Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session BM10: Mini-Conference: Collisionless Shocks in Laboratory and Space Plasmas IOn Demand
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Chair: Nikolai Pogorelov, University of Alabama, Huntersville Room: Room 406 |
Monday, November 8, 2021 9:30AM - 9:50AM |
BM10.00001: The microphysics of turbulent collisionless shocks: recent advances from laboratory experiments and kinetic simulations Frederico Fiuza Collisionless shocks are ubiquitous in space and astrophysical plasmas and play an important role in magnetic field amplification and particle acceleration. The plasma microphysics that controls magnetic field amplification, energy partition, and particle acceleration is not yet well understood. I will present results from recent laser-driven high-energy-density experiments at the National Ignition Facility (NIF) where we have observed for the first time high-Mach number Weibel-mediated collisionless shocks and measured the electron temperature downstream of the shock and the associated nonthermal electron acceleration. I will also discuss results from large-scale particle-in-cell simulations of counter-streaming plasma flows for the conditions of the NIF experiments, which help us elucidate the heating and injection mechanisms associated with turbulent collisionless shocks. |
Monday, November 8, 2021 9:50AM - 10:10AM |
BM10.00002: Laboratory Studies of Laser-Driven, Magnetized Collisionless Shocks Derek B Schaeffer, William R Fox, Jackson V Matteucci, Kirill Lezhnin, Amitava Bhattacharjee, Gennady Fiksel, Chikang Li, Kai Germaschewski, Daniel J Haberberger, Russell K Follett, Daniel H Barnak, Suxing X Hu We present results from experiments and simulations on the formation and evolution of quasiperpendicular collisionless shocks created through the interaction of a supersonic laser-driven magnetic piston and magnetized ambient plasma. Time-resolved, two-dimensional imaging of plasma density and magnetic fields shows the formation and evolution of a supercritical shock propagating at magnetosonic Mach number Mms ≈ 12. By directly probing particle velocity distributions, additional measurements reveal the coupling interactions between the piston and ambient plasmas that are key steps in the formation of magnetized collisionless shocks. Particle-in-cell simulations constrained by experimental data further detail the shock formation process, the role of collisionality, and the dynamics of multi-ion-species ambient plasmas. The development of this experimental platform complements, and in some cases overcomes, the limitations of similar measurements undertaken by spacecraft missions, and allows novel investigations of energy partitioning and particle acceleration by high-Mach-number shocks. |
Monday, November 8, 2021 10:10AM - 10:30AM |
BM10.00003: Strongly magnetized parallel collisionless shocks in pair plasmas Antoine Bret, Colby C Haggerty, Damiano Caprioli AB was supported by grants ENE2016-75703-R from the Spanish Ministerio de Economía y Competitividad and SBPLY/17/180501/000264 from the Junta de Comunidades de Castilla-La Mancha. CH was supported by the FDSS NSF AGS-1936393 grant from the Institute for Astronomy of the University of Hawaii. DC was partially supported by NASA (grants 80NSSC18K1218, 80NSSC20K1273, and 80NSSC18K1726) and by NSF (grants AST-1714658, AST-2009326, AST-1909778, PHY-1748958, and PHY2010240). |
Monday, November 8, 2021 10:30AM - 10:50AM |
BM10.00004: Studying Quasi-Parallel Collisionless Shocks in the Laboratory Peter V Heuer, Yu Zhang, Chuang Ren, Jonathan R Davies, Derek B Schaeffer, Martin S Weidl, Christoph Niemann, William R Fox, Damiano Caprioli Quasi-parallel collisionless shocks are common in heliospheric and astrophysical systems. Measurements show that quasi-parallel shocks are capable of accelerating particles (including ions) to high energies through diffusive shock acceleration. This process intrinsically depends on the structure of quasi-parallel shocks, which are more turbulent, unstable, and spatially extended than quasi-perpendicular shocks. We review recent laboratory experiments combining a large, moderate-density ambient plasma and a laser-produced plasma to study some of the beam instabilities involved in quasi-parallel shock formation. These experiments reproduced waves observed by spacecraft upstream of the Earth's quasi-parallel bow shock. However, to date, no experiment has reproduced a full quasi-parallel shock or observed diffusive ion acceleration. We describe scaling relations and simulations that suggest that smaller-scale, higher-density experiments may provide a path to creating quasi-parallel collisionless shocks in the laboratory. |
Monday, November 8, 2021 10:50AM - 11:10AM |
BM10.00005: Multiple species laser-driven ion-shock acceleration Brandon K Russell, Paul T Campbell, Alexander G Thomas, Louise Willingale The particle-in-cell code OSIRIS was used to study the effects of multiple ion species on laser-driven collisionless shock acceleration. Two-dimensional plasma slab simulations composed of protons, carbon, and electrons were used to study the effects of plasma composition, ion change state, and the ratio of downstream to upstream plasma density on the generation of collisionless shocks. Two shocks were formed at large density ratios with the faster primary shock reflecting protons and the slower secondary shock reflecting carbon ions. The velocities of these shocks cannot be accurately predicted by current collisionless shock theory because a kinetic model is required to describe the interaction of the ion populations. Laser-driven shock simulations were performed where the ion composition was varied. However double shocks were only formed when steep density profiles were used. The implications for laser-driven ion shock experiments will be discussed. |
Monday, November 8, 2021 11:10AM - 11:30AM |
