Session BO04: Plasma Astrophysics I

Initial Aggregation of Fine-Grained Rim Dust Layers

            Hints of what occurred in the early protoplanetary disk can be obtained by examining meteorites and specifically the mm-sized pebbles found inside called chondrules. The majority of chondrules contain a fine-grained dust rim that is believed to dampen the impact energy between chondrules during collisions allowing them to more easily stick together. A proper understanding of a fine-grained rim’s structure is essential to providing insight into the early environment of a protoplanetary disk [1][2]. It is currently assumed that in such an environment, dust layer growth occurs naturally starting with the initial deposition of material. The mechanism driving these dust deposits is informed by several factors including, surface chemistry, particle size, particle shape and particle charge. However, little research data exists for the initial deposition phase at protoplanetary impact velocities. In this presentation, experimentally driven data will be employed to explain dust behavior during the initial dust deposition and restructuring phase. A technique for studying dust aggregation behavior will also be presented. As a result, this research has application elsewhere including dust mitigation on the lunar and Martian surfaces as well as the soiling of solar panels in both Martian and Earth environments.

 

¹ C. Xiang, A. Carballido, R.D. Hanna, L.S. Matthews, and T.W. Hyde, Icarus 321, 99 (2019).

² Hanna, R.D., Ketcham, R.A., 2018. Earth Planet Sc Lett 481, 201–211.    

Stable magnetically driven plasma jets on the 1-MA COBRA generator

Collimated plasma jets appear to be a ubiquitous feature of the universe, developing over a vast range of scale lengths and source energies. However, common features suggest universal mechanisms may be responsible for jet formation, collimation, and stability. No single model of jet formation is universally accepted to account for the extreme length and stability observed in many jets; however, theory, astrophysical observations, and recent laboratory experiments suggest that some jets may represent magnetically driven configurations that form self-organized equilibria with stabilizing shear flows. To test this hypothesis, a platform has been developed for the 1-MA, 220-ns rise time COBRA generator. In contrast to previous high-energy-density laboratory jet experiments that use radial/conical wire arrays or foils, this experiment uses azimuthally symmetric gas-puff injection. This avoids the ablation phase from a solid target, provides a continuous mass source, and allows for free rotation of the jet foot-points. A permanent magnet provides an initial poloidal magnetic field which links the two concentric electrodes and mimics the boundary conditions of a star-accretion disk system. Here we present the design of the experiment and measurements of the resulting stable, high-aspect ratio jets taken using optical Thomson scattering, laser interferometry, Faraday polarimetry, and B-dot probes. Results are compared to 3D simulations using the PERSEUS extended magnetohydrodynamics (XMHD) code.   

Laboratory Evidence of Fluctuation Dynamo in Supersonic Turbulence

Highly compressible magnetized turbulence is prevalent in the interstellar and intergalactic mediums. Stochastic fluctuations in turbulent, supersonic plasmas have a marked effect on the magnitude of the magnetic fields that permeate them, namely, dynamo action. Tapping into the plasma kinetic energy reservoir, turbulent motions cause a sequence of transformations in the magnetic fields that results in the amplification of the magnetic energy density. While fluctuation dynamo is commonplace in astrophysical systems, it is hard to realize in terrestrial laboratories. Here we demonstrate, using laser-driven experiments at the Omega Laser Facility, that supersonic turbulence is indeed capable of fluctuation dynamo action. The experiments exploit the TDYNO experimental platform, which demonstrated turbulent dynamo in the laboratory for the first time [Tzeferacos et al. Nat. Comm. 9, 591, 2018], meticulously characterized it [Bott et al. PNAS 118, e2015729118, 2021] in the subsonic regime, and was extended to study supersonic turbulence [Bott et al. Phys. Rev. Lett. 127, 175002, 2021]. We detail the experiments that led to this demonstration, as well as the FLASH simulation campaigns that we executed for the design and interpretation of the experiments.   

