulse in laser wakefield acceleration Cui Ye, Ma Yan-Yun, Zhang Guo-Bo, Feng Lei The ionization injection in laser wakefield acceleration has been a promising injection scheme. Nevertheless, it usually produces electron beam with large energy spread because the ionization injection occurs continuously. By using particle-in-cell (PIC) simulations, we propose a method to truncate ionization injection process with a chirp laser pulse. Due to the dynamically evolving of the chirp laser pulse when it propagating in plasma, the ionization injection length can be decreased to a few hundred micrometers determined by the chirp parameter. As a result, a quasi-monoenergetic electron with the narrow energy spread can be produced. The energy spread is expected to be decreased further by optimizing the quadratic chirp parameter. |
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GP11.00027: Neutral flow field structure of partially ionized plasma in the HYPER-I and HYPER-II devices Kenichiro Terasaka, Emika Abe, Shinji Yoshimura, Mitsutoshi Aramaki It has been widely recognized that plasma-neutral coupling plays an important role in atmosphere-ionosphere coupling, transport phenomena in laboratory plasmas, and plasma acceleration in plasma propulsion systems. In weakly ionized plasmas, a change in the plasma fluid induces a change in the neutral fluid. However, the neutral gas flow has been usually treated as too much simplified because of the difficulty of spatial profile measurement of neutral gas flow. We have experimentally studied the effect of neutral gas flow on plasma structure formation in the HYPER-I device (NIFS) and the HYPER-II device (Kyushu Univ.). An ECR plasma was produced with a 2.45 GHz microwave and the high-density plasma (typically 10% ionization degree) was used. The local velocity distribution function measurement for the neutral particles in plasma carried out with a high accuracy LIF system composed with a tunable diode laser. We have observed the generation of vortex pattern and axial flow reversal of neutral gas flow associated with neutral depletion. These results indicate that plasma and neutral fluids affect each other to make the characteristic spatiotemporal structures. |
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GP11.00028: Simulation Studies of Neon Pellets and SPI for Plasma Disruption Mitigation in Tokamaks Nicolas Bosviel, Roman V. Samulyak, Paul B Parks Numerical studies of the ablation of neon pellets and multiple pellet fragments in tokamaks in the plasma disruption mitigation parameter space have been performed using time-dependent pellet ablation models based on the front tracking code FronTier-MHD and the Lagrangian Particle code. Both codes use the same kinetic models for the electronic heat deposition, pellet surface ablation models, equations of state with multiple ionization support, and radiation models. The Lagrangian particle model is highly adaptive and capable of simulating a large number of fragments in 3D. The Lagrangian treatment of ablated material eliminates numerical difficulties of dealing with the tokamak background plasma. Both codes achieve good agreement with theory for spherically symmetric ablation flows. Simulations predict processes in ablation clouds and the dependence of pellet ablation rates on background plasma parameters and the magnetic field. |
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GP11.00029: 3D cylindrical particle-in-cell simulations for the plasma instability in the cross-field condition Young Hyun Jo, Vladimir V. Mikhailenko, HaeJune Lee There are various devices using cross-field discharges such as Penning traps, hall thrusters, magnetron sputtering systems, and so on. One characteristic of these devices is that there are magnetized electrons which increase the plasma density and unmagnetized ions emitted from the devices. In other words, they are partially ionized plasmas with lower magnetic field compared with the fully ionized plasmas in tokamaks. The complex physics including plasma turbulence and transport of this kind of system has not been fully revealed yet. This study focuses on the investigation of plasma instability occurred in this condition. Basically, it has a kind of Kelvin-Helmholtz instability due to the velocity shear caused by the ExB drift, and the collisions of charged particles with neutral gases affect the physics as well obviously. In this study, 3D cylindrical particle-in-cell simulations are performed to analyze the kinetics of plasmas including the perpendicular motion to the cross-field with not only axially periodic but also bounded conditions. |
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GP11.00030: Dynamic Stabilization of Filamentation Instability Shigeo Kawata, Takahiro Karino, Yanjun Gu The paper presents a study on dynamic stabilization of filamentation instability driven by an electron beam in a plasma. The results show that the filamentation instability is stabilized by the dynamic stabilization mechanism, in which the electron beam axis oscillates. The dynamic stabilization mechanism for plasma instabilities was proposed in the paper [PoP 19, 024503 (2012)]. Instabilities emerge from the perturbations. Normally the perturbation phase is unknown so that the instability growth rate is discussed. If the perturbation phase is known, the instability growth can be controlled by a superimposition of perturbations: if the perturbation is introduced by, for example, a driving beam axis oscillation, the perturbation phase can be controlled and the instability growth is mitigated. When the electron beam axis oscillates in a plasma, the perturbations carried by the electron beam also oscillate. The superimposition of the perturbations mitigates the filamentation growth [PoP 25, 011601 (2018)]. |
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GP11.00031: The transverse kink instability of electron phase-space holes Ian H Hutchinson Electron holes (solitary BGK modes) routinely form in one dimension as the stable, long-lived, non-linear result of two-stream or bump-on-tail unstable electron distributions. In multiple dimensions, however, they break up unless there is a strong magnetic field, by a 'transverse instability', which kinks and often destroys the hole. Many prior simulations have observed the instability, but its mechanism has not previously been correctly identified. I show that the instability mechanism is in fact kinematic, arising from the jetting of electrons as the kink accelerates in the trapping electric field direction. The growth rate at long wavelength can be deduced by a conceptually simple consideration of conservation of total hole momentum. PIC simulations of unmagnetized-hole kink growth show wavelengths and growth rates in agreement with heuristic estimates of the fastest growing mode2. In addition, a comprehensive linear kinetic analytic calculation at non-zero magnetic field has been completed for the relevant shift eigenmode, providing the full dispersion relation of the instability, and verifying the heuristic estimates3. |
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GP11.00032: The Richtmyer-Meshkov Instability with Unknown Magnetic Field Topology Michael Lavell The stability of a shocked interface in magnetohydrodynamics (MHD) is investigated through linear stability analysis and two-dimensional simulations. The Richtmyer-Meshkov instability (RMI) occurs when a shock interacts with a rough surface and leads to the growth of a penetrating spike and fluid mixing. This process inhibits energy transport in inertial confinement experiments and is responsible for early mixing of heavy elements in supernova explosions. Numerical experiments and linear stability analysis demonstrated an applied magnetic field of sufficient strength aligned transverse or tangential to the shock front impedes vorticity generation at a fluid interface [1,2]. This study explores the evolution of a shocked interface in MHD without placing any assumptions on the topology of the magnetic field. Numerical studies performed with FLASH verify results discovered through linear stability analysis. A field that is tangent to the shock front minimizes vorticity generation at the interface due to field line tension, and all field orientations effectively suppress penetration and mixing that are characteristic of the RMI. [1] Samtaney, R., Physics of Fluids 15, L53 (2003). [2] Wheatley, V., Samtaney, R., Pullin, D., and Gehre, R., Physics of Fluids 26, 016102 (2014). |
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GP11.00033: Reduction in the energy of an Alfvén wave propagating through a longitudinal inhomogeneity in Alfvén speed Sayak Bose, Troy Carter, Walter N Gekelman, Michael Hahn, Shreekrishna Tripathi, Stephen T Vincena, Daniel Wolf Savin We have explored the effectiveness of longitudinal Alfvén speed gradients in reducing the energy of propagating Alfvén waves under conditions scaled to match a solar coronal hole. The experiments were conducted in the Large Plasma Device located at the University of California, Los Angeles. Our results show that the energy of the transmitted Alfvén wave decreases as the inhomogeneity parameter, λ/LA , increases. Here, λ is the wavelength of the Alfvén wave and LA is the scale length of Alfvén speed gradient. The waves are observed to lose up to ≈ 91% of their energy while propagating through gradients similar to those in coronal holes. Contrary to theoretical expectations, the reduction in the energy of the transmitted waves is not accompanied by a reflected wave. Nonlinear effects are ruled out as the amplitude of the initial wave is too small. However, collisional and Landau damping are found to contribute to wave damping in the experiment. After removing these effects, we find a gradient driven reduction in transmission of ≈ 86%. |
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GP11.00034: Zonal flow dynamics: phase-space simulations beyond the ray approximation Yao Zhou, Hongxuan Zhu, Ilya Y Dodin Traditional phase-space (wave-kinetic) modeling of drift-wave turbulence and zonal flows invokes the geometrical-optics (ray) approximation, i.e., treating the drift waves as point particles interacting with the zonal flow fields. Recently, a Wigner-Moyal model of this system has been developed, which retains full-wave effects by treating the drift waves as quantum-like particles (Ruiz et al. 2016, PoP 23, 122304). We present numerical simulations, based on the Wigner-Moyal model, of zonal flow formation (zonostrophic instability), deterioration (tertiary instability), and oscillation (of predator-prey type) (Zhu et al. 2018, PRE 97, 053210). Certain types of stationary and propagating nonlinear coherent structures are also investigated. We demonstrate the importance of full-wave effects by comparing these results with wave-kinetic simulations. |
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GP11.00035: The full-wave effects behind the predator-prey oscillations and its relation to the finite Dimits shift Hongxuan Zhu, Yao Zhou, I Y Dodin As known commonly, some drift-wave (DW) turbulence can be suppressed above the linear threshold of homogeneous-plasma "primary" instabilities. This threshold modification, called the Dimits shift, is attributed to the secondary instability, which drains out the turbulence energy by spontaneously forming zonal flows (ZFs). The finite value of the Dimits shift is often explained in terms of the tertiary instability (TI), which makes intense ZFs unstable. However, the TI theory does not explain the so-called predator-prey oscillations in the DW--ZF system. We present an analytical model and quasilinear simulations of the predator-prey oscillations using a quantumlike Wigner--Moyal formalism [Zhu et al., PRE 97, 053210 (2018)]. We show that these oscillations can be governed by full-wave effects and thus may not be adequately captured by the wave kinetic equations, contrary to some literature. In particular, the predator-prey oscillations occur only when the ZF scale is less than the ion sound radius. The influence of these oscillations on the ZF saturation and the Dimits shift is discussed. |
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GP11.00036: Mode conversion in cold low-density plasma with a sheared magnetic field Ilya Dodin, Daniel E Ruiz, Shin Kubo A theory is proposed that describes mutual conversion of two electromagnetic modes in cold low-density plasma, specifically, in the high-frequency limit where the ion response is negligible [Phys. Plasmas 24, 122116 (2017)]. In contrast to the classic (Landau-Zener-type) theory of mode conversion, the region of resonant coupling in low-density plasma is not necessarily narrow, so the coupling matrix cannot be approximated with its first-order Taylor expansion; also, the initial conditions are set up differently. For the case of strong magnetic shear, a simple method is identified for preparing a two-mode wave such that it transforms into a single-mode wave upon entering high-density plasma. The theory can be used for reduced modeling of wave-power input in fusion plasmas. In particular, applications are envisioned in stellarator research, where the mutual conversion of two electromagnetic modes near the plasma edge is a known issue. |
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GP11.00037: Structure formation in MHD turbulence as instability of effective quantumlike plasma Vasileios Tsiolis, Ilya Y Dodin Within the quasilinear approximation, structure formation in turbulence can be viewed as instability of effective collisionless plasma formed by linear waves, which serve as particles. These "particles" are quantumlike in the sense that their wavelengths can be nonnegligible and so is their polarization, which produces "spin" effects. The kinetic equation of this "plasma" is the Moyal equation for the Wigner tensor of the turbulent field, and the non-turbulent field governs collective interactions. We apply this formulation to MHD turbulence, where the role of effective particles is played by Alfven waves. Potentially, MHD structure formation can be described on the same footing with standard linear instabilities of conventional plasma. The Alfven-wave Hamiltonian, which is pseudo-Hermitian, is inferred from [A. Brizard, Phys. Lett. A 168, 357 (1992)]. We seek to explicitly formulate the theory of Alfven-wave plasma stability and, as an application, to describe quasilinear turbulent dynamo from first principles. |
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GP11.00038: Wave—mean-field interaction in two-fluid plasmas near the magnetohydrodynamic slow manifold Daniel E. Ruiz, Joshua W. Burby The magnetohydrodynamic (MHD) model of a plasma omits a variety of important physical effects, e.g., the nonlinear effect of waves oscillating on timescales much shorter than the characteristic MHD timescales. Starting from the two-fluid—Maxwell model, we construct an asymptotic theory of high-frequency waves interacting with MHD mean fields. The resulting asymptotic model is a generalization of MHD that incorporates (a) the ponderomotive forcing of nominally MHD plasma motions by the high-frequency wave fields and (b) the self-consistent transfer of energy and momentum between the wave-like and MHD-like dynamics. Consequences of the theory are discussed. |
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GP11.00039: A Low-Cost, High Power RF Resonance System Phase II Results Chris Bowman, Paul Melnik, James R Prager, Timothy Ziemba, Kenneth E Miller, Alex Henson Eagle Harbor Technologies, Inc. (EHT) has completed a Phase II program to develop a pulsed RF system for plasma heating and heat pulse diagnostic. The RF burst is produced by a nonlinear transmission line (NLTL) driven by an inductive adder. EHT designed several unique diode-based NLTLs with voltages up to 30 kV at output frequencies of several hundred megahertz to several gigahertz. To demonstrate RF plasma heating experiments were conducted in the EHT testing chamber. The EHT experimental system consists of a high-power helicon, which can generate high density hydrogen plasmas that are confined in a magnetic bottle configuration. We will present results showing RF generation and details of the plasma heating experiments. |
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GP11.00040: Compressional magnetic field fluctuation in global gyrokinetics Shu-Wei Tsao, Moritz J Pueschel, David R Hatch The gyrokinetic framework has been successful in simulating a variety of phenomena in fusion and astrophysical plasmas. Past research has mostly focused on electrostatic and shear-magnetic fluctuations, neglecting compressional magnetic fluctuations. We derive a radially global gyrokinetic framework which includes compressional fluctuations. Using finite element functions, we construct the field equations. The low-β constraint is relaxed by including the full coupling of electrostatic, shear-magnetic, and compressional-magnetic fluctuations. We present an initial implementation of this new set of equations in the gyrokinetic turbulence code GENE. With the compressional magnetic fluctuations included in GENE, the Gradient-driven Drift Coupling instability found in the high-β experiments in the Large Plasma Device, as well as magnetic reconnection in the solar corona, can be studied to finer precision. |
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GP11.00041: Dispersion relations for parallel propagating waves in magnetized quantum plasma Chang Ho Woo, Min Ho Woo, Cheong R. Choi, Kyoung Wook Min We derived the quantum Vlasov equation in a differential form based on the Wigner distribution function and applied the equation to parallel propagating waves in magnetized quantum electron plasma. Whereas the resulting electrostatic dispersion relation for a Langmuir wave is the same as the one obtained previously from the fluid approach, the behaviours of the electromagnetic dispersion relations are different from those of the fluid ones, which have been obtained to be the same as the classical dispersion relations. It is found that the degeneracy effect is to increase both the phase and the group velocities of the upper branches of the L and R waves whereas it decreases those of the whistler wave. It is also seen that the quantum effect on the R wave is larger than that on the L wave. In the long wavelength limit, the degeneracy effect is shown to be dominant over the quantum kinetic effect. |