BM10.00006: Modeling the effects of α particles on collisionless oblique heliospheric shocks Leon Ofman, Lynn B Wilson, Adam Szabo, Andriy Koval The α particles in the solar wind are the second most abundant ion, and can carry significant energy, momentum and mass flux. We investigate the effects of α particles on the dynamics and the oscillations in high-Mach number (M>3) oblique heliospheric shocks. However, detailed in-situ observations of α particle properties in these shocks are rare, in particular at high cadence on-par with the magnetic field measurements. The downstream magnetic oscillations in oblique collisionless heliospheric shocks were detected by Wind with 10.9 samples/s and recently by DSCOVR spacecraft with high temporal resolution of 50 samples/s. The ions were also detected by Wind, albeit with lower temporal resolution then the magnetic oscillation. It is expected that Parker Solar Probe and Solar Orbiter will observe shocks in the inner heliosphere with detailed proton and α particle data with the expected increase of solar activity. Meanwhile, we report the results of 2.5D and 3D hybrid models of high Mach number shocks, where we investigate several α particle typical relative abundances, Mach numbers, and shock normal directions, and compare the results for the various shock parameters. In particular we model the effects of α particles on the shock ramp, wake, and downstream oscillations and study the kinetic properties of proton and α particle velocity distributions function (VDFs) downstream of the shocks. The modeling results demonstrate that with typical α particle solar wind abundances of 5% the dynamics and the oscillations of high-Mach number shocks is significantly affected, evident from comparison to proton only shock models. We discussed the implication of our modeling results to the interpretation of spacecraft observations. |
Monday, November 8, 2021 11:30AM - 11:50AM |
BM10.00007: Laboratory evidence for proton energization by collisionless shock surfing Julien Fuchs Collisionless shocks are ubiquitously in the Universe and are held responsible for the production of non-thermal particles and high-energy radiation. Without particle collisions, theoretical works show that microscopic instabilities are able to mediate energy dissipation and allow for shock formation. Using our platform where we couple high-powerful lasers (JLF/Titan and LLNL, and LULI2000) with high-strength magnetic fields, we have investigated the generation of magnetized collisionless shock and the associated particle energization. We have diagnosed the plasma density, temperature, as well as the electromagnetic field structures and particle energization in the experiments, under various conditions of ambient plasma and B-field. We have also modelled the formation and interpenetration of the shocks using both macroscopic hydrodynamic simulations and kinetic particle-in-cell simulations. By varying the parameters of the expanding plasma launched in the ambient gas, as well as those of the background magnetic field, we investigate the bridge between the simulated dissipation mechanisms and observed particle energization, as will be reported here. |
Monday, November 8, 2021 11:50AM - 12:10PM |
BM10.00008: Particle Acceleration and Transport at Collisionless Shocks: Effects of Superdiffusion Deduced from Data Analysis and Numerical Simulations Gaetano Zimbardo Collisionless shocks in space and astrophysical plasmas are among the main candidates to explain the acceleration of particles to very high energies. Direct evidence of shock acceleration is given by spacecraft in the heliosphere, but how the properties of accelerated particles, like the energy spectrum, intensity, and maximum energy, depend on the shock parameters (i.e., Mach numbers, compression ratio, plasma beta, and shock normal angle) defies a satisfactory understanding. In recent years, we have extended the standard scenario of diffusive shock acceleration (DSA) to the case of superdiffusive transport, formulating what we call superdiffusive shock acceleration (SSA). The latter allows us to better interpret spacecraft observations, since it predicts an upstream energetic particle profile that decays as a power-law rather than as an exponential. Further, SSA also accounts for (i) a downstream particle flux which is not constant but decays, (ii) energy spectral indices harder than the ones predicted DSA, and (iii) shorter acceleration times. We show how the predictions of SSA are tested with a number of heliospheric shock crossings, and how a numerical implementation of superdiffusive transport can lead to a very good fit of the observed energetic particle time profiles, both upstream and downstream. We also show how SSA may lead to a simple interpretation of the extended precursor of relativistic electrons at supernova remnant shocks, and how it may provide a solution to the apparent discrepancy in shock Mach numbers, as deduced from radio and from X-rays observations, for galaxy cluster merger shocks. These results imply that superdiffusive transport should be consistently taken into account when considering energetic particle transport and acceleration at shocks. |
Monday, November 8, 2021 12:10PM - 12:30PM |
BM10.00009: Shock Propagation and Associated Particle Acceleration in the Presence of Ambient Turbulence Fan Guo We discuss the influence of magnetic turbulence on shock propagation and its consequence on the acceleration and transport of energetic particles at shocks. As the shocks sweep through a turbulent medium, the shock surfaces fluctuate and ripple in a range of different scales. We discuss particle acceleration at rippled shocks in the presence of ambient turbulence. This strongly affects particle acceleration and transport of energetic particles at shock fronts. In particular, we point out that the effects of upstream turbulence are critical for understanding the variability of energetic particles at shocks. Moreover, the presence of pre-existing upstream turbulence significantly enhances the trapping near the shock of low-energy charged particles, including near the thermal energy of the incident plasma, even when the shock propagates normal to the average magnetic field. |
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