Laboratory Study of Magnetorotational Instability in a Swirling Partially Ionized Plasma

The magnetorotational instability (MRI) has been proposed as a plasma instability to transport angular momentum to enable fast accretion in astrophysical disks, and its standard form (SMRI) has recently been detected in a laboratory setting [1]. However, for weakly-ionized protoplanetary disks, it remains unclear whether the combined non-ideal magnetohydrodynamic (MHD) effects of Ohmic resistivity, ambipolar diffusion, and the Hall effect make these disks MRI-stable. While much effort has been made to simulate non-ideal MHD MRI, these simulations make simplifying assumptions and are not always in agreement with each other. Here, we present our proposed concept of a swirling weakly-ionized argon-plasma experiment between two concentric cylinders threaded with an axial magnetic field [2]. We derive the equilibrium flow and a dispersion relation for MRI that includes the three non-ideal effects. We solve this dispersion relation numerically for the parameters of our proposed experiment. We find that it should be possible to produce MRI in such an experiment because of the Hall effect, which increases the MRI growth rate when the vertical magnetic field is antiparallel to the rotation axis. As a proof of concept, we also present experimental results for gas flow in an unmagnetized prototype. We find that our prototype has a small, but non-negligible, alpha-parameter that could serve as a baseline for comparison to our proposed magnetized experiment, which could be subject to additional effects from the MRI.   

Study of astrophysical collisionless shocks in the laboratory

High Mach number astrophysical plasmas can create collisionless shocks via plasma instabilities and turbulence that are responsible for magnetic field generations and cosmic ray acceleration.  With the advent of high-power lasers, laboratory experiments with high-Mach number, collisionless plasma flows can provide critical information to help understand the mechanisms of shock formation by plasma instabilities and self-generated magnetic fields. A series of experiments were conducted on Omega and the National Ignition Facility to observe: the filamentary Weibel instability that seeds microscale magnetic fields [1, 2]; collisionless shock formation (seen by an abrupt ~4x increase in density and ~6x increase in temperature); and electron acceleration distributions that deviated from the thermal distributions [3]. In addition to the case of collisionless shock formation under unmagnetized initial condition, shock formation under magnetized environment is also being studied. Experimental results along with theoretical interpretations aided by particle-in-cell simulations will be discussed.

 

[1] H.-S. Park et al., HEDP 8, 38 (2012)

[2] C. Huntington et al., Nature Physics 11, 173 (2015)

[3] F. Fiuza et al., Nature Physics, 16, 916 (2020)   

FLASH Simulations for the Redesign of the OMEGA TDYNO Experimental Platform

Magnetized turbulence is a key process in astrophysical environments, and small-scale turbulent dynamo action is currently our best working hypothesis to account for the cosmic magnetic fields measured in galaxy clusters. Guided by high-fidelity FLASH simulations, the TDYNO (turbulent dynamo) collaboration has conceived, designed, and successfully executed experimental campaigns at the Omega Laser Facility, which demonstrated turbulent dynamo in a terrestrial laboratory for the first time [Tzeferacos et al. Nat. Comm. 9, 591, 2018]. Inspired by our recent experiments at the GSI Helmholtz Centre for Heavy Ion Research [Campbell et al. Bull. Am. Phys. Soc. 2022; Moczulski et al. Bull. Am. Phys. Soc. 2022], we used two- and three-dimensional simulation campaigns with the FLASH code to craft a novel variant of the TDYNO platform which is (1) simpler to assemble, (2) easier to align, and (3) cheaper to manufacture, whilst maintaining the desired plasma conditions achieved by the original platform. We discuss the platform characteristics in detail and recount the design choices that led us to the final configuration, which will be deployed at the Omega Laser Facility through the Laboratory Basic Science Program.    

Diffusion of Cosmic Rays in MHD Turbulence with Magnetic Mirrors

As the fundamental physical process with many astrophysical implications, the diffusion of cosmic rays (CRs) is determined by their interaction with magnetohydrodynamic (MHD) turbulence. We consider the magnetic mirroring effect arising from MHD turbulence on the diffusion of CRs. Due to the intrinsic superdiffusion of turbulent magnetic fields, CRs with large pitch angles that undergo mirror reflection, i.e., mirroring CRs, are not trapped between magnetic mirrors, but move diffusively along the turbulent magnetic field, leading to a new type of parallel diffusion, i.e., mirror diffusion. This mirror diffusion is in general much slower than the diffusion of CRs with small pitch angles that undergo gyroresonant scattering. We find that the mirror diffusion of CRs is important for confining CRs in star-forming regions and their strong coupling with the interstellar media in active galaxies.   