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GP11.00042: Magnetized Plasma Relaxation in the Presence of a Mass-Dependent Potential Elijah Kolmes, Mikhail Mlodik, Ian E Ochs, Nathaniel J Fisch The relaxation of a multiple-species magnetized plasma under the influence of an external potential is characterized by several distinct timescales during which different physical effects dominate.1 In appropriate parameter regimes, these timescales can be substantially separated from one another. The resulting behavior is notable both because of how differently it affects different ion species and because of how different it is from the relaxation behavior of other materials, such as neutral gas.2,3 We discuss these problems analytically and simulate them numerically using a multiple-fluid model, including the behaviors of different density and temperature distributions. [1] E. J. Kolmes, I. E. Ochs, and N. J. Fisch, Phys. Plasmas 25, 042703 (2018). [2] V. I. Geyko and N. J. Fisch, Phys. Rev. Lett. 110, 150604 (2013). [3] V. I. Geyko and N. J. Fisch, Phys. Rev. E 94, 042113 (2016). |
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GP11.00043: Collisional Relaxation in Multi-Ion Species Plasma Mikhail E Mlodik, Elijah J Kolmes, Ian E Ochs, Nathaniel J Fisch When an external forces, such as gravity or centrifugal forces, are applied to a magnetized multi-ion species plasma, the relaxation of ion density profiles has several distinct timescales. Here we identify and explore intermediate timescales and describe metastable states of the plasma. In particular, we are interested in differential ion transport[1]. |
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GP11.00044: A consistent and numerically stable family of plasma fluid models Federico David Halpern, Ronald Edward Waltz, Sanjay Chatterjee, Mark Kostuk, Igor Sfiligoi Plasma fluid models satisfy conservation laws, implying physical consistency, that are seldom exactly satisfied by numerical codes. We demonstrate how to derive fluid models that combine consistency and numerical stability with a simple and flexible implementation. We exploit the anti-symmetric form of the plasma flow operator, which becomes apparent when the equations are written using generalized moments related to conserved quantities [1]. The plasma velocity generates infinitesimal rotations of the fluid, implying time reversibility. The resulting models possess discrete analogs that inherit important properties of the analytic forms, including conservation laws and positivity preservation. An advantage of this approach is its simplicity and flexibility. We apply our methodology to the Braginskii model, the MHD equations, and also drift-ordered models. Conservation properties are verified using single seeded blob motion and the Orzsag-Tang vortex, obtaining high-fidelity simulation results with negligible dissipation. [1] F.D. Halpern and R.E. Waltz, Phys.Plasmas 25, 060703 (2018). |
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GP11.00045: Numerical convergence tests for a kinetic closure of fluid moments1 Ronald E. Waltz -R.E. WALTZ and F.D. HALPERN, General Atomics. A method for a kinetic closure of fluid moments for a magnetized plasma with arbitrary collisionality has been developed[1]. The velocity distribution functions are expanded in 8 Gaussian Radial Basis Functions (GRBFs) which are shifted Maxwellians at eight representative 3D-velocity points of drift. The 8 fluid moments (for particle density, 3 particle fluxes, total energy density, and 3 energy fluxes) time advanced in conservative form with external sources determine the 8 GRBF density weighs in 3D real space. The two closure moments (for the stress tensor and the energy weighted stress tensor) are linearly determined from the GRBF weighs. Moments of the nonlinear Coulomb Fokker-Planck collision operator are evaluated from the GRBF weights. Generalization from 8 to a 12, 16, 20... energy weighted moment hierarchy is straightforward. The method is applied to the problem of ion temperature gradient turbulent transport with adiabatic electrons and drift approximated ions. [1] R.E. Waltz, F.D. Halpern, Zhao Deng, and J. Candy, “Kinetic fluid moments closure for a magnetized plasma with collisions”, to be published |
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GP11.00046: A New Look at the Transport in 2D MHD Turbulence Xiang Fan, Patrick Henry Diamond, Luis Chacon We revisit the study of transport of mean magnetic potential - an active scalar field - in 2D MHD turbulence with a large scale magnetic field B, using numerical simulation. Previous studies found that the turbulent transport is suppressed even when the large scale B field is much weaker than the equipartition level. We further notice that the real space structure of the B field contains important information, which is ignored by previous studies that mainly focused on spatially averaged quantities. The B field in the suppression stage is strongly localized in some linearly extended domains. These strong B domains are effectively transport barriers. The barrier width is studied. One characteristic of the existence of transport barriers is found in the PDF of the magnetic potential field A: this PDF is bimodal when there are transport barriers in the suppression stage. If the PDF of A field is designed to be unimodal in the initial condition while <|B|> is held fixed, barriers never form. Therefore, the suppression stage is significantly shorter than the case with bimodal initial condition. This result implies that local transport barriers play an important role in the suppression of transport in 2D MHD. |
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GP11.00047: Generation of Sheared Plasma Flow in a Linear Device Yong Lang, Cong Meng, Bo Li The sheared mean flows and toroidal intrinsic rotations are found to be very important to tokamak plasmas, but their origins are not well understood yet. To get a clear understanding of the mechanism of plasma flow generation, we develop a new 3D code to simulate electrostatic plasma turbulence in linear devices. The code uses drift-reduced Braginskii equations and can self-consistently evolve global full density, pressure, vorticity and parallel velocities. A sheared azimuthal flow is found without any momentum input. The possible mechanisms of the generation and saturation of the azimuthal flow will be discussed. |
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GP11.00048: Bicoherence Analysis of Electrostatic Interchange Mode Coupling in a Turbulent Laboratory Magnetosphere M. C. Abler, Michael E Mauel, Vivian Liu Plasmas confined by a strong dipole field exhibit interchange turbulence, which previous experiments have shown respond locally to active feedback [1]. On the Collisionless Terrella Experiment (CTX), this turbulence is characterized by low frequency, low order, quasi-coherent modes with complex spectral dynamics. We apply bicoherence analysis [2] to study nonlinear phase coupling in a variety of scenarios. First, we study the self-interaction of the naturally occurring interchange turbulence; this analysis is then expanded to include the effects of driven modes in the frequency range of the background turbulent oscillations. Initial measurements of coupling coefficients are presented, as well as evidence for an inverse cascade. Future work is also discussed, including use of wavelet bicoherence analysis and applications to planetary magnetospheres. [1] Roberts, Mauel, and Worstell, Phys Plasmas (2015). [2] Grierson, Worstell, and Mauel, Phys Plasmas (2009). |
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GP11.00049: Turbulence spreading effects in the Landau-Ginzburg theory of Transport and Relaxation Rameswar Singh, Patrick Henry Diamond Gil and Sornette (1996) introduced a 2-field, bi-stable continuum model of avalanching, consisting of a bistable oder parameter(OP) and a control parameter(CP). For sub-critical bifurcation dynamics of OP and diffusive dynamics of CP it was demonstrated that avalanching and other SOC-like dynamics appear when diffusive relaxation of CP is faster than the instability growth rate of the OP and in the other limit of slow diffusion, avalanches comparable to the system size become dominant. A recent experiment (Inagaki et al (2013)) has reported bistable nature of turbulence. This makes this Landau-Ginzburg theory applicable to confinement problems, where the OP is turbulence intensity and CP is mean density. Turbulence spreading then naturally becomes an important concern. Hence Landau-Ginzburg theory à la Gil and Sornette is revisited with turbulence spreading. Novel findings including a quasi-periodic limit cycle state will be presented in detail in the meeting. Special attention will be focused on studies of the effective Prandtl Number dependence, which measures the relative strength of transport and spreading. We aim to understand how spreading modifies the avalanche distribution and spreading. |
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GP11.00050: Plasma turbulence, mm-wave scattering by turbulence and fast ion transport studies in TORPEX Marcelo Baquero-Ruiz, Oulfa Chellai, Ambrogio Fasoli, Ivo Furno, Fabian Manke, Paolo Micheletti The TORPEX device is a basic plasma physics experiment located at the Swiss Plasma Center in Lausanne, Switzerland. Due to its ample diagnostic coverage and its toroidal geometry, TORPEX allows for plasma studies of interest in tokamak physics without the demanding requirements of operation of fusion grade devices. Two such areas of research are turbulence and particle transport, in which TORPEX has made important contributions and continues to prove its usefulness and versatility. We report our findings in recent experiments on millimeter-wave scattering by field-aligned structures (so-called blobs), time-resolved detection of fast ions propagating in a turbulent plasma, and high spatial resolution observations of blobs. This last item is the first application of a diagnostic developed to image plasma structures with high spatial resolution. |
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GP11.00051: Analytical and numerical evidence of the cascade reversal due to electron inertia George Miloshevich, Santiago J Benavides, Philip J Morrison, Emanuele Tassi Astrophysical plasmas exist in a large range of length-scales throughout the universe. At sufficiently small scales, one must account for many two-fluid effects, such as the ion or electron skin-depths, as well as Larmor radii. These effects occur when ignoring electron mass, for instance, is no longer possible. We are interested in studying homogeneous turbulence in the context of such plasma models. In particular, we look at a 2D extended MHD model, where the effect of electron inertia may be non-negligible . This model has been applied to understanding collisionless reconnection in past. Two-dimensional simulations are less computationally intensive and thus allow us to perform a parameter study of many runs, in which we look at the cascade of conserved quadratic quantities as we vary the effective electron skin-depth. We find that the cascade directions depend strongly on whether the length scale is relevant in the system, and, furthermore, that the transition in cascade directions happens in a critical way, as was previously observed in other studies of the kind but in different systems. |
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GP11.00052: Role of Stable Modes in Driven Shear-Flow Turbulence Adrian Everett Fraser, M.J. Pueschel, Paul Willis Terry, Ellen Gould Zweibel Unstable shear flows are found in a variety of fusion and astrophysical systems, where they may become turbulent, and have recently been shown to nonlinearly drive large-scale damped modes [Fraser et al. PoP (2017)]. These previously-neglected stable modes remove energy from the fluctuations before it cascades to small scales, suggesting they are a key component of shear-driven turbulence, where typical models assume that the largest scales take the form of the most unstable modes. Here we compare gyrokinetic simulations of forced, shear-driven turbulence where fluctuations are subject to scale-independent radiative damping, which suppresses the impact of stable modes relative to unstable ones in a manner consistent with the expected effect of a flow-aligned magnetic field in a free shear layer. We construct a simple model for how Reynolds stress scales with driving, showing that the inclusion of stable modes yields significant improvements to the model except at high radiative damping. Informed by these results, we then compare to the free shear layer system, and investigate how the turbulence scales with the flow-aligned magnetic field. |
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GP11.00053: Maser-like mode driven by flow shear in a heated magnetized plasma G. J. Morales, M. J. Poulos, S. Jin, B. G.P. Van Compernolle Results of a basic heat transport experiment at LAPD involving an off-axis heating source are presented. A ring-shaped electron beam source injects low energy electrons (below ionization energy) along a strong magnetic field into a pre-existing, large and cold plasma, resulting in a long, hollow, cylindrical region of elevated plasma pressure. Azimuthal and axial flows with shear are present as a consequence of the boundary conditions at the ring-source. The source configuration allows for active control of the azimuthal flows and flow shear. At high flow shear the system suddenly transitions from broadband drift-wave fluctuations into a regime dominated by a single frequency (~ 0.04fci), maser-like mode with Alfvenic characteristics, which causes enhanced transport. The large mode amplitude (delta n / n > 0.5 ) peaks where the shear in the azimuthal flow is largest. The mode remains active for times long compared to typical transport time scales. Results are presented on the mode structure, the threshold for mode growth and its dependence on the magnitude of the flow shear. |
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GP11.00054: Plasma flows generated by an annular thermionic cathode in a magnetized plasma Suying Jin, Bart G.P. Van Compernolle, Matthew J Poulos, George J Morales A LaB6 thermionic emitter of annular shape is used in the LAPD device to create off-axis heating conditions for various transport studies. Since the emitter is biased relative to a distant anode (15 m away) the entire magnetized plasma develops a self-consistent, potential structure that simultaneously generates transverse and axial flows with shear. This study uses swept Langmuir probe techniques and Mach probes to map the three-dimensional flow patterns and their dependence on bias and plasma parameters. By implementing additional biasing configurations it is possible to control the magnitude of the flows and their shear strength. The experimental measurements are compared to predictions of a Braginskii code that incorporates the boundary conditions associated with thermionic injection. |
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GP11.00055: Producing structures at the Larmor-radius scale via the interaction of shear Alfvén waves and magnetic flux ropes Stephen T Vincena, Shreekrishna Tripathi, Walter N Gekelman, Patrick Pribyl Understanding the transfer of energy from larger to smaller scales remains an active subject of study in plasma physics. Two types of system-sized structures are magnetic flux ropes and shear Alfvén waves; and, one of the dissipation scales is the ion Larmor radius. These experiments are carried out in the Large Plasma Device (LAPD) at UCLA. Flux ropes are generated using a LaB6 cathode-anode discharge ( L = 18 m, n=3x1012cm-3, Te=6eV, 10-3 < β < 0.1) within a larger, r = 30 cm, plasma. Alfvén waves are launched with antennas inside the ropes. Magnetic field frequency spectra reveal multiple sidebands centered on |
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GP11.00056: Time domain structures in magnetic flux rope experiments Shawn Wenjie Tang, Walter N Gekelman, Patrick Pribyl, Stephen Vincena Time Domain Structures (TDS) are narrow, intense spikes that appear in the electric potential/field measurements of numerous space observations and laboratory experiments. They are believed to be associated with end state of turbulence and may have connections to the chaotic behavior of Lorentzian spikes. In an ongoing investigation at UCLA, TDS have been observed near the surface of two magnetized flux ropes produced within the LArge Plasma Device (LAPD). Two 11 m long kink-unstable flux ropes were created by a lanthanum hexaboride (LaB6) source and are encapsulated by a 18 m long background plasma produced by a barium oxide (BaO) cathode. A preliminary analysis of the TDS suggest that they may be associated with the motion of the ropes and it appears that they emanate from the reconnection region between the ropes. In addition, these structures appear to have Lorentzian character (an indicator of chaotic behavior) and can couple to the kinking of the ropes when more power is delivered to the ropes. As the ropes become more chaotic under higher magnetic guide fields (~1kG), the effect of this on the TDS will be explored. |
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GP11.00057: Turbulence and transport in mirror geometries in the LAPD Phil Travis, Troy Carter Measurements of turbulence and transport in varying magnetic mirror ratios have been performed using the flexible magnetic geometry of the Large Plasma Device (LAPD). Fluctuations in density (ion saturation current), floating potential, and magnetic field were recorded, with amplitudes peaking, as expected, on the edge pressure gradient. Planar correlation functions were also recorded. Density and magnetic field fluctuation amplitudes decreased with increasing mirror ratio, while potential fluctuation amplitudes remained similar. The cross-phase between potential and density fluctuations varies with increasing mirror ratio, suggesting a shift in the underlying linear instability as the mirror ratio is increased and magnetic curvature is introduced. Changes in energy confinement, zonal flows, and turbulence as a function of mirror ratio will be presented.
