Session GO6: Astrophysical Plasmas and Laboratory Astrophysics

Exploring the universe through Discovery Science on NIF*

New regimes of science are being experimentally studied at high energy density facilities around the world, spanning drive energies from microjoules to megajoules, and time scales from femtoseconds to microseconds. The ability to shock and ramp compress samples to very high pressures and densities allows new states of matter relevant to planetary and stellar interiors to be studied. Shock driven hydrodynamic instabilities evolving into turbulent flows relevant to the dynamics of exploding stars (such as supernovae), accreting compact objects (such as white dwarfs, neutron stars, and black holes), and planetary formation dynamics (relevant to the exoplanets) are being probed. The dynamics of magnetized plasmas relevant to astrophysics, both in collisional and collisionless systems, are starting to be studied. High temperature, high velocity interacting flows are being probed for evidence of astrophysical collisionless shock formation, the turbulent magnetic dynamo effect, magnetic reconnection, and particle acceleration. And new results from thermonuclear reactions in hot dense plasmas relevant to stellar and big bang nucleosynthesis are starting to emerge. A selection of examples of frontier research through NIF Discovery Science in the coming decade will be presented.   

Formation of High Mach-Number Magnetized Collisionless Shocks in Laser-Produced Plasmas

Magnetized collisionless shocks commonly occur in the heliosphere and interstellar medium. Collective collisionless processes mediating such shocks can now be studied in the laboratory. We carry out an experiment to observe the formation of a high Alfven Mach number (Ma) magnetized collisionless shock on OMEGA-EP facility. In the experiment, a laser-produced plasma flow penetrates into a pre-existing magnetized plasma. Proton radiography shows a moving region of proton deficit followed by a caustic enhancement of proton density. These features are produced by a propagating front of compressed magnetic field. We use a particle tracing code to model the proton radiography and determine the speed of the compressed field from a series of proton radiographs. Modeling of the shape of the proton deficit region allows us to constrain the amount of magnetic compression. When compared to particle-in-cell simulations of magnetized shocks, we find that the amount of observed magnetic compression is well explained by a magnetized perpendicular collisionless shock propagating with Ma=4. These experiments create a platform for further study of physical processes in the transition region of collisionless magnetized shocks.   

Producing Beam Instabilities Relevant to Parallel Shock Formation in a Magnetized Laboratory Plasma

Simulations have identified several electromagnetic beam instabilities that play an essential role in the formation of Alfv\' enic parallel shocks, but have never been studied in that context in the laboratory. We present measurements of one such instability (the Right-Hand Resonant Instability, or RHI) from a series of recent experiments at the University of California, Los Angeles. Instabilities are observed between a field-parallel super-Alfv\' enic ($M_A = 5$) \lq beam\rq ~of laser-produced plasma expanding over 80 $\delta_i$, and the large, magnetized ambient plasma of the Large Plasma Device (LAPD), and are diagnosed with an array of 3-axis magnetic flux \lq bdot\rq probes and Langmuir probes. Measurements are compared to hybrid simulations of both the experiment and of fully formed parallel shocks.   

Investigation of Weibel-filament growth in the nonlinear regime using laser-irradiated foils of different materials

M.J.-E. MANUEL \textit{GENERAL ATOMICS}, C.M. HUNTINGTON, D.P. HIGGINSON, B.B. POLLOCK, B.A. REMINGTON, H. RINDERKNECHT, J.S. ROSS, D. RYUTOV, G. SWADLING, S. WILKS, A.B. ZYLSTRA, H.-S. PARK \textit{LLNL}, F. FIUZA, S. TOTORICA\textit{ SLAC}, G. GREGORI\textit{ OXFORD}, J. PARK, A. SPITKOVSKY\textit{ PRINCETON}, Y. SAKAWA, H. TAKABE\textit{ OSAKA}, H. SIO\textit{ MIT}, A.B. ZYLSTRA\textit{ LANL}. The Weibel instability is presently the leading mechanism proposed to amplify magnetic fields necessary to form `collisionless' shocks in weakly magnetized astrophysical systems, including young supernova remnants and gamma-ray bursts. These systems rely on the presence of strong self-generated magnetic fields to mediate shock formation since the typical collisional mean-free-path is much larger than the system size. The work presented here investigates the development of the Weibel instability in the nonlinear regime through experimental variation of plasma parameters using different ion species and separation distances. Our goal is to investigate the underlying physical mechanism that may allow the formation of collisionless shocks in astrophysical objects. Recent experimental and computational results will be presented and discussed.~   

Kinetic inhibition of MHDshocks in the vicinity of a parallel magnetic field

According to MHD, the encounter of two collisional magnetized plasmas at high velocity gives rise to shock waves. Investigations conducted so far have found that the same conclusion still holds in the case of collisionless plasmas. For the case of a flow-aligned field, MHD stipulates that the field and the fluid are disconnected, so that the shock produced is independent of the field. We present a violation of this MHD prediction when considering the encounter of two cold pair plasmas along a flow-aligned magnetic field. As the guiding magnetic field grows, isotropization is progressively suppressed, resulting in a strong influence of the field on the resulting structure. A micro-physics analysis allows us to understand the mechanisms at work. Particle-in-cell simulations also support our conclusions and show that the results are not restricted to a strictly parallel field. [Bret at al., Journal of Plasma Phys. (2017), vol. 83, 715830201]   

The structure of low Mach number, low beta, quasi-perpendicular collisionless shocks