Ion-electron energy partition in low Mach number magnetized collisionless shocks

Energy partition between ions and electrons in magnetized, collisionless shocks is an unresolved issue. 2-D kinetic simulations show that ions and electrons equilibrate on a time scale of a few ion gyro-times in low Mach number, magnetized, quasi-parallel, collisionless shocks. A multi-fluid model shows a resonance between electron whistler waves and ion magnetohydrodynamic waves that may be responsible for the energy transfer from drifting ions to thermal electrons.   

Turbulent Saturation of the Acoustic Resonant Drag Instability

The acoustic resonant drag instability (RDI) is a recently proposed instability of relevance to a range of dusty astrophysical environments. The instability results from a resonant interaction between dust streaming through gas and acoustic waves in the gas, producing amplification of sound waves and large fluctuations of dust density.[1] The nonlinear evolution of this instability has been shown in simulations to produce clumping of the dust into filaments, the generation of turbulence, and, under some circumstances, the formation of dust jets.[2] Understanding these behaviors is of relevance to modelling dusty winds such as those around active galactic nuclei, in planetary nebulae, and in molecular clouds.[3] We present simulations of the acoustic RDI under varied Mach number, dust/gas mass ratio, and wavelength. These are compared against both novel and previously proposed analytic estimates of saturation amplitude across different length scales, and the phenomenology of the turbulence produced under different regimes is considered.

[1] P. F. Hopkins and J. Squire, The Resonant Drag Instability (RDI): Acoustic Modes, MNRAS 480, 2813 (2018)

[2] E. R. Moseley, J. Squire, and P. F. Hopkins, Non-Linear Evolution of Instabilities between Dust and Sound Waves, MNRAS 489, 325 (2019)

[3] B. Y. Israeli, A. Bhattacharjee, and H. Qin, Resonant instabilities mediated by drag and electrostatic interactions in laboratory and astrophysical dusty plasmas, arXiv:2303.13640 (2023) (to appear in Phys. Plasmas)   

Whistler Lion Roars within Mirror Modes in Galaxy Clusters

Lion roars are bursts of whistler waves associated with low magnetic field regions of mirror modes. They are observed in plasmas near Earth, Saturn and the solar wind. In the intracluster medium (ICM) of galaxy clusters, mirror instability is also expected to be excited, but it is not yet clear whether whistler lion roars can also be present in this high-β environment. In this work, we perform fully kinetic particle-in-cell simulations of a plasma subject to a continuous amplification of the mean magnetic field to study the nonlinear stages of the mirror instability and the ensuing excitation of whistler lion roars under ICM conditions. Once mirror modes reach nonlinear amplitudes, a simultaneous excitation of whistler lion roars and ion-cyclotron waves (IC) is observed, with sub-dominant amplitudes and quasi-parallel propagation. We show that the underlying mechanism of excitation is the pressure anisotropy of electrons and ions trapped in mirror modes with loss-cone type distributions. We observe that IC waves play an essential role in regulating the global ion pressure anisotropy at nonlinear stages. We argue that lion roars are a concomitant feature of mirror instability even at high β, therefore expected to be present in the ICM. We discuss the implications of this work on the ICM turbulent heating via magnetic pumping.   

Magnetogenesis in a collisionless plasma: from Weibel instability to turbulent dynamo

Astronomical observations suggest pervasive micro-gauss magnetic fields in our Galaxy and in the intracluster medium (ICM) of galaxy clusters. It is widely believed that such dynamically important magnetic fields are produced by plasma dynamos acting upon some ``seed'' magnetic fields. However, a complete understanding of this process in a weakly collisional plasma is still lacking. We report a first-principles numerical and theoretical study of plasma dynamo in a fully kinetic framework. By applying an external mechanical force to an initially unmagnetized plasma, we develop a self-consistent treatment of the generation of ``seed'' magnetic fields, the formation of turbulence, and the inductive amplification of fields by fluctuation turbulent dynamo. The driven large-scale motions in an unmagnetized, weakly collisional plasma are subject to strong phase mixing, which in turn leads to the development of thermal pressure anisotropy. The Weibel instability is then triggered and produces filamentary, micro-scale ``seed'' magnetic fields. The plasma is thereby magnetized, enabling the stretching and folding of the fields by the plasma motions and the development of pressure-anisotropy instabilities. The scattering of particles off these microscale magnetic fluctuations provides an effective viscosity, impacting the field morphology and turbulence. During this process, the seed fields are further amplified by the fluctuation dynamo until they attain equipartition with the turbulent flow. This work has important implications for magnetogenesis in dilute astrophysical systems by demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through large-scale turbulent flows.   