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GP11.00058: Plasma heating during turbulent kinetic magnetic reconnection in two dimensions. Ryusuke Numata, Nuno F Loureiro The magnetic reconnection process and associated heating of plasma may be altered by the existence of background plasma turbulence. To study the effects of turbulence on magnetic reconnection, we have developed a method of driving turbulence by adding random forcing in gyrokinetic simulations. The developed method injects energy into the system at a constant rate. Without reconnection, a statistical steady state is achieved where the energy injection and the collisional dissipation balance. We perform turbulent kinetic magnetic reconnection simulations, and discuss how turbulence affects the heating process during reconnection. A preliminary result shows a remarkable difference with the MHD results: In the gyrokinetic model, turbulence does not accelerate the reconnection process very much because kinetic reconnection is already fast. We estimate how much fraction of the initial magnetic energy is converted into heat via phase mixing [1] in kinetic plasmas, and compare it with and without turbulence. [1] R. Numata and N. F. Loureiro, J. Plasma Phys. 81, 305810201 (2015). |
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GP11.00059: Role of the plasmoid instability in magnetohydrodynamic turbulence Chuanfei Dong, Liang Wang, Yi-Min Huang, Luca Comisso, Amitava Bhattacharjee The plasmoid instability in evolving current sheets has been widely studied due to its effects on the disruption of current sheets, the formation of plasmoids, and the resultant fast magnetic reconnection. In this study, we investigate the role of the plasmoid instability in two-dimensional magnetohydrodynamic (MHD) turbulence by means of high-resolution numerical simulations. At sufficiently large magnetic Reynolds number (Rm=10^6), the combined effects of dynamic alignment and turbulent intermittency lead to a copious formation of plasmoids in a multitude of intense current sheets. The disruption of current sheet structures facilitates the energy cascade towards small scales, leading to the breaking and steepening of the energy spectrum. In the plasmoid-mediated regime, the energy spectrum displays a scaling that is close to the spectral index -2.2 as proposed by recent analytic theories. We also demonstrate that the scale-dependent dynamic alignment exists in 2D MHD turbulence and the corresponding slope of the alignment angle is close to 0.25. [1] C. Dong et al., Role of the Plasmoid Instability in Magnetohydrodynamic Turbulence, arXiv:1804.07361. |
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GP11.00060: Hard X-Ray Spectrum Measured in a Solar-Relevant Lab Experiment That Undergoes Fast Magnetic Reconnection Michael J Flynn, Ryan S Marshall, Paul M Bellan The Caltech MHD jet experiment creates a collimated jet that kinks causing lateral acceleration which then instigates a fast secondary Rayleigh-Taylor instability. This breaks the jet and a hard X-ray burst is detected. A CMOS camera modified to function as an X-ray spectrometer reveals that the pulse consists of a non-uniform distribution of X-ray energies centered around 8 keV. The camera has an aluminum foil sheet blocking visible light and no lens. X-ray photons transiting the foil deposit energy into random camera pixels so that a histogram of the pixels gives the energy spectrum. These measurements complement a plastic scintillator having nanosecond time resolution but poor energy resolution. It is proposed that despite the short collision mean free path, an inductive electric field associated with jet breaking accelerates a subgroup of electrons to keV energies without any of these electrons undergoing collisions. It is further proposed that the fast electrons suddenly decelerate via collisions and radiate X-rays. Extrapolation to the solar corona and/or chromosphere predicts the acceleration of a small subset of electrons to very large super-thermal energies by sub-Dreicer electric fields. |
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GP11.00061: Relativistic tearing mode and plasmoid instability Shu-Di Yang, Amitava Bhattacharjee Relativistic linear tearing mode instability in pair plasmas is investigated, and a dispersion relation is obtained including the effects of both resistivity and thermal-inertia. The dispersion relation is then further employed to study the evolution of the relativistic plasmoid instability. The basic properties of linearly unstable plasmoids, the dominant modes, and the number of plasmoids are investigated for both static and slow-evolving current sheets. The implications of the theory for relativistic tearing and plasmoid instabilities for astrophysical objects are discussed. |
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GP11.00062: A Kinetic Model for Electron Heating in Antiparallel Magnetic Reconnection Exhausts Blake A Wetherton, Jan Egedal, William S Daughton In antiparallel reconnection, parallel beams of cold electrons are often seen streaming into the reconnection region, accelerated inward by a parallel potential. We investigate a method of electron bulk heating wherein energy is exchanged through the parallel potential into bulk streaming energy in beams and is then thermalized through an effective scattering process. This scattering process is based on the breakdown of the electron magnetic moment as an adiabatic invariant in the reconnection exhaust, which causes the distribution to be independent of the magnetic moment in that region. A simplified differential equation has been derived to explain this thermalization. Results from VPIC simulations designed to provide constraints for two model parameters related to the efficiency of first- and second-order Fermi acceleration across a range of upstream conditions will be presented. This model retains first-order parallel electron dynamics and can be extended to a scale-invariant model applicable to large-scale systems that are not amenable to kinetic simulation, such as the solar corona. |
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GP11.00063: Kinetic Entropy as a Diagnostic in Particle-in-Cell Simulations Haoming Liang, Paul Cassak, Matt Argall, Gian Luca Delzanno, John Dorelli, James Drake, Michael Hesse, William Henry Matthaeus, Tai Phan, Vadim Roytershteyn, Earl Scime, Sergio Servidio, Michael A Shay, Marc Swisdak, Roy Torbert While entropy has been used broadly in fluid and gyrokinetic models, kinetic entropy of fully kinetic plasma systems has been vastly under-utilized. It is the natural metric of irreversible dissipation since it is conserved in ideal closed systems and increases when there is dissipation. This suggests kinetic entropy can address important questions on the nature of dissipation. In this work, we carry out an initial study to develop the diagnostic in collisionless particle-in-cell (PIC) simulations, using 2.5 D anti-parallel reconnection as a test case. First, we calculate the traditional kinetic entropy and the full Boltzmann entropy. We show kinetic entropy can be decomposed into a sum of a velocity space and position space entropies. We find that total entropy in the simulations is preserved quite well - within two percent - and use the departure from conservation to quantify the effective numerical dissipation. Finally, we use kinetic entropy to identify regions with non-Maxwellian distributions and compare it to other approaches including J dot E', agyrotropy, and Pi-D. The infrastructure developed here will be useful for studies of weakly collisional systems, including reconnection, turbulence and shocks, and is being applied to Magnetospheric Multiscale (MMS) data. |
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GP11.00064: Fast, strong ion heating in collisionless magnetic reconnection via an intrinsic stochastic mechanism Young Dae Yoon, Paul M Bellan A long-standing problem in collisionless magnetic reconnection is its accompaniment by fast, strong ion heating that cannot be explained by classical mechanisms. We explain this anomaly by showing that stochastic ion heating is inherent to collisionless reconnection and is sufficiently fast and strong to account for the observed anomalous ion heating. In a typical collisionless reconnection geometry, i.e., a perturbation to a Harris equilibrium with a shear length smaller than the ion skin depth, magnetic field lines reconnect while canonical vorticity field lines remain connected. This fact is exploited to approximate the electric and magnetic fields in the inflow and outflow directions and then demonstrate that they meet the stochastic ion heating criteria. The analytical calculation is verified by an electron fluid simulation. Test-ion simulations and comparisons to experiments confirm the existence of this mechanism. |
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GP11.00065: Particle acceleration in magnetically-driven reconnection using laser-powered capacitor coils Abraham Chien, Hantao Ji, Lan Gao, Gennady Fiksel, Eric Blackman, Quanming Lu, Kenneth Wayne Hill, Brian F Kraus, Philip Charles Efthimion, Philip Michael Nilson Collisionless magnetic reconnection events in astrophysical plasmas have been observed to generate particles with energies far higher than the thermal background. For instance, nonthermal acceleration has been observed in the Earth's magnetosphere, where nonthermal electrons have energies of up to 105 eV, compared to thermal electrons of a few eV; additionally, in solar flares, a large portion of released energy can be attributed to energetic electrons. The mechanisms behind nonthermal particle acceleration are not well understood: several theories have been hypothesized and tested in particle-in-cell simulations, but they have not been experimentally confirmed. Recently, laser-powered capacitor coils have emerged as a novel technique to generate strong external magnetic fields. These can be arranged to create a laboratory magnetic reconnection setup with low-β, collisionless reconnection, as well as measurement of the energetic electron energy spectrum. Preliminary results on the Omega EP laser facility at the Laboratory for Laser Energetics demonstrate evidence of particle acceleration due to magnetic reconnection; further experiments, combined with particle-in-cell simulations, will provide a better understanding of the specific acceleration mechanisms. |
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GP11.00066: Evolution of the Reconnecting Internal Kink Mode Paolo Buratti, Bruno Coppi The best agreement between theory and experiments concerning the onset of magnetic reconnection is (probably) represented by the theory of the resistive internal kink mode [1]. Now there remains a need to explain the following observed evolution of the reconnection rate that involves the formation of a relatively large magnetic island and a local steepening of the electron temperature gradient. Thus, the effects that characterize a magneto-thermal reconnecting mode [2] are proposed to be compounded with those (e.g. total plasma pressure gradient) that lead to the excitation of the original $\left( {{m}^{0}}=1 \right)$ mode. [1] B. Coppi, R. Galvão, R. Pellat, M. Rosenbluth, P. Rutherford, Fizika Plazmy (Soviet Journal of Plasma Physics) 2, 961 (1976). [2] B. Coppi, B. Basu, and A. Fletcher, Nucl. Fusion, 57, 7 (2017). |
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GP11.00067: Threshold for the torus instability of arched, line-tied flux ropes Andrew D Alt, Clayton Myers, Hantao Ji Coronal mass ejections occur when long-lived magnetic flux ropes anchored to the solar surface destabilize and erupt away from the Sun. One mechanism that drives this eruption is the ideal magnetohydrodynamic torus instability [1]. The torus instability has previously been considered in axisymmetric fusion devices where instability criterion is given by the decay index of the confining magnetic field, n = -∂ln(B)/∂ln(R) > ncr, where ncr = 1.5 in the large aspect ratio limit. In recent laboratory experiments performed on the Magnetic Reconnection Experiment (MRX), however, the critical decay index in solar-relevant, line-tied flux ropes was instead found to be ncr ≈ 0.8 [2]. In this work, we investigate how line-tying and aspect ratio effects modify the predicted torus instability criterion. We then compare these predictions to the MRX flux rope eruption database. This work motivates future laboratory experiments in continued investigation of line-tied flux rope eruption mechanisms, including the role of magnetic self-organization in erupting and non-erupting flux ropes. |
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GP11.00068: Laboratory measurement of whistler wave pulse from impulsive reconnection Magnus A Haw, Byonghoon Seo, Paul M Bellan We present measurements of coherent whistler wave pulses generated by isolated, impulsive magnetic reconnection events on the Caltech jet experiment. The wave pulses are measured with a new multicluster Bdot-probe located more than 45 cm away from the reconnection location. The whistler character of these pulses is confirmed by measurements of the background parameters, the wave polarization, and the wave dispersion. The results provide evidence that whistler waves are a consequence rather than a cause of collisionless reconnection. |
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GP11.00069: A 2D particle-in-cell simulation of anomalous transport in Hall effect thrusters CheongBin Cheon, Young Hyun Jo, HaeJune Lee Hall effect thrusters (HETs) have been utilized for a diverse space mission with its high specific impulse. Both the anomalous transport phenomenon and the enhanced mobility were reported in the previous study in a rectangular coordinate system [1]. In this study, we report the transport phenomena in HETs investigated with a 2D GPU particle-in-cell (PIC) method. In a low-pressure range, it is observed that the cross-field mobility does not have classical characteristics. Instead, an enhanced mobility relation which differs from the former is observed. It means that an anomalous transport phenomenon that is explained as azimuthal fluctuations due to large electron drift velocities affects instabilities. An enhanced electron transport theory shows that it is irrelevant to electron-neutral collisions when pressure decreases. This instability effect plays a more important role in the cross field transport than the secondary electron emission. [1] Lafleur et al. Physics of Plasmas 23, 053502 (2016). |
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GP11.00070: Electron heating in non-relativistic perpendicular shocks Anatoly Spitkovsky, Vasileios Tsiolis, Patrick Crumley An on-going question about astrophysical collisionless shocks is whether electrons and ions reach different temperatures downstream of a supernova remnant shock or whether they equilibrate. We investigate the electron heating mechanism in collisionless, non-relativistic, quasi-perpendicular, electron-ion shocks from first principles. We perform fully kinetic, 2D particle-in-cell simulations to follow the shock formation until the downstream steady state is reached. Our simulations are performed for a range of Alfvénic Mach numbers (MA) from 2 to 50, and of sonic Mach numbers (MS) from 2 to 36. The two species tend to reach equipartition as MS approaches unity, independently of MA value. For larger MS, electron-ion temperature ratio weakly depends on MA and falls in the range 0.1-0.5, much larger than the number of the order of mass ratio expected in the absence of collisionless heating. Electrons gain energy in the shock foot due to the electric field associated with the cross-shock potential. The shock transition region becomes filamentary for larger MA, indicating that Weibel instability may be responsible for electron scattering and isotropization. We have checked the convergence of our results by performing a parametric study on mass ratio and particles per cell number. |
