Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session CI2: Invited Basic II: Magnetic Reconnection and Shocks |
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Chair: Bill Daughton Room: Floridian Ballroom AB |
Monday, October 21, 2019 2:00PM - 2:30PM |
CI2.00001: Roles of plasmoid instability in magnetic reconnection and magnetohydrodynamic turbulence Invited Speaker: Yi-Min Huang The ubiquitous thin current sheets in high-Lundquist-number space and astrophysical plasmas are known to be unstable to the plasmoid instability, which disrupts current sheets to form smaller structures such as flux ropes and secondary current sheets. The plasmoid instability thus plays a significant role in magnetic reconnection and magnetohydrodynamic (MHD) turbulence. In the presence of a large-scale magnetic field, the three-dimensional plasmoid instability can lead to self-generated turbulence within the reconnection layer without the need of external forcing. We show that this turbulent state in a highly inhomogeneous reconnection layer does not conform to the classic Goldreich-Sridhar (GS) theory.[1] Most notably, (1) the scale-dependent anisotropy predicted by GS is not observed, and (2) the turbulence energy spectrum is steeper with a power-law index approximately -2.2 rather than -5/3. Similar steepening of the energy spectrum is also observed in 2D homogeneous MHD turbulence at small scales.[2] We clarify the roles of seed noise on the condition of current sheet disruption.[3] This key ingredient enables us to develop a self-consistent theory of plasmoid-mediated MHD turbulence, where the turbulence fluctuation at smaller scales provides the seed noise to disrupt current sheets at larger scales.[4] In addition to these theoretical developments, we will present evidence of the plasmoid instability in solar observations, as well as new results from models beyond resistive MHD, including the Hall effect. [1] Huang, Y.-M. and Bhattacharjee, A., Astrophys. J. 818, 20 (2016) [2] Dong, C., Wang, L., Huang, Y.-M., Comisso, L., and Bhattacharjee A., Phys. Rev. Lett. 121, 165101 (2018) [3] Huang, Y.-M., Comisso, L., and Bhattacharjee, A., Astrophys. J. 849, 75 (2017) [4] Comisso, L., Huang, Y.-M., Lingam, M., Hirvijoki, E., and Bhattacharjee, A., Astrophys. J. 854, 103 (2018). [Preview Abstract] |
Monday, October 21, 2019 2:30PM - 3:00PM |
CI2.00002: The Collisonal-Collisionless Phase Transition in Partially Ionized Magnetic Reconnection Invited Speaker: Jonathan Jara-Almonte Magnetic reconnection is the fundamental process responsible for topological rearrangement of fieldlines and an important process in nearly all magnetized plasmas. While commonly studied in fully ionized plasmas, neutrals are important in many astrophysical environments such as the solar chromosphere or interstellar medium, but comparatively little attention has been given towards understanding reconnection physics in partially ionized systems. In particular, the transition from slow, collisional reconnection to fast, collisionless reconnection is not well understood. Analytic models have predicted that, when neutrals are present, this transition occurs when the current sheet thickness reaches the total inertial length (defined with the total mass density), but multi-fluid simulations have not seen this effect. In this work, fully kinetic, particle-in-cell simulations of weakly-collisional, partially ionized magnetic reconnection are performed. The transition from collisional to collisionless reconnection is found to occur when the current sheet thins below the ion-inertial length (defined by only the ion mass density). Within the collisionless regime, and in sufficiently large systems, the peak reconnection rate scales with the total Alfvén speed in agreement with experimental results. Neutral viscosity is important and plays a significant role in momentum transport through the ion diffusion region. To clarify the role of kinetic and collisionless effects, well-matched fluid simulations are compared with the particle-in-cell simulations. Finally, the fully ionized case is studied within the context of phase-transition theory. By identifying good order parameters, the transition is shown to consist of both a second-order phase transition at the ion-scale and an additional, first-order transition at the electron scale. [Preview Abstract] |
Monday, October 21, 2019 3:00PM - 3:30PM |
CI2.00003: First Principles Simulations of Microscale Turbulence in the Solar Wind Invited Speaker: Daniel Groselj Recent advances in observational and computational techniques have enabled significant progress in the understanding of kinetic-scale turbulent cascades in magnetized plasmas. These findings have important implications for models of turbulent heating and particle energization in weakly collisional space and astrophysical plasmas, including the observationally accessible solar wind. Here, a set of massively parallel particle simulations are performed to investigate the nature of turbulent fluctuations below the ion spectral break in the solar wind. Using a fully kinetic description in three spatial dimensions, the field polarization properties and a turbulence anisotropy analysis are employed to identify the self-consistently cascaded fluctuations as kinetic Alfven waves. Special attention is paid to the role of coherent structure formation, which gives rise to intermittent statistics of the magnetic field and of the electron density. A longstanding debate on the relative importance of waves and coherent structures is addressed. Based on a set of novel diagnostic measures, combined with in situ spacecraft observations, it is argued that the kinetic-scale structures themselves carry wavelike features. [Preview Abstract] |