Exact von-Kármán-Howarth relations for the Hosking integral in decaying, non-helical magnetically-dominated turbulence

The Hosking integral (Hosking & Schekochihin 2021, PRX 11, 041005) has recently been recognised as a key invariant that constrains the decay of magnetic fields that are statistically homogeneous, isotropic and non-helical, such as would have existed in the early Universe under certain primordial magnetogenesis scenarios. In this talk, we present new von-Karman-Howarth-Monin relations and corresponding exact scaling relations for the two-point magnetic-helicity-density correlation function in both incompressible and compressible magnetohydrodynamic (MHD) turbulence. We demonstrate with high-resolution numerical simulations of such turbulence that the condition of rapid decorrelation of the velocity and magnetic fields that — according to our new relations — is required for the conservation of the Hosking integral is, indeed, satisfied. Thus, we provide new evidence in support of the importance of the Hosking integral in constraining turbulent MHD decay.   

Formation of Magnetic Fields on Grand Scale Distances Involving Anisotropic Electron Distributions*

The emergence of significant magnetic fields in cosmic plasmas over large scale distances is an important issue to deal with and is still unresolved. One of the difficulties is that known and potentially applicable theories, such as those based on the Weibel instability, suffer from involving unrealistically small distances (in particular, $c/omega_{pe}$). The presently proposed theory starts from considering the electron density and temperature fluctuations [1] which can be excited in circumbinary disks sustained by pairs of black holes. These low frequency fluctuations can drive a “magneto-thermal alternator” of the kind introduced in Ref. [2] which can produce a slowly varying and sheared magnetic field structure. The shearing component of this field can then be amplified by a magneto-thermal reconnection process [2,3] up to substantial amplitudes following the emergence of electron populations with non-thermal distributions in momentum space and significant (spatial) gradients. The simplest case treated is that of an electron population with anisotropic temperatures. An important feature of magneto-thermal reconnection is that the width of the layer where reconnection takes place remains significant relative to the involved macroscopic distances [2] unlike the case of the weakly collisional tearing mode identified in Ref. [4]. *Sponsored in part by the Kavli Foundation (MIT) and by CNR of Italy.

1. B. Coppi, Fundamental Pl. Phys., 100007 (2023).

2. B. Coppi, and B. Basu, Phys. Lett. A, 397, 127265 (2021).

3. B.Coppi,J.Mark,G.Bertin and L.Sugiyama,Ann.Phys.119,370(1979)

4. B. Coppi, Phys. Fluids (1964).   

A unified treatment of mean-field dynamo and angular-momentum transport in magnetorotational instability-driven turbulence

Magnetorotational instability (MRI)-driven turbulence and dynamo phenomena are analyzed using direct statistical simulations. Our approach begins by developing a unified mean-field model that combines the traditionally decoupled problems of the large-scale dynamo and angular-momentum transport in accretion disks. The model consists of a hierarchical set of equations, capturing up to the second-order correlators, while employing a statistical closure approximation for three-point correlators. We highlight the web of interactions that connect different components of stress tensors---Maxwell, Reynolds, and Faraday---through shear, rotation, mean fields, and nonlinear terms. We determine the dominant interactions crucial for the development and sustenance of MRI turbulence. Our unified mean-field model allows for a self-consistent construction of the electromotive force, accounting for inhomogeneities and anisotropies. Regarding the large-scale magnetic field dynamo, we identify two key mechanisms: the rotation-shear-current effect and the rotation-shear-vorticity effect, responsible for generating the radial and vertical magnetic fields, respectively. We provide explicit expressions for the transport coefficients associated with each of these dynamo effects. Notably, both mechanisms rely on the intrinsic presence of a large-scale vorticity dynamo within MRI turbulence.