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GP11.00071: Observing the characteristic velocity-space signature of particle energization mechanisms using modern spacecraft instrumentation J. L. Verniero, G. G. Howes, D. E. Stewart Understanding how turbulent energy is transferred from large to small scales and is eventually dissipated into plasma heat, or some other form of particle energization, is a grand challenge problem at the forefront of space and astrophysical plasma physics. In weakly collisional space environments, the collisionless interactions between electromagnetic fields and individual plasma particles dictate the removal of energy from turbulent fluctuations. The novel Field-Particle Correlation technique determines how turbulent energy dissipates into plasma heat by identifying which particles in velocity-space experience a net gain of energy. Using data from a gyrokinetic simulation, we map field-particle correlations to realistic phase-space resolutions of modern spacecraft instruments to determine the limitations on resolving the velocity-space signature of field-particle correlations for ions and electrons. We incorporate statistical models of the Poisson noise to establish the number of particle counts needed for sensible signal-to-noise ratios which facilitates a thorough investigation of implementation restraints. |
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GP11.00072: The Parker Instability with Cosmic Ray Streaming Evan Heintz, Ellen Gould Zweibel The Parker Instability is a Rayleigh-Taylor like mode where the magnetic field and cosmic rays support the thermal gas against gravity. The effect of cosmic ray transport on stratified systems had not been analyzed. Therefore we performed a stability analysis on the Parker Instability for three different cosmic ray transport models: decoupled from the gas, coupled with $\gamma_c=4/3$ but not streaming, and finally coupled with streaming at the Alfv\'{e}n speed. When the compressibility of the cosmic rays is decreased the system becomes much more stable, but the addition of cosmic ray streaming to the Parker Instability severely destabilizes it. Through comparison of these three cases and analysis of the work contributions for the perturbed quantities of each system, we demonstrated that cosmic ray heating of the gas is responsible for the destabilization of the system. Currently, we are working to run numerical simulations of the instability. Through these simulations, we aim to add in more effects like radiative cooling and diffusion as well as observe any nonlinearity that appears in the instability. |
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GP11.00073: Soft Cosmic Ray Spectra from Supernova Remnants Felix Aharonian, Mikhail Malkov It is becoming increasingly evident that the spectra of cosmic rays accelerated in supernova remnants (SNRs) are noticeably softer than the commonly used diffusive shock acceleration (DSA) theory predicts. We propose a simple physical explanation for this discrepancy by considering a spherical shock expanding into an interstellar medium with a quasi-homogeneous magnetic field. Particle injection into the DSA is efficient only on two opposite sides of a spherical shock where its normal does not strongly deviate from either parallel or antiparallel field direction. These regions (cusps) grow in time, as the shock expands, thus continually adding particles with a shorter acceleration history and, therefore, lower energies. A proper integration over their acceleration history, indeed, gives considerably softer spectra than what the standard DSA predicts for strong shocks. We also argue that similar spectral softening mechanism is pertinent to shocks and magnetic fields with more complicated geometry where the field-shock normal angle, \vartheta_{nB}, varies on the shock surface. |
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GP11.00074: Weibel Instability in a Foreshock Region Keita Todoroki, Mikhail Medvedev Collisional shocks, especially the relativistic ones, in weakly-magnetized plasmas are accompanied by an extended magnetized region in front of it -- the foreshock. The foreshock is produced by outgoing particles (cosmic rays) interacting with the ambient medium via plasma instabilities. Weibel Instability (WI) is known to be particularly important in GRB and early SN shocks. Here we discuss how WI is proceeding in the 'pre-conditioned' medium of the foreshock, when 'fresh' cosmic rays are continuously ejected from the shock front into the turbulent plasma with fairly strong and small-scale magnetic fields. |
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GP11.00075: Maser radiation from electrons accelerated by magnetised collisionless shock waves Robert Bingham, D C Speirs, Kevin Ronald, Alex Rigby, Fabio Cruz, Ruth Bamford, R.A. Cairns, A D.R. Phelps, Mark E Koepke, Barry Kellett, Ricardo Fonseca, Luis O Silva, Sergey V Lebedev, Gianluca Gregori In this paper we describe a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetised collisionless shocks. It is inspired by the work of Begelman, Ergun and Rees [1] who argued that the cyclotron maser instability occurs in localised magnetised collisionless shocks such as those expected in Blazar jets. We report on two recent laboratory experiments and numerical simulations carried out to investigate electron acceleration at collisionless shocks and the maser radiation mechanism [2-3]. We describe how electrons accelerated by lower-hybrid waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. Magnetic compression and conservation of magnetic moment lead to the formation of an electron velocity distribution having a horseshoe or ring shape as the electrons are accelerated along the magnetic field. We show that under certain conditions the horseshoe or ring electron velocity distribution is unstable to the cyclotron maser instability. [1] M. C. Begelman, R. E. Ergun, and M. J. Rees, Astrophys. J. 625, 51 (2005). [2] F. Cruz et al., Physics of Plasmas 24, 022901 (2017). [3] D. C. Speirs et al., Phys. Rev. Lett. 113, 155002 (2014).
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GP11.00076: Axisymmetric PIC simulations of pulsar magnetospheres with ab initio photon emission and pair production Fabio Cruz, Thomas Grismayer, Luis O Silva, Alexander Y Chen, Anatoly Spitkovsky Global particle-in-cell (PIC) simulations of pulsar magnetospheres performed in recent years comprise a multitude of phenomena, including surface charge injection, general relativity frame dragging corrections to Maxwell’s equations, and QED gamma ray emission and pair production. The QED processes, thought to be critical in the development of pulsar magnetospheric plasmas, are, however, only considered phenomenologically. In this work, we present simulations performed with a recently developed 2D axisymmetric spherical version of the OSIRIS PIC code that treat the photon emission and pair production processes from first principles, leveraging on recent OSIRIS ab initio QED modules. |
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GP11.00077: Particle orbits and electric fields in astrophysical jets Paul M Bellan Astrophysical jets cannot power themselves so the power must come from elsewhere. It was recently shown [1] that neutral-ion clumps having a critical q/m in a weakly ionized protostellar accretion disk spiral inwards and accumulate at small radius to produce a positively charged cloud. Because the spiral motion depends on q/m, electrons do not similarly spiral in and, instead, try to neutralize the positive charge cloud by flowing along poloidal magnetic flux surfaces outside the disk. This creates a complete poloidal electric current circuit where the inward spiraling positive clumps are the power source and the jets are the load. Toroidal magnetic field links the poloidal current so resultant J x B forces drive bi-directional jets while also providing a Z-pinch radial equilibrium. A radial EMF must supply the continuously increasing toroidal flux in the lengthening, constant-current jets. Lorentz transformation to the lab frame of Harris's Bennett pinch solution [2] supports this point of view by showing that a Z-pinch moving with axial center of mass velocity Vz has a lab-frame electric field Er = VzBphi. [1] P.M. Bellan, MNRAS 458 (2016) 4400; PPCF 60 (2018) 014006 [3] E. G. Harris, Nuovo Cimento 23 (1962) 115 |
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GP11.00078: A Bragg Diffraction Based 6 keV X-ray Imaging System for a Laboratory Plasma Jet Yi Zhou, Paul Murray Bellan A simple x-ray imaging system based on Bragg diffraction has been designed to image approximately 6 keV x-ray bursts from a lab plasma jet undergoing an instability that then breaks the jet. The imaging system consists of a spherically bent silicon (Si) crystal capable of reflecting and focusing 6 keV x-ray photons having near 90-degree angle of incidence, an aluminum foil to block visible light, and a cerium-doped yttrium aluminum garnet (YAG: Ce) scintillator that converts the x-ray image to a visible light image. The converted x-ray images will be recorded by a high-speed movie camera. The diagnostic will be placed outside a vacuum chamber and will view the x-ray source through a Kapton window. Comparison between the x-ray images produced by the system and visible light images of the plasma jet will identify the time and location of the x-ray bursts and show if, as expected, they are associated with the breaking off of the jet. |
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GP11.00079: Magnetically Induced Current Piston for Generating Extreme-Ultraviolet Fronts in the Solar Corona Pakorn Wongwaitayakornkul, Magnus A Haw, Hui Li, Paul M Bellan EUV waves are single-pulse, globally-propagating coronal fronts first observed in 1995 and their physical mechanism has been a subject for debate for two decades. Two distinct types of theories have been used to describe EUV waves, depending on the presumed driving mechanism: wave or pseudo-wave. We propose a hybrid model where EUV waves are compressional fronts driven by a reverse electric current layer induced from the coronal mass ejections (CMEs). The CMEs are considered as erupting flux ropes that induce the layer of electrical current. The reverse electric current layer, flowing in the opposite direction with respect to the current in the driving CME, is an eddy current layer that is necessary to maintain magnetic flux conservation in the coronal regions above the layer. The opposing CME and reverse currents mutually repel via magnetic forces with the result that the moving induced reverse current layer acts as a piston that drives a compressional perturbation in the coronal regions above the layer. This model is supported and motivated by results from 3D ideal magnetohydrodynamics (MHD) simulations and from laboratory experiments. |
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GP11.00080: Numerical modeling of magnetic self-organization at the top of the solar convection zone John O'Bryan, Thomas Jarboe Nonlinear, numerical computation with the NIMROD code is used to explore magnetic self-organization at the top of the solar convection zone. When considered together, two related effects (localized convective turbulence and rotational induction) create a robust mechanism for magnetic field generation, regardless of its magnitude. In the vicinity of the thin, shallow superadiabatic layer, convective turbulence and subsequent magnetic self-organization generate a radially-localized, latitudinally-elongated magnetic structure. The convective turbulence drive is stabilized by magnetic field. While this limits the achievable magnetic field from localized turbulence alone, the resulting structure is self-healing: should any part of the magnetic structure be perturbed away, the localized convective turbulence will rapidly regenerate it. Differential rotation of the sun creates an inductive electric field which also causes growth of the magnetic field within the structure, the rate of which scales with its magnitude. The nonlinear evolution of such a shallow magnetic structure may be able to explain some key surface magnetic features, including the behavior of sunspots, granules, etc.
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GP11.00081: The Shear-Current Effect: An Extended Analysis Evan L Yerger, Jonathan Squire, Amitava Bhattacharjee Following the work of J. Squire and A. Bhattacharjee[1], we investigate the shear-current effect (SCE) as a mean-field dynamo (MFD) mechanism in accretion disks. We first verify the previous results using a new code, Athena++, by performing an ensemble of unstratified zero-net-vertical-flux shearing box simulations. The SCE is then verified using both test-field and least-squares methods to compute the dynamo coefficients, assuming the standard closure model for the electromotive force. We subsequently investigate the effects of shearing box vertical aspect ratio and spatial resolution, following [2]. Further extension to boxes with net vertical flux, stratification, and the inclusion of non-ideal MHD terms is also considered. |
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GP11.00082: Localized Measurements of the Galactic Magnetic Field with Photoionized Interstellar Plasmas (HII Regions) Steven Spangler, Allison Costa The Milky Way galaxy has a large scale magnetic field that is in the plane of the Galactic disk, is mainly azimuthal with a smaller radial component, and has a magnitude of approximately 4 microGauss. Most astrophysical plasma measurements are path integrals along the entire line of sight through the plasma. In this paper, we discuss a method which provides information on the Galactic magnetic field on scales much smaller than the Galactic radius (tens of parsecs rather than thousands of parsecs). We measure Faraday rotation due to HII regions, roughly spherical regions of photoionized plasmas surrounding hot stars. We have made measurements of Faraday rotation on lines of sight passing through two HII regions, the Rosette Nebula and W4. In both cases, the Faraday rotation measure (RM) is dominated by the HII region, and in both cases the sign of the RM is consistent with the sign of the large scale Galactic field in that direction. We will discuss the implications of these measurements for interstellar turbulent magnetic fluctuations on spatial scales of 10 - 100 parsecs. |
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GP11.00083: Plasma coupling to gravitational waves: Developing a variational approach Deepen Garg, Ilya Y Dodin The recent detection of gravitational waves (GW) has stimulated increased interest in GW interactions with astrophysical plasmas, which are generally expected to involve electromagnetic effects. Existing theories of these interactions are cumbersome already for homogeneous waves; hence, they are not easily extendable to time-dependent effects that may involve mode conversion and nonlinear phenomena. We are developing a variational formulation to GW-plasma interactions to make such effects more tractable. |
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GP11.00084: Theoretical Model for the Structure of the Gamma-ray Bubbles in Our Galaxy Alessandro Cardinali, Bruno Coppi A flux of nuclei emerging from the center of the Galaxy is assumed to reach a magnetic field “arch” with dimensions of the same order of magnitude as that of the Galaxy [1]. Then a lower hybrid mode is envisioned to be excited by the non-thermal features of the nuclei distribution [2]. This mode can then transfer energy, parallel to the magnetic field to high velocity electrons which can be held responsible for the observed high energy radiation emission at large distances from the center of the galaxy defining the edge of each bubble. 1. H.-Y. K. Yang, M. Ruszkowski, E.G. Zweibel, Galaxies 6 29 (2018). 2. T. Chang and B. Coppi, Geophys. Res. Lett. 8, 1253 (1981).