Monday, October 21, 2019 3:30PM - 4:00PM |
CI2.00004: Ion Acceleration and Substructure of Hall Shocks in the Big Red Ball Invited Speaker: Douglass Endrizzi The non-stationarity of quasi-perpendicular collisionless shocks makes satellite observations of the earth's bowshock challenging. Repeatable laboratory experiments can test our understanding of the shock formation/reformation process in a simpler setting. A Hall-regime, high-beta ($\beta \geq 1 $) theta-pinch collisionless shock experiment is performed on the Big Red Ball. A supersonic magnetic piston moves radially inwards at $30 - 100$ km/s, compressing the target plasma column ($T_e \simeq 2$ eV, $n_e\simeq 10^{18}$ m$^{-3}$, $B\simeq 0.5$ mT). The sound and Aflv\'en speeds ($c_s\simeq 15$ and $v_A\simeq 10$ km/s) are controlled by adjusting the plasma density and magnetic field. Together, the control over the drive and characteristic speeds enables a wide range of magnetosonic Mach numbers ($1 < M_{MS} < 10 $). Several predictions of a Hall model (specular reflection of ions, parallel energization of electrons, out-of-plane Hall magnetic fields) are observed in the experiment. Cylindrical VPIC simulations are used to optimize the experiment, and independently reproduce the above experimental observations. Results from both the experiment and simulations, as well as comparison to other Hall-regime shock experiments, will be presented. [Preview Abstract] |
Monday, October 21, 2019 4:00PM - 4:30PM |
CI2.00005: Experimental Observation of Shock-front Separation in Multi-ion-species Collisional Plasma Shocks Invited Speaker: Tom Byvank In the context of inertial-confinement-fusion and high-energy-density experiments, shock-driven ion species separation in the fuel potentially leads to neutron yield degradation [1], and this has in part motivated the recent strong interest in studying multi-species plasma transport and shocks. In this work, we directly observe shock-front separation and species-dependent shock widths in collisional plasma shocks. The shock-front is produced by obliquely merging plasma jets with initial 97\%-He and 3\%-Ar atomic concentrations on the Plasma Liner Experiment [2]. Cameras with narrow bandpass filters directly observe line emission from the distinct ion species. We experimentally infer both shock-front separation and individual shock widths (for the He and Ar) to be of order several tens of post-shock thermal ion-ion mean free paths. These observations agree reasonably with results from 1D multi-fluid simulations using the Chicago code [3]. Moreover, the experimental and simulation results are consistent with first-principles theoretical predictions [4] that the lighter He ions diffuse farther ahead (toward the pre-shock region) within the overall shock-front than the heavier Ar ions. Our fundamental experimental data can be used to benchmark first-principles-based multi-fluid or kinetic simulations of multi-ion-species collisional plasma shocks, for which there have been recent known discrepancies between models [5].\\ Work performed in collaboration with Sam Langendorf, Scott Hsu, and Carsten Thoma, and supported by FES and ARPA-E of the DOE. LA-UR-19-26018. [1] P. Amendt et al., PRL 105, 115005 (2010) [2] S. C. Hsu et al., IEEE Trans. Plasma Sci. 46, 1951 (2018) [3] C. Thoma et al., Phys. Plasmas 18, 103507 (2011) [4] G. Kagan and X.-Z. Tang, Phys. Plasmas 19, 082709 (2012) [5] B. D. Keenan et al., Phys. Rev. E 96, 053203 (2017) [Preview Abstract] |
Monday, October 21, 2019 4:30PM - 5:00PM |
CI2.00006: \textbf{Collisionless shocks driven by supersonic plasma flows with self-generated magnetic fields} Invited Speaker: Chikang Li Collisionless shocks are ubiquitous in the universe, a consequence of supersonic plasma flows sweeping through interstellar and intergalactic media. It has been speculated that these shocks are the cause of many astrophysical phenomena, but details of shock structure and behavior have remained controversial because of a lack of ways to study them experimentally. Generation of astrophysically relevant, collisionless shocks in the laboratory has therefore been an important experimental goal during the last several decades for scientists wanting to explain numerous fascinating astrophysical phenomena and developing an incisive platform for studying a broad range of fundamental physics. Laboratory experiments reported here, with astrophysically relevant plasma parameters, demonstrate the formation of a quasi-perpendicular magnetized collisionless shock. In the upstream it is fringed by a filamentated turbulent region, a rudiment for a secondary Weibel-driven shock. This turbulent structure is found responsible for electron acceleration to energies exceeding the average energy by two orders of magnitude. These experiments mimic the scenario of collisionless shocks in nonrelativistic astrophysical contexts, and provide insight into shock physics in relativistic contexts. This work was performed in collaboration with V. T. Tikhonchuk, Q. Moreno, H. Sio, E. D'Humi\`{e}res, X. Ribeyre, Ph. Korneev, S. Atzeni, R. Betti, A. Birkel, E.M. Campbell, R. K. Follett, J. A. Frenje, S. X. Hu, M. Koenig, Y. Sakawa, T. C. Sangster, F. H. Seguin, H. Takabe, S. Zhang, and R. D.Petrasso. [Phys. Rev. Lett. (2019)]. [Preview Abstract] |
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