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GP11.00085: Assessment of Gradient Effects on Soft X-ray Spectral Measurements on the Z Machine David Dunkum, Cuyler Betty, Theodore Lane, Pawel M Kozlowski, Mark E Koepke Accurate assessment of temperature, density, magnetic field, opacity, etc. can be complicated by a spatial gradient or rapid temporal variation, among other factors. The Z Pinch Dynamic Hohlraum (ZPDH) radially emits X-rays that are used for backlighting and for heating a plastic (CH)-tamped sub-micron-radially-thick magnesium sodium-flouride (MgNaF) foil. Simulations were performed over a range of temperature and density values to match observed absorption spectra and thereby determine experimental plasma parameters. Impurity-oxygen lines were included, using our recently developed isoelectronic line-ratio technique for soft x-ray absorption spectra as a temperature diagnostic on Z [Lane, et al., ICHEDS 2017] to overcome previous discrepancies between theoretical and experimental results. |
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GP11.00086: 3D Nonaxisymmetric Simulations of the free Stewartson-Shercliff Layer with Insulating and Conducting Axial Boundaries Dahan Choi, Fatima Ebrahimi, Kyle J Caspary, Erik Gilson, Jeremy Goodman, Hantao Ji Results are presented from 3-D numerical nonaxisymmetric simulations of the Princeton MRI Experiment, which is a modified Taylor-Couette device with GaInSn as its working fluid to study rotational MHD flows. Spectral Finite Element Maxwell and Navier Stokes (SFEMaNS) code is utilized to investigate the effect of axial boundary conductivity on a free Stewartson-Shercliff Layer (SSL). The free SSL is established by a sufficiently strong magnetic field imposed axially across the differentially rotating fluid with two rotating rings enforcing the boundary conditions. Numerical simulations show that the response of the bulk fluid flow is vastly different in the two different cases of insulating and conducting endcaps. We find that for the insulating endcaps, there is a transition from stability to instability of a Kelvin Helmholtz-like mode that saturates at an azimuthal modenumber m = 1 and at an Elsasser number for GaInSn of unity, while for the conducting endcaps, the reinforced coupling between the magnetic field and the bulk fluid generates a strong shear in the azimuthal velocity resulting in Rayleigh-like modes even at reduced thresholds for the axial magnetic field. Numerical results will further be compared with the experimental measurements [K. Caspary et al. PRE 2018]. |
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GP11.00087: Substorm Onset from Sheared Zonal Flow Vortices and Interchange Dynamics Jason Derr, Wendell Horton, Richard Wolf Kalmoni et al.[JGR 2016 and Substorm Conference 2017] propose a theoretical model to interprete their extensive statistical analysis of early phase of substorm onsets. The analysis shows that a chain of small size (<100km) weak auroral brigthenings called “beads” develops along the latitude of the magnetic field foot points that defined by early auroral brightening and subsequent substorm auroral arcs. We study and extent the wave equations that include the ion energy distribution functions with the lower and higher energy ion components in auroral flux tubes along with the horizontal sheared EXB flows. The resulting stability wave equations are similar to that in the Kalmoni analysis with some apparent differences in the terms that combine the Kelvin-Helmholtz meridional sheared flows with interchange dynamics in the auroral magnetic fields. The model gives conditions for the growth of the auroral beads and the release of energy from the local pressure gradients. Comparisons are made between the dynamics driven by the interchange and sheared flow dynamics. The results extent conditions for the onset of the beads and the substorm dynamics. |
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GP11.00088: An overview of the diagnostic developments for M3, a 2.5 MJ low inductance capacitor discharge machine. Paul Holligan, Luis S Caballero Bendixsen, Thomas Clayson, James Parkin, Jamie Darling, Simon N Bland, Savva Theocharous, Jonathan Skidmore, Hugo W Doyle, Guy Burdiak, Peta Foster, Matthew R Betney, Peter Allan, Tim J Ringrose First Light Fusion Ltd (FLF) is a privately funded company in the early stages of researching energy generation using inertial confinement fusion. FLF currently operates a two stage light gas gun and two low inductance pulsed power drivers delivering 1 MA and 3.5 MA which are used for electromagnetic launch. Experimental campaigns are focused on validation of Hytrac and Code B, two simulation tools developed by FLF. Ongoing pulsed power developments, simulations and experimental data feed into the design and construction of our new 2.5 MJ low inductance 200 kV capacitor discharge pulsed power driver, Machine 3 (M3). This has been designed to allow us to demonstrate fusion in the laboratory. World class diagnostic capabilities have been established at FLF, including ultra-high speed imaging (~ 3 ns exposure), streaked spectroscopy, VISAR and dynamic x-ray radiography. Machine diagnostics include a fibre optic Faraday rotation system, V-dot and B-dot probes. This enhances our understanding of the target physics as well as electromagnetic launch techniques. An overview of both experimental and machine diagnostics will be presented along with M3 construction and commissioning progress. |
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GP11.00089: A taxonomy for the hundreds of diagnostics on the US Inertial Confinement Fusion facilities Joseph Kilkenny The three large US ICF facilities, NIF, Z and OMEGA have a large number, 200 to 300, of operating diagnostics. These diagnostics have been built up over scores of years and it can therefore be difficult to quickly understand their function and their mode of operation. This paper will show that nearly all the ICF diagnostic instruments can be categorized in a two dimensional array, one axis representing the front end which is either an imager or a spectrometer, and an orthogonal axis representing the detector of the emitted “particles” ( light, x-ray, neutrons etc.) after they have been through the front end. The diagnostic instruments can be used in emission, absorption or scattering experimental platforms making a simple third axis of this way of looking at the ICF diagnostics. Many examples of this taxonomy will be shown for the diagnostics and platforms for NIF, the National Ignition Facility.
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GP11.00090: Next Generation Gamma-ray Cherenkov Detectors for ICF Hans W. Herrmann, Yongho Kim, Alex Zylstra, Hermann Geppert-Kleinrath, Kevin Meaney, Jorge Carrera Fusion reaction history and ablator areal density measurements for Inertial Confinement Fusion experiments at the National Ignition Facility are conducted using the Gamma Reaction History diagnostic (GRH-6m). Future Gas Cherenkov Detectors (GCD) will ultimately provide ~200x more sensitivity, reduce the effective temporal resolution from ~100 to ~10 ps and lower the energy threshold from 2.9 to 1.8 MeV, relative to GRH-6m. The first phase consisted of inserting the existing coaxial GCD-3 detector into a reentrant well which put it within 4 meters of the implosion. This diagnostic platform has allowed assessment of the x-ray radiation background environment within the well which will be fed into the shielding design for a follow-on “Super” GCD. It has also enabled use of a revolutionary new pulse-dilation photomultiplier tube (PD-PMT) to improve the effective measurement bandwidth by >10x relative to current PMT technology. The next phase is to improve sensitivity by increasing solid angle. This can be accomplished with a single GCD w/ PD-PMT on a TANDM diagnostic insertor, or multiple smaller GCDs on a TANDM with standard PMTs. The PD-PMT version would provide unprecedented temporal resolution, while the multiple GCD concept would allow for time-resolved mix measurements. |
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GP11.00091: Optimized shielding design for the Magnetic Recoil Spectrometer time (MRSt) on the NIF Alexander Sandberg, Brandon J Lahmann, Johan Frenje, Cody E Parker, Maria Gatu Johnson, Chikang Li, Fredrick Seguin, Richard David Petrasso, Terance Joseph Hilsabeck, Joseph David Kilkenny, Robin Hibbard, Hesham Khater, Andy Mackinnon For successful implementation of the MRSt on the NIF for time-resolved measurements of the neutron spectrum with a time resolution of ~20 ps and accuracy of ~5%, shielding will be required to reduce the neutron and gamma-ray background flux to an acceptable level at the MRSt detector. Expanding upon previous work done by C. W. Wink et al. [1], an MCNP model of the final MRSt design on the NIF was developed to assess the required shielding for a down-scattered-neutron measurement with a S/B > 5. This work accounts for additional and final changes in the engineering design of MRSt and its placement within the NIF target bay. [1] C. W. Wink et al., Rev. Sci. Instrum. 87, 11D808 (2016). The work was supported by DOE, LLNL and LLE. |
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GP11.00092: Comparison of DT neutron and x-ray induced instrument response functions for a current-integrated neutron-time-of-flight detector Jedediah Styron, Chad Forrest, Carlos Ruiz, Kelly D Hahn, Owen Mannion, Gordon A Chandler, Gary Wayne Cooper, Vladimir Glebov, Clark Highstrete, Brent M Jones, James P Knauer, Bruce McWatters, Sara Pelka, Christian Stoeckl, Jeremy Vaughan, Colin Weaver Neutron time-of-flight (nTOF) detectors are widely used in inertial confinement fusion to infer kinetic effects and the bulk ion temperature within the plasma. Extracting these parameters from the digitized nTOF signal requires knowledge of the detector instrument response function (IRF). Traditionally the detector IRF has been measured using short-pulse x rays as a surrogate for a neutron response. For this work, the neutron IRF measured at Sandia National Laboratories’ Ion Beam Laboratory using single-event DT neutron interactions and the x-ray IRF measured at the OMEGA and MTW laser facilities at the University of Rochester’s Laboratory for Laser Energetics are compared for the same nTOF detector. |
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GP11.00093: Optimizing neutron imaging system design for the National Ignition Facility using synthetic imaging data Jacquelynne Vaughan, Verena Geppert-Kleinrath, Petr L Volegov, Carl Wilde, Christopher R Danly, Aidan Crilly, Brian Appelbe The Los Alamos National Laboratory Advanced Imaging team has been providing neutron imaging of inertial confinement fusion implosions at the National Ignition Facility (NIF) since 2011. Now, the team is designing two additional active detector systems adding two more lines-of-sight to allow three-dimensional reconstruction of the fuel assembly. Together these systems will deliver shape information on the burning hot spot, surrounding cold fuel, and the remaining ablator by adding gamma imaging capabilities. The design of the new detectors is optimized using synthetic neutron imaging data from simulations in the Chimera rad-hydro code together with a detailed forward model of the complete neutron imaging system including the pinhole aperture array. The simulation results will not only inform the design requirements for the new systems, but also improve future physics interpretation of NIF imaging data from both the existing and the planned imaging systems. |
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GP11.00094: CR-39 Neutron Sensitivity Study for Improved ODIN Neutron Response Function Analysis Jeremy Vaughan, Carlos Ruiz, David Neal Fittinghoff, Mark Joseph May, David J. Ampleford, Brandon J Lahmann, Gary Wayne Cooper, Gordon A Chandler, Kelly D Hahn, Jedediah D Styron, Bruce McWatters, Jose Torres The one-dimensional imager of neutrons (ODIN) at the Sandia Z facility was designed to determine the size, shape, and location of the neutron producing region in Sandia’s baseline ICF concept, namely magnetized liner inertial fusion (MagLIF). Modeling efforts with subsequent IBL experiments continue to advance the neutron imager using an HDPE converter and CR-39 plastic detector. Sandia’s Ion Beam Laboratory (IBL) was used to experimentally determine CR-39 DD and DT neutron sensitivity (with and without an HDPE (n,p) converter foil), edge spread function, and resolution. The HDPE converter will produce roughly four times as many tracks as the unfiltered CR-39, increasing the CR-39 sensitivity of 1e-4 tracks per neutron. This will produce an edge spread function that can be analyzed with current MCNP6.1 point spread model. Finally, modeling efforts of proton recoil (DD and DT neutrons entering CR-39 with and without HDPE converter) will be shown to further understand the resolution of the CR-39 detector. |
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GP11.00095: The MIT HEDP Accelerator Facility for Diagnostic Development for OMEGA, Z, and the NIF Fredrick Seguin, Maria Gatu Johnson, Patrick J Adrian, Andrew Birkel, Neel Kabadi, Brandon J Lahmann, Cody E Parker, Raspberry Simpson, Hong Sio, Graeme Sutcliffe, Arijit Bose, Irving Doeg, Robert Frankel, Johan Frenje, Chikang Li, Richard David Petrasso, Ramon J Leeper, Kelly D Hahn, Carlos Ruiz, Thomas C Sangster, Terance Joseph Hilsabeck The MIT HEDP Accelerator Facility utilizes a 135-keV, linear electrostatic ion accelerator; DT and DD neutron sources; and two x-ray sources for development and characterization of nuclear diagnostics for OMEGA, Z, and the NIF. The accelerator generates DD and D3He fusion products through the acceleration of D+ ions onto a 3He-doped Erbium-Deuteride target. Capability to generate T3He fusion products through the acceleration of 3He2+ ions onto an Erbium-Tritide target has also been recently been developed and will be discussed in this contribution. Accurately characterized fusion product rates up to 106 s−1 are routinely achieved. The DT and DD neutron sources generate up to 6´108 and 1´107 neutrons/s, respectively. One x-ray generator is a thick-target W source with a peak energy of 225 keV; the other uses Cu, Mo, or Ti elemental tubes to generate x-rays with a maximum energy of 40 keV. Diagnostics developed and calibrated at this facility include CR-39-based mono-energetic particle radiography, charged-particle spectrometers, neutron detectors, and the particle Time-Of-Flight (pTOF) CVD-diamond-based bang time detector. The accelerator is also a valuable hands-on tool for student education at MIT. This work was supported in part by the U.S. DoE, SNL, LLE and LLNL. |
(Author Not Attending)
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GP11.00096: Achieving Thin Glue Gaps in Target Fabrication Pegah Bagheri, Lane Carlson, Luis Gonzalez The target fabrication group at General Atomics has developed a method using different vapor glues and liquids to bond components utilized for inertial fusion energy and high-energy density physics. The investigation associates with current targets fabricated for planar target assemblies and equation of state experiments. These components are constructed with high tolerances in respect to its surface flatness and roughness, the precision machining, and various elements that include high-density carbon to low-density foams. The thickness can range from 10’s to 100’s of microns. An essential characteristic to compare the experimental data to the simulated results is the specific glue applied despite being in a 3- dimensional build or a planar stack form. The most practical glue bond is to be very thin (<3 μm). The laser experiment may suffer from instabilities and shock reflections if too thick or non-uniform. Experiments are underway to characterize minimum glue bonds achievable in target assemblies with surface profilimetry, a precision assembly station, and high-resolution thickness measurement techniques. |
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GP11.00097: First refraction enhanced radiography experiments for probing inflight density profiles of ICF capsule implosions Eduard L. Dewald, Otto L Landen, Laurent Pierre Masse, Darwin Ho, Yuan Ping, Daniel Thorn, Nobuhiko Izumi, Laura Berzak Hopkins, Jeremy Kroll, Abbas Nikroo, Jeffrey Allan Koch Possible explanations for reduced compression of deuterium-tritium (DT) fuel capsule implosions on the National Ignition Facility are higher than expected ablator preheat that can increase the ablator-DT ice density jump and induce mix, more high-mode interface mix, and unexpected strength of reverberating shocks. We have hence implemented 8 keV streaked x-ray refraction enhanced radiography to infer the inflight density profiles in indirectly-driven TH layered fuel capsule implosions. The first data taken at 5 µm, 25 ps resolution reveal sharp fringe features associated with the ablation front, shocks and ice-ablator interface that are not visible in traditional absorption radiographs. The inferred density profiles will be compared to simulations. |
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GP11.00098: Spectroscopic Measurements of the Equatorial Electron Temperature in a NIF Hohlraum Using Mid-Z Tracer Elements* Klaus Widmann, Duane A Liedahl, Stephan A MacLaren, Maria A Barrios, Marilyn Beth Schneider, Hui Chen, William A Farmer, Leonard C Jarrott, Oggie Jones, Robert L Kauffman, Otto L Landen, Nathan Meezan, Mark Sherlock, Daniel Thorn, John D Moody Spectrally and temporally resolved x-ray emission measurements of highly charged mid-Z ions are used to determine the electron temperature (Te) in the equatorial region of a laser-driven ignition-type NIF hohlraum. A mid-Z tracer “dot” of 400 µm in diameter and 0.16 µm thickness is coated onto the outside of a high-density carbon (HDC) capsule. The measured x-ray spectra are fit using SCRAM simulations and a genetic algorithm to give a peak Te of (2.7 ± 0.4) keV at the hohlraum equator between the HDC capsule and the gold wall. The measurements show a decrease in Te near the hohlraum equator even before the laser drive is turned off giving evidence that the equatorially-directed laser beams are losing their energy before they reach the hohlraum equator. While we find agreement with post-shot simulations when the plasma reaches its peak temperature, some disagreement between the measured and simulated Te remains in the earlier part of the laser heating and also in the temporal signature of the cooling phase near the end of the laser drive. We present a detailed error analysis of the spectroscopic measurements and a discussion of the uncertainty in the derived Te. |
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GP11.00099: High-Resolution X-ray Spectroscopy of a Polar Direct Drive Exploding Pusher on NIF Brian Francis Kraus, Lan Gao, Kenneth Wayne Hill, Manfred Ludwig Bitter, Philip Charles Efthimion, Marilyn Beth Schneider, Hui Chen, Jay Ayers, Duane A Liedahl, Andrew G MacPhee, Heather D Whitley, Charles L Ellison, Hai P. Le, Ronald Bettencourt, Robert L Kauffman, David Nelson An absolutely-calibrated, high-resolution x-ray Bragg crystal spectrometer has been deployed at the National Ignition Facility (NIF) to diagnose plasma conditions in a polar direct drive exploding pusher (PDXP) implosion near stagnation. Two conical crystals focus the Kr He-α and He-β complexes onto a streak camera for time-resolved spectroscopy, and spectral intensities are calibrated via a cylindrical von Hamos crystal that time-integrates the intervening energy range between 12.8 and 15.6 keV. The evolution of electron density and temperature are inferred though Stark broadening and the relative intensities of dielectronic satellites. Results from collisional-radiative codes SCRAM and CRETIN are compared to relative and absolute line intensities and widths, which enables benchmarking of predicted plasma parameter profiles from 1D radiation hydrodynamic simulations of the PDXP platform. |
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GP11.00100: Inference of electron density in NIF capsules from krypton helium-β Stark lineshapes K. W. Hill, M. Bitter, Lan Gao, B. Kraus, P. C. Efthimion, M. B. Schneider, D. B. Thorn, H. Chen, R. L. Kauffman, D. A. Liedahl, A. G. MacPhee, P. Beiersdorfer, H. D. Whitley, R.C. Mancini, R. Doron, E. Stambulchik, Y. Maron The dHIRES x-ray spectrometer measures Kr Heα and Heβ spectra from NIF compressed capsules with 10-eV spectral and 30-ps temporal resolution. Theoretical calculations of the Stark-broadened lineshape of the Heβ complex (3 3P1, 1P1, 1D2) show monotonic variations with density of the line widths, line energies, and intensity of the 3 3P1 and 3 1D2 lines relative to the main, 3 1P1 peak. Comparison of the measured Kr Heβ complex line profiles with the theoretical lineshapes provides a measure of the time history of the electron density. These comparisons will be shown for four NIF shots with Kr-doped capsules |
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GP11.00101: Measuring CH-Au interface in gas-filled hohlraum using Thomson scattering on SG-III prototype Hang Zhao, Zhichao Li, Dong Yang, Xiaohua Jiang, Xin Li, Yongkun Ding An experiment was conducted on the SG-III prototype laser facility to measure the plasma conditions in pentane-gas-filled gold hohlraum using Thomson scattering. The time-resolved plasma conditions are measured 400um off the hohlraum wall. In the measured ion feature, the transition from scattering from CH to scattering from Au is clearly resolved. Plasma parameters are obtained by fitting the ion feature with a theoretical form factor, with electron density given by radiation-hydrodynamic simulation (LARED code). A discrepancy is observed in flow velocity and electron temperature between experiment and 2D-simulation, indicating that the impact of the probe beam on the gold-bubble and 3D-effects are not negligible. A post-simulation was done to evaluate the influence of the probe beam on the plasma conditions, and the result shows, when considering probe-beam heating, (1) the CH/Au interface moves much faster, (2) the electron density inside the gold bubble decreases significantly, and (3) a density increase occurred at the CH/Au interface. |
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GP11.00102: High-Order Low-Order (HOLO) Nonlinear Convergence Accelerator for the Rosenbluth-Fokker-Planck Collision Operator William Tsubasa Taitano, Luis Chacon, Andrei Simakov A fully implicit solver strategy is required for the nonlinear integral-differential Fokker-Planck equation when ε-1=Δt/τcol>>1, owing for the need to simultaneously, ensure exact conservation properties (mass, momentum, and energy), as well as the correct asymptotic convergence to the Maxwellian [1]. However, developing an effective solver is challenging, due to the integral-differential nature of the formulation (via the so-called Rosenbluth potentials), leading to a dense linear system. To effectively deal with these numerical challenges, we explore a multiscale iterative strategy based on a HOLO convergence accelerator scheme [2]. HOLO employs an LO (fluid) moment system to accelerate the convergence of the HO (kinetic) system. The LO quantities (n, u, T) inform the Maxwellian component of the potentials, which also contain a perturbation term [of O(ε)<<1] computed from the HO solution. This reformulation shifts the non-local contributions through the potentials from the HO system to the LO one, where they can be dealt with efficiently. Numerical experiments in challenging applications (ICF implosions) demonstrate the enabling capabilities of the HOLO scheme. [1] W.T. Taitano et al., JCP, 297, 357-380 (2015). [2] L. Chacón et al., JCP, 330, 21-45 (2017). |
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GP11.00103: A fully implicit, conservative, multi-scale algorithm for the multi-species Vlasov-Ampère system Steven Anderson, William T. Taitano, Luis Chacon, Andrei N. Simakov, Brett D Keenan We discuss the addition of a fully kinetic electron capability into the 1D-2V Vlasov-Fokker-Planck code iFP.1 The proposed algorithm utilizes an approach similar to that in Taitano & Chacón,2 but offers several advancements over this earlier 1D-1V study. Our approach utilizes a time- and space-adaptive velocity mesh, which dynamically expands/contracts individual species’ meshes as they heat/cool, and shifts to center on their bulk velocity. Additionally, we achieve full conservation of mass, momentum, and energy for an arbitrary temporal integration scheme. To demonstrate the effectiveness of the approach, we present simulation results for several canonical collisionless kinetic problems. These include the two-stream instability, and the ion acoustic shockwave, where the inverse plasma frequency is a very stiff timescale. The results demonstrate the promise of this scheme for application to complex multiscale kinetic plasma systems, such as inertial confinement fusion capsule implosions.
1W. T. Taitano, et al., J. Comp. Phys., 365, 173-205 (2018) 2W. T. Taitano, L. Chacón, J. Comp. Phys. 284, 718-736 (2015) |
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GP11.00104: Synthetic analysis of X-ray Images using a neural network Bradley T Wolfe, John L Kline, Zhehui Wang X-ray imaging is widely used for ICF experiments. It is necessary to extract information such as convergence, density profiles, turbulent mixing for modeling and predictive code development. X-ray imaging analysis is a complex task since an X-ray image is a convoluted product of the X-ray source, the dynamic scene evolution, and responses of detectors. Although static images and target preparation can help with information extraction, powerful data analysis techniques provide new options for X-ray imaging in ICF and elsewhere. Here describe a machine-learning analysis technique using convoluted neural network and sparsity constraint. A training data set is used to generate a dictionary for new image interpretation and feature extraction (edge detection). The new images are excluded from the training set. To compensate for the relatively small number of images available for the training set, we introduce synthetic data from computer simulations and analytic models for neural network training. The results are compared with other techniques based on polynomial fitting and extrapolation. |
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GP11.00105: A low-dispersion, exactly energy-charge-conserving semi-implicit relativistic particle-in-cell algorithm Guangye Chen, Luis Chacon, David Stark, Lin Yin, Brian James Albright, Robert F Bird Leap-frog based explicit algorithms, either energy-conserving or momentum-conserving, do not conserve energy discretely. Fully implicit algorithms can conserve discrete energy exactly (Lapenta and Stefano. POP 2011), which is desirable for long-term simulations, but introduce large dispersion errors in the light-wave modes, regardless of timestep sizes. This can lead to intolerable simulation errors where highly accurate light propagation is needed (e.g. laser-plasma interactions, LPI). In this study, we selectively combine the leap-frog and Crank-Nicolson methods to produce a low-dispersion, exactly energy-and-charge-conserving PIC algorithm. Specifically, we employ the leap-frog method for Maxwell equations, and the Crank-Nicolson method for particle equations. Such an algorithm admits exact global energy conservation, exact local charge conservation, and preserves the dispersion properties of the leap-frog method for the light wave. The algorithm has been implemented in a code named iVPIC, based on the VPIC code (https://github.com/losalamos/vpic) developed at LANL. We will present numerical results that demonstrate the properties of the scheme with sample test problems (e.g. Weibel instability run for 10^7 timesteps, and LPI applications). |
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GP11.00106: Super-configuration-accounting opacity of CH and its applications Teck Lee, Michel P Busquet, Marcel Klapisch, Jason Wilson Bates, Andrew J Schmitt, John L Giuliani Polystyrene CH is an essential material in inertial-confinement-fusion (ICF) target designs [1]. Based on super transition arrays (STA) calculations [2], we generate a wide range polyethylene (CH) opacity table of density ρ= 1.0 × 10-6 to ρ = 500 g/cm3 and temperature T = 0.015 to 2.5 keV. Contrasting the STA calculated opacities to the results obtained using the first principle quantum molecular dynamics (QMD) code developed at the Laboratory for Laser Energetic of University of Rochester and the ATOMIC code from Los Alamos National Laboratory [3], we find that the STA results are in reasonable agreement with the data from QMD and ATOMIC codes down to plasma temperature of about 20 eV where comparisons are possible. Additionally, we present samples of assessment on the reliability of the present CH dataset through a radiation hydrodynamic simulation. [1] D.T. Michel, V.N. Goncharov, I.V. Igumenshchev, R. Epstein and D.H. Froula, Phys. Rev. Lett. 111, 245005 (2013). [2] A. Bar-Shalom, J. Oreg, W. H. Goldstein, D. Shvarts, and A. Zigler, Phys. Rev. A, 40, 3183, (1989). [3] S.X. Hu et al., Phys. Rev. B., 96, 144203 (2017) |
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GP11.00107: Kinetic Simulations of Plasma-Ion Stratification in Exploding-Pusher Experiments Andrei N. Simakov, William T. Taitano, Luis Chacon, Brett D. Keenan, Steven E. Anderson, Hans G. Rinderknecht Recent OMEGA experiments [1] imploded thin spherical glass shells filled with D and 3He mixtures to explore stoichiometric effects on fusion reactivity. As the deuterium atomic fraction in the gas was varied hydro-equivalently, the experiments observed anomalous (w.r.t. single-fluid rad-hydro simulations) reduction of the fusion yield (by up to 50%) and scaling of burn-averaged ion temperatures. Differential heating of D and 3He ions and partial expulsion of deuterium from the capsule center by the shock [2,3], both neglected in rad-hydro simulations, were proposed as the explanations. Semi-analytical theory and reduced-ion-kinetics (RIK) simulations [4] seem to confirm the hypothesis. Herein, we employ Vlasov-Fokker-Planck code iFP [5] to carry out the first kinetic simulations of the experiments to validate, or disprove, the proposed explanations. [1] H. G. Rinderknecht et al., Phys. Rev. Lett. 114, 025001 (2015). [2] W. T. Taitano et al., Phys. Plasmas 25, 056310 (2018). [3] B. D. Keenan et al., Phys. Plasmas 25, 032103 (2018). [4] N. M. Hoffman et al., Phys. Plasmas 22, 052707 (2015). [5] W. T. Taitano et al., J. Comp. Phys. 365, 173 (2018). |
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GP11.00108: Quantifying Uncertainties in Predictive Models of Inertial Confinement Fusion Gemma Anderson, Jim A Gaffney Inertial confinement fusion (ICF) hydrodynamic simulations are crucial for understanding the implosion of the fuel target and are used to design future experiments at the National Ignition Facility. Typically, these simulations are computationally expensive to run. Deep learning can be used to build powerful predictive models mapping the simulation inputs (e.g. physics parameters and laser inputs) to outputs (such as neutron yield and bang time). However, most deep learning techniques yield point estimates with no information on how certain the model is in its prediction. As the model architecture increases in complexity, it becomes more unclear how to propagate and quantify uncertainties. We present current efforts to identify the various sources of uncertainty in deep learning models trained on ICF simulation data and quantify their effects on the overall uncertainty of model-based predictions of key ICF quantities relevant for assessing the performance of the implosion. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE- AC52-07NA27344, and released under LLNL-ABS-753983. |
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GP11.00109: Indirect-drive Inertial Confinement Fusion Simulations at the Centre for Inertial Fusion Studies. Jeremy Chittenden, Brian Appelbe, Kristopher McGlinchey, Christopher Alexander Walsh, Jon Tong, Aidan Crilly, Michael Zhang, Griffin Farrow Results are presented from simulations of High-Foot and High Density Carbon capsule implosions on the National Ignition Facility, using the ‘Chimera’ 3D radiation hydrodynamics code. Novel synthetic diagnostic tools construct detailed neutron spectra and images of the primary and scattered neutrons and gamma rays to establish characteristic diagnostic signatures of both capsule engineering features and radiation drive asymmetries. Detailed 3D radiation hydrodynamics simulations explore the effect of drive asymmetry and the capsule support tent on fusion performance in High-Foot capsules as well as the effect of the fill tube in HDC capsules. Ignition and propagating burn in highly perturbed hotspots is examined using a 3D Monte-Carlo alpha particle model both for present day experiments and possible future experiments with higher laser energies. The contribution of self-generated magnetic fields to hotspot thermal insulation is explored using extended MHD models incorporating full Braginskii transport. The increase in fusion performance with externally applied fields is also explored. Preliminary results from magnetised alpha particle burn calculations are used to explore how applied fields can influence the laser energy required to progress along an ignition cliff.
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GP11.00110: A Simple ODE Model for ICF Gain by Volume Ignition Nicholas Hawker, David Chapman, Nicolas P.L. Niasse There are several basic fuel configurations for ICF. Hot spots aim to reach high gain by propagating a burn wave into surrounding cold fuel. The small hot spot size means they are optically thin and the required temperature is >5 keV. Volume ignition has fuel in a single phase and relies on the trapping of radiation. This reduces the required temperature to ~2 keV, allowing high gain. The model includes energy deposition by alpha particles as instantaneous but non-local. The radiation loss smoothly transitions from pure bremsstrahlung to blackbody. Degeneracy effects are included and found to be significant. Verification of the model against detailed simulations from literature indicates good accuracy. The critical trade-off between self-heating time and confinement time is captured. Confinement time is eventually the limiting factor. Were it possible to increase the confinement time without deleterious additional effects, required temperature can be reduced to <1keV. Finally, constraints coming from reactor design are related back to the plasma properties. The union of parameter space where no constraint applies is called the “island of viability” and it is shown that for this island to exist, a very high coupling efficiency of 2% is required. |
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GP11.00111: Measurements of X-ray Pre-Heat in NIF Double Shell Capsules Douglas Wilson, Elizabeth Merritt, Eric Loomis, Tana Cardenas, Marty Marinak, Joshua P Sauppe, Ryan F Sacks, David S Montgomery, Evan Dodd, Willow Wan, Brian Michael Haines, Dru Renner, Sasikumar Palaniyappan Hohlraum generated X-rays penetrate the ablator of a double shell capsule and are absorbed in the inner surface of both the ablator and central capsule. Los Alamos has executed two NIF shots to study the impact of preheat. Both used VISAR in a keyhole geometry to measure the motion of one inner surface. The first of these observed the motion on the inner surface of a beryllium ablator. Sensitive to the hohlraum gold M-band (~>2keV) penetrating the ablator, the expansion of both pole and equator agreed with HYDRA calculations. The second experiment observed the inner surface of a tungsten central shell. Here the calculated velocities, due to harder gold L-band X-rays (~ 9-40 keV) were a factor of several too high. This expansion responds to ~15-20 keV X-rays, a spectral region calculated without strong lines in the gold L-band. The entire gold L-band emission or just in this spectral region may be calculated too high. |
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GP11.00112: Compression and Burn in the Presence of Low-Mode Asymmetries for Double Shells D.S. Montgomery, W.S. Daughton, E.N. Loomis We previously reported a metric for robust burn in double shell implosions where the specific power deposited by alpha-heating must exceed the specific power due to expansion cooling losses of the hot spot [1, 2]. This criterion results in a minimum hot spot temperature at stagnation in the absence of burn. Margin is obtained for designs by the degree that they exceed this minimum no-burn temperature. In this present work, we extend this model to include simple low-mode asymmetry using a quasi-adiabatic approximation, and compare predictions of this model to 2D HYDRA simulations of double shell capsules. We further examine requirements during the compression phase to achieve the requisite hot spot conditions.
[1] D.S. Montgomery, W.S. Daughton et al., BAPS.2017.DPP.NO7.1; [2] D.S. Montgomery, W.S. Daughton et al., submitted to Phys. Plasmas (2018). |
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GP11.00113: Utilizing multiple fusion-reaction histories, x-ray-emission histories, and charged-particle stopping to evaluate high-energy-density-plasma (HEDP) transport properties at OMEGA Patrick J Adrian, Johan Frenje, Neel Kabadi, Hong Sio, Raspberry Simpson, Maria Gatu Johnson, Graeme Sutcliffe, Chikang Li, Fredrick Seguin, Richard David Petrasso, Sean P Regan, Vladimir Glebov, Paul Eric Grabowski We present a new technique that is used to evaluate HEDP transport properties, including stopping power and ion-electron equilibration, in D3He thin-glass exploding pushers. This technique is based on measurements of DD and D3He reaction histories, and x-ray emission histories in different energy bands from which time-resolved ion and electron temperatures are inferred. It is also based on measurements of energy loss of the low-velocity ions: DD-3He, DD-triton and D3He-alpha, which is directly related to ion-electron equilibration. From these two complementary measurements, the ion-electron equilibration rate can be determined. Long term, this technique will be extended to more broadly explore transport properties in HEDP at various conditions. The work was supported by DOE, LLNL and LLE. |
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GP11.00114: Diffusive tunneling in an isobaric but non-isothermal fuel-pusher mixture Todd Elder, Xianzhu Tang, Chris McDevitt, Zehua Guo Hydrodynamic mix of fusion fuel and inert pusher can simultaneouly generate smaller fuel pockets and finer pusher layers that separate them. Smaller fuel pockets have greater local Knudsen numbers, which tend to excerberate the Knudsen layer reactivity reduction. A thinner pusher layer separating the neighboring fuel pockets, on the other hand, can enable the diffusive tunneling of Gamow fuel ions through the pusher layer and hence alleviate the Knudsen layer reactivity degradation. Here the diffusive tunneling phenomenon describes a |
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GP11.00115: VISRAD, 3-D Target Design and Radiation Simulation Code James L Sebald, Joseph J MacFarlane, Igor E Golovkin The 3-D view factor code VISRAD is widely used in designing HEDP experiments at major laser and pulsed-power facilities, including NIF, OMEGA, OMEGA-EP, ORION, LMJ, Z, and PLX. It simulates target designs by generating a 3-D grid of surface elements, utilizing a variety of 3-D primitives and surface removal algorithms, and can be used to compute the radiation flux throughout the surface element grid by computing element-to-element view factors and solving power balance equations. Target set-up and beam pointing are facilitated by allowing users to specify positions and angular orientations using a variety of coordinates systems (e.g., that of any laser beam, target component, or diagnostic port). Analytic modeling for laser beam spatial profiles for OMEGA DPPs and NIF CPPs is used to compute laser intensity profiles throughout the grid of surface elements. We will discuss recent improvements to the software package and plans for future developments. |
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GP11.00116: Absorption Spectroscopy of direct-drive implosions with multiple dopants Franck Philippe, Maxime Comet, Catherine Esnault, Stephane Laffite, Jean-Christophe Pain, Charles Reverdin, Roberto Claudio Mancini, Dylan T Cliche The need to accurately predict radiation transport in stellar interiors and ICF targets provides a strong motivation for spectral opacities measurements of hot dense plasmas. In principle, laser-driven implosions could provide a platform to access extreme plasma conditions (ρ>ρ0, Te>400 eV), hard to reach by other methods. In this scheme, an element embedded in the shell is compressed by convergence, and heated by conduction from the central hotspot, which also provides a bright broadband x-ray source for absorption spectroscopy. Additionally, opacity measurements require knowledge of the source spectra, plasma conditions, and areal mass. We present an experimental study of the feasibility of such a measurement in a simple case, associating two tracer elements in the shell of a direct-drive implosion. The first element is used as a thermometer to infer plasma conditions, while the second element is the sample of interest. Self-Emission Shadowgraphy of the implosion enables us to infer areal density. As a first step toward the future study of more astrophysically relevant elements, Titanium and Vanadium were used as tracers in this experiment performed on the OMEGA laser facility |
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GP11.00117: Detailed analyses of x-ray-imaging spectroscopy data from OMEGA direct-drive inertial confinement fusion experiments to infer interspecies ion separation Tirtha R Joshi, Peter Hakel, Scott C Hsu, Nelson M Hoffman We present the analyses of x-ray-imaging spectroscopy data obtained from LANL’s interspecies ion separation campaign ‘IonSepMMI-17A’ on the Omega laser facility. This campaign consisted of series of direct-drive ICF experiments and aimed for more symmetric implosions than our prior campaign.1,2 The targets were Ar-doped, D2-filled spherical plastic shells of varying D2-Ar relative and total gas pressures. We used a time- and space-integrated spectrometer, streaked crystal spectrometer, and three gated multi-monochromatic x-ray imagers (MMI) fielded along quasi-orthogonal lines of sight to record x-ray spectral features obtained from the implosions. The timing of the recording of the experimental data in the implosion was in between first-shock convergence and slightly before neutron-bang time. Detailed analyses of MMI data revealed observations of weak and strong ion species separations depending on the target fill, temporal evolution of ion densities and species separations, and implosion asymmetries in the observed results along different lines of sight. Experimental results are compared against the predictions of hydrodynamic simulations, which incorporate multi-ion species transport model. 1S. C. Hsu et al, EPL 115, 65001 (2016). 2T. Joshi et al., PoP 24, 056305 (2017). |
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GP11.00118: The role of incidence angle in the laser ablation of ICF targets Brett Scheiner, Mark Jude Schmitt The effect of the laser incidence angle on the mass ablation rate and ablation pressure of inertial confinement fusion (ICF) targets is explored using an idealized planar model. NIF, in its current configuration, has its 192 laser beams clustered within 50 degrees of the poles of its target chamber. Therefore, non-normal incidence angles are encountered in all direct drive ICF experiments currently being carried out at the NIF. In this work, a modified version of the textbook model of laser ablation [Manheimer et al. Phys. Fluids 25, 1644 (1982)] is used to illustrate that the mass ablation rate and ablation pressure scale with the 4/3 and 2/3 power of the cosine of laser incidence angle. This implies that a uniform shell mass and velocity cannot be simultaneously obtained when beams intercept the target with a variety of incidence angles. However, with the correct intensity variations, uniform dynamic pressure can be achieved approximately. Additionally, the conduction zone length is found to increase with increasing incidence angle, resulting in decreased laser imprint. Predictions of the scaling of imprint efficiency are found to be in good agreement with prior experimental measurements. |
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GP11.00119: Measurements of shock velocity nonuniformities imprinted by the Nike laser Jaechul Oh, Max Karasik, Victor Serlin, Stephen P. Obenschain With broad bandwidth and induced spatial incoherence (ISI) beam smoothing, the Nike KrF laser delivers most uniform illumination on targets among all existing high-energy lasers for inertial confinement fusion research. This gives particular interest to the laser-imprint levels that Nike actually provides, anticipating higher and more uniform ablation pressures in targets. We are building a high resolution 2D-VISARa on the Nike facility as a sensitive optical diagnostic for the laser imprint studies. Using a short pulse probe laser, the 2D-VISAR takes snapshots of 2-dimensional velocity fields at shock fronts. The velocity perturbations in the snapshot images will be used to evaluate the laser imprinting and its effect on target performance. We plan initial 2D-VISAR measurements on planar CH targets irradiated by various numbers of Nike main beams (1-44) to investigate multibeam irradiation effect on imprinting. This poster will present summary of the configuration of the 2D-VISAR with other target diagnostics including the line-VISAR and the 5th harmonic grid image refractometer (Nike-GIR), and results of the initial measurements of imprint at Nike. a P.M. Celliers, et al, Rev. Sci. Instrum. 81, 035101 (2010). |
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GP11.00120: Analysis of Simulation Predictivity of Hohlraum Conditions and Implosion Performance in the Two-Shock Layered Experiments on the NIF Jay Salmonson, Stephan A MacLaren, Laurent Pierre Masse, Shahab Khan, George A Kyrala, Tammy Yee Wing Ma, Jesse E Pino, Joseph E Ralph, Robert E Tipton We analyze recent layered implosion experiments using the two-shock laser pulse platform[1] wherein the capsule and hohlraum linear dimensions and laser energy have been scaled up from the original experiments to push the implosion into the self-heating regime. Preliminary results indicate that these scaled-up implosions were not as spherically symmetric as the layered implosion at the original two-shock dimensions[2] due to a P2 swing. This series of shots, along with a series of two-shock experiments that deliberately alter P2 symmetry, offers a dataset unprecedented in its simplicity and completeness. Our analysis attempts to draw conclusions about the fidelity and predictivity of our radiation-hydro design codes in experiments at different scales.
[1] S.F. Khan et al. Phys. Plasmas 23, 042708 (2016) [2] S.A. Maclaren et al. Phys. Plasmas 25 056311 (2018) |
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GP11.00121: Self-Similar Solutions With Electro-Thermal Processes for Plasmas of Arbitrary Beta* John L Giuliani, Alexander L. Velikovich Self-similar, magnetohydrodynamic (MHD) solutions are developed for a cold planar wall next to a hot plasma with an embedded magnetic field parallel to the wall, including electro-thermal terms. Velikovich et al. [1] studied the Nernst electro-thermal effect for such a problem under the assumption that the ratio of thermal to magnetic pressure (plasma beta) was large. Other electro-thermal processes, such as the Ettingshausen and plasma Thomson effects, vary inversely with beta, as does Joule heating, and may impact the plasma evolution at low beta. To study all these processes we have extended the self-similar formulation to allow for an arbitrary beta. Self-similar solutions are presented with conditions characteristic of the plasma immediately following laser pre-heat in the MagLIF experiment. Solutions for the density, temperature, and velocity are largely insensitive to the electro-thermal effects. However, the profile of the magnetic field changes significantly with electro-thermal effects. One interesting case is simulated with a 1D MHD code. The solutions are proposed as verification tests for advanced, multi-physics, MHD codes. [1] A.L. Velikovich, J.L. Giuliani, and S.T. Zalesak, Phys. Plasmas, vol. 22, 042702, 2015. |
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GP11.00122: Experimental investigation of the collision of a magnetized plasma jet with a gas target cloud to simulate the compression in magnetized target fusion Byonghoon Seo, Hui Li, Paul M Bellan We report detailed experimental results and interpretation of an experiment investigating the physics underlying compression of a magnetized plasma jet. This investigation exploits a change of reference frame such that an MHD-driven plasma jet and a gas target cloud impacted by the jet correspond respectively to the compressed magnetized plasma and to the liner in an actual magnetized target fusion configuration. Measurements of the jet-cloud impact are made using several diagnostics including an axially translatable interferometer, Thomson scattering, a magnetic probe array, a spectrometer, and a radiated power detector. These measurements indicate that the impact causes both compression and heating of the jet plasma as well as compression of the magnetic field frozen into the plasma. The temperature initially increases in a manner consistent with adiabatic scaling, but then rapidly drops. Analysis reveals that this temperature drop results from UV radiation emitted by neutral hydrogen atoms which are spontaneously formed during the compression by the process of threebody recombination. A criterion for how fast compression must be to outrun radiative loss is discussed for a wide range of plasma parameters including the fusion regime. |
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GP11.00123: Assessment of High-β Magnetized Target Formation and Plasma-Liner Nonuniformities in Plasma-Jet-Driven Magneto-Inertial Fusion using the FLASH Code Tom Byvank, Samuel Langendorf, Scott C Hsu, Petros Tzeferacos, Y C Francis Thio The Plasma Liner Experiment (PLX) is being used to explore a potentially lower-cost fusion-energy concept by which an imploding spherical plasma liner will compress a magnetized plasma target to fusion conditions [1,2], a concept known as plasma-jet-driven magneto-inertial fusion (PJMIF). The liner is produced by merging of supersonic plasma jets, and each jet is generated by a coaxial plasma gun. Using the FLASH code, we study possible formation of a high-β magnetized target (β > 1, ωiτi > 1) by merging pre-magnetized jets [3]. Furthermore, we characterize how liner nonuniformities degrade the implosion during target compression. Adaptive mesh refinement in FLASH permits study of shock structures formed by the merging jets and their consequences in 2D and 3D. We compare FLASH results for target compression with recent relevant 1D simulation results [4,5]. The FLASH results will help guide experiments and continued assessment of the PJMIF concept. [1] S. C. Hsu et al., IEEE Trans. Plasma Sci. 40, 1287 (2012); [2] S. C. Hsu et al., ibid 46, 1951 (2018); [3] S. C. Hsu and S. J. Langendorf, J. Fusion Energy, Accepted (2018); [4] C. E. Knapp and R. C. Kirkpatrick, Phys. Plasmas 21, 070701 (2014); [5] S. J. Langendorf and S. C. Hsu, ibid 24, 032704 (2017). |
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GP11.00124: Six- and Seven-Jet Merging Experiments as an Intermediate Step to Spherical Plasma Liner Formation Kevin C Yates, Samuel Langendorf, Scott Chia Hsu, John P Dunn, Mark Gilmore The Plasma Liner eXperiment (PLX) at Los Alamos National Laboratory aims to demonstrate the formation of spherically imploding plasma liners as a low-cost, high-shot-rate, standoff driver for plasma-jet-driven magneto-inertial fusion (PJMIF). We report results of jet-merging experiments using 2, 3, 5, 6, and 7-gun configurations with Ar, N, Xe, and Kr plasma jets. The jets were diagnosed using high-resolution and survey spectroscopy, 12-chord heterodyne interferometry, photodiodes, and single- and 12-frame intensified CCD imaging. Diagnostics were fielded to determine ion and electron temperature, ionization state, electron density, plasma jet velocity, and jet-to-jet balance. Results show that the jets form shocks that heat the ions up to tens of eV, which then equilibrate with the cooler electrons over several μs. Electron temperatures are measured to range from 1-3 eV for all gas species. Interferometry captures non-uniformities in density due to the shocks between merging jets, which could seed eventual instabilities at the liner-target interface . These results allow for continued assessment of the PJMIF concept. |
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GP11.00125: Improving current coupling to the load in magnetized liner inertial fusion Matthew R. Gomez, Christopher A. Jennings, Brian T. Hutsel, Derek C. Lamppa, Stephen A. Slutz, George R. Laity Magnetized Liner Inertial Fusion (MagLIF) is a magneto-inertial fusion scheme under investigation on the Z facility. In MagLIF, an axial magnetic field is applied to a metal cylinder containing gaseous fusion fuel, a multi-kJ, few-ns-duration laser is used to heat the magnetized fuel, and then the Z-Machine current flows through the metal cylinder, causing it to implode. Simulations of the MagLIF concept predict that 100 kJ DT yields are possible with 25-30 T, 4-6 kJ laser energy deposited, and 22-24 MA. In initial experiments the input parameters were only 10 T, 0.3-0.7 kJ, and 16-18 MA peak load current, resulting in 0.2-0.4 kJ DT-equivalent yield. Focused efforts to improve all three parameters are underway. A significant improvement in peak load current was demonstrated with a new final transmission line and return can design, which reduced the inductance while simultaneously increasing the anode-cathode gap. A peak load current of approximately 20 MA has been measured, and 22+ MA appears feasible with further improvements to the design. |
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GP11.00126: Experimental Aspects of MagLIF Pre-Heat Studies Matthias Geissel, Adam Harvey-Thompson, David Bliss, Jeffrey R Fein, Benjamin R Galloway, Michael E Glinsky, Matt Gomez, Christopher Jennings, Mark W. Kimmel, Kyle J Peterson, Patrick Rambo, Jens Schwarz, Jonathon E. Shores, Stephen A Slutz, Ian C. Smith, Shane Speas, Matthew Weis, Daniel Woodbury, John L Porter Dedicated experiments in the Pecos target area at Sandia National Laboratories investigate and optimize the pre-heat phase of Magnetized Liner Inertial Fusion (MagLIF). The primary observables are energy deposition of the Z-Beamlet laser into deuterium gas, X-ray emission from the gas, and the analysis of backscattered light as indication of laser plasma instabilities (LPI). The latter is observed for stimulated Brillouin scatter and stimulated Raman scatter. Over the course of the last two years, SBS and SRS were significantly reduced while simultaneously doubling the gas density. Increased density and coupling of the laser energy to the gas enables higher neutron yield and is expected to provide more stability for the implosion phase of MagLIF. The presentation will describe the instrumentation suite at Pecos and the various milestones on the way from the initial, high LPI preheat scenario to the latest one, which enabled the highest yields for MagLIF experiments to date. |
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GP11.00127: Estimates of Fusion Gain of Plasma Jet Driven Magneto-Inertial Fusion Peter Stoltz, Scott Hsu, Samuel Langendorf, Tom Byvank We present 2D simulations of imploding plasma liners as a way to estimate fusion gain for these systems. The goal of this work is to guide design of high gain configurations for devices similar to the PLX-Alpha project, currently operating at Los Alamos National Laboratory. The liners are spherically symmetric xenon plasma with implosion velocities of 60-120 km/s formed by a series of high speed jets. At the center of the liner is a DT target that is compressed by the liner and the resulting high density and temperature creates neutron yield. We investigate, in particular, the effect of equation of state, radiation, and liner perturbation on the neutron yield. |
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GP11.00128: Modification of Transport in Magnetized Burning Plasmas due to alpha-electron Collisions Brian Appelbe, Mark Sherlock, Daniel E Ruiz, Jon Tong, Jeremy Chittenden Ignition in MIF requires alpha particles to transfer energy to the deuterium-tritium plasma via Coulomb collisions. For plasma temperatures < ~25 keV, the alphas transfer energy predominantly to the plasma electrons. The electron-electron collision time is much shorter than the alpha-electron collision time since the alpha number density is a small fraction of the electron number density. Therefore, it is usually assumed that the electron distribution function remains Maxwellian when alpha particles are present. In this work it is shown that a net flux of alpha particles can perturb the electron distribution function from Maxwellian. The electron kinetic equation is solved in the presence of arbitrary populations of alpha particles and an external magnetic field to quantify this perturbation. This is used to derive a set of modified transport coefficients for magnetized burning plasmas. It is shown that a flux of alpha particles can increase the heat flow from hot regions of the plasma. Transport of the magnetic field is also affected by this process. |
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GP11.00129: Pulsed Power Switch Development for the HJ1 Coaxial Plasma Accelerator for PJMIF Samuel J Brockington, Y C Francis Thio, Ajoke Williams, Edward J Cruz, Marco Luna, Andrew Case, F. D. Witherspoon HyperJet Fusion Corporation is developing spark gap and field distortion switches for their HJ1 plasma railgun accelerators for PJMIF. Each plasma gun acceleration bank is expected to contain about 7.5kJ of energy. Thus, to ease switch requirements, each acceleration bank has been segmented into 6 modules each requiring a switch. Since investigating PJMIF liner formation requires deploying significant arrays of plasma guns, which necessitates deploying even larger arrays of switches, emphasis has been placed on the reliability and repeatability of designs, as well as reducing jitter in switch closing times. Experimental results will be discussed, along with future plans for switch development. |
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GP11.00130: Initial data from the One-Dimensional Imager of Neutrons on Magnetized Liner Inertial Fusion experiments David Ampleford, Carlos Ruiz, David Neal Fittinghoff, Jeremy Vaughan, Kelly D Hahn, Brandon J Lahmann, Adam Harvey-Thompson, Matt Gomez, Patrick F Knapp, Jose Torres We have recently developed a One-Dimensional Imager of Neutrons (ODIN) for Magnetized Liner Inertial Fusion (MagLIF) experiments on Z. When coupled to diagnostics, including x-ray imaging, we anticipate that locally diagnosing neutron emission will aid in characterizing regions of mix within the target. In this poster we describe the instrument, which consists of a 10-cm thick rolled edge W slit and CR39 detectors. We also show initial data obtained with ODIN and compare to other data from MagLIF experiments. |
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GP11.00131: Simulation Study of Influence of Experimental Variations on Plasma Liners for the PLX-α Project Roman Samulyak, Wen Shih, Scott Chia Hsu Simulation studies of the influence of experimental variations on the merger of supersonic plasma jets and the formation and implosion of plasma liners for the PLX-α project have been performed using the FronTier code. Simulations operate with six supersonic plasma jets with parameters closely matching the PLX-α experiment at Los Alamos. A series of simulations have been performed by randomly varying the initial mass and shot time of each jet as well randomly varying the initial mass and velocity of each jet in such a way that their kinetic energy remained constant. Simulations demonstrate the influence of experimental variations on the structure and strength of oblique shock waves, generated by the collision of supersonic jets, liner non-uniformities, averaged Mach numbers and self-implosion pressures, as well as synthetic quantities, experimentally observable by a multi-chord interferometer and a high-speed camera. The inclusion of experimental variations in simulations improve their agreement with the interferometry reading while shock-wave structures observed in simulations agree very well with images obtained by the high-speed camera. |
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GP11.00132: Modeling of Laser Heated Gases Including MHD Matthew Robert Weis, Matthias Geissel, Michael E Glinsky, Adam Harvey-Thompson, Christopher Jennings, Kyle J Peterson, Joseph M Koning, Michael M Marinak This presentation expands upon a general session talk for the MIF audience. Magnetic field effects in laser heated gases are examined in two and three dimensions utilizing the HYDRA code. HYDRA features a full 3D MHD package including anisotropic thermal conduction, as well as the Nernst/Ettingshausen and Righi-Leduc terms. Results of 2D and 3D calculations are presented at the Z-Beamlet (2-4 kJ) and the NIF (single quad) scale (> 20 kJ). A key signature of magnetization during preheat, is higher electron temperature due to restriction of thermal conduction. With Bz the fuel is magnetized as it is heated, then quickly cools as a blast wave develops, limiting thermal conduction effects. Both 2D and 3D simulations show the potential for whole beam self-focusing and steering from thermal effects, that could lead to laser induced mix. General implications for MagLIF are discussed. |
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GP11.00133: Effects of plasma jet initial conditions and number on peak ram pressure and uniformity for plasma jet driven magneto-inertial fusion experiments on PLX-$\alpha$ Jason Cassibry, Kevin J Schillo, Roman V. Samulyak, Scott Chia Hsu, Y C Francis Thio Numerical simulations of spherically imploding plasma liners formed by merging supersonic plasma jets have been performed using the smooth particle hydrodynamics (SPH) codes in support of the PLX-$\alpha$ project. The physics includes radiation, Braginskii thermal conductivity and ion viscosity, separate ion and electron temperatures, and tabular EOS (LTE and non-LTE). Variation in the initial plasma state, number of jets, and velocity will be explored. Solid-angle-averaged and standard deviation of liner ram pressure and Mach number reveal variations in these properties during formation and implosion. Spherical plasma targets are introduced into the cavity to study the uniformity as the liner interacts with and compresses the target. Spherical-harmonic mode-number analysis of the target surface at various radii and times provide a quantitative means to assess the evolution of liner non-uniformity. A preliminary look at burn physics in three dimensions will be provided based on previous 1D studies. |
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