A study of the structure of 145 low Mach number ($M \leq 3$), low beta ($\beta \leq 1$), quasi-perpendicular interplanetary collisionless shock waves observed by the \emph{Wind} spacecraft has provided strong evidence that these shocks have large amplitude whistler precursors. The common occurrence and large amplitudes of the precursors raise doubts about the standard assumption that such shocks can be classified as laminar structures. This directly contradicts standard models. In 113 of the 145 shocks ($\sim$78\%), we observe clear evidence of magnetosonic-whistler precursor fluctuations with frequencies $\sim$0.1–7 Hz. The presence or absence of precursors showed no dependence on any shock parameter. The majority ($\sim$66\%) of the precursors propagate at $\leq$45$^{\circ}$ with respect to the upstream average magnetic field, most (~87\%) propagate $\geq$30$^{\circ}$ from the shock normal vector, and most ($\sim$79\%) propagate at least 20$^{\circ}$ from the coplanarity plane. The peak-to-peak wave amplitudes are large with a range of maximum values of $\sim$0.2–13 nT with an average of $\sim$3 nT. When we normalize the wave amplitudes to the upstream averaged magnetic field and the shock ramp amplitude, we find average values of $\sim$50\% and $\sim$80\%, respectively.   

Phase-mixing of electrostatic modes in pair-ion plasma

In a homogeneous plasma, an excited electrostatic mode can face breaking even if the perturbation amplitude is kept well below the threshold value through the process of phase-mixing . In various physical situations (for example inhomogeneous ion density background, inhomogeneous magnetic field, relativistic effects etc), associated characteristic mode frequency of the oscillation becomes space dependent and the mode is called phase-mixed. As the phase-mixed mode evolves through the space and time, particles located at different positions oscillate with different local frequencies. In next scenario, the phase difference between two adjacent oscillators begins to increase secularly with time and, finally, trajectories of the adjacent oscillators start to intersect their paths. It costs a gradual loss in the phase coherence of the constituting oscillators and eventually the relevant mode breaks. The appearance of spiky density profile signifies the occurrence of phase mixing. In our works, phase-mixing process has been studied in pair ion plasmas (fullerene-ion plasma, electron-positron plasma) and the effects of different parameters (like temperature, external magnetic field, ion concentration) on the phase-mixing process have been explored analytically.   

On pair plasma instability due to pressure gradients in homogeneous magnetic fields

With the advent of laboratory experiments on collective effects in electron-positron plasmas, theoretical prediction of their stability properties becomes increasingly relevant. Prior work without compressional magnetic fluctuations [Helander \& Connor, JPP 82, 905820301 (2016)] predicted complete stability of pair plasmas to density or temperature gradients in a homogeneous magnetic guide field. Here, it is shown that the inclusion of such fluctuations produces a Gradient-driven Drift Coupling (GDC) instability [Pueschel et al., PoP 22, 062105 (2015)] also seen in helium plasma experiments [Pueschel et al., PPCF 59, 024006 (2017)]. An analytical growth rate expression applicable to a wide range of plasma parameters is derived, and a subdominant, finite-$k_z$ GDC is discussed. Overall, the GDC is shown to have a potential impact on systems ranging from Gamma Ray Bursts to magnetic confinement experiments like APEX, to laser-based setups. In all of these configurations, GDC growth times are much shorter than plasma lifetimes.   

Holistic Framework for Understanding the Evolution of Stellar Coronal Plasmas

Understanding how how the coronal X-ray activity of stars depends on magnetic field strength, dynamos, rotation, mass loss and age is of interest not only for the basic plasma physics of stars, but also for stellar age determination and implications for habitability. Approximate relations between field strength, activity, spin down, mass loss and age have been measured, but remain to be understood theoretically. The saturation of plasma activity of the fastest rotators and the decoupling of spin-down from magnetic field strengths for slow rotators are particular puzzles. To explain the observed trends, I discuss our minimalist holistic theoretical framework that combines a Parker wind with (i) magnetic dynamo sourcing of thermal energy, wind energy and x-ray luminosity (ii) dynamo saturation based on magnetic helicity conservation and shear-induced eddy shredding and (iii) coronal equilibrium to determine how the magnetic energy divides into wind, x-ray, and thermal conduction sinks. We find conduction to be important for older stars where it can reduce the efficacy of wind angular momentum loss, offering an alternative explanation of this trend to those which require dynamo transitions. Overall, the framework shows promise and provides opportunity for further   

Plasma Constraints on the Cosmological Abundance of Magnetic Monopoles and the Origin of Cosmic Magnetic Fields

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 place new tight constraints on the cosmological abundance of monopoles. In particular, the existence of micro-Gauss magnetic fields in galaxy clusters and radio relics implies that the scales of these structures are below the Debye screening length, thus setting an upper limit on the cosmological density parameter of monopoles, $\Omega_M\le 3\times 10^{-4}$, which precludes them from being the dark matter. Future detection of Gpc-scale coherent magnetic fields could improve this limit by a few orders of magnitude. In addition, we predict the existence of magnetic Langmuir waves and turbulence which may appear on the sky as ``zebra patterns'' of an alternating magnetic field with ${\bf k\cdot B} \not =0$. We also show that magnetic monopole Langmuir turbulence excited near the accretion shock of galaxy clusters may be an efficient mechanism for generating the observed intracluster magnetic fields.   

Astrophysical Applications of Relativistic Shear Flows

We review recent PIC simulation results of relativistic collisionless shear flows in both 2D and 3D. We apply these results to spine-sheath jet models of blazars and gamma-ray-bursters, and to shear flows near the horizon of rapidly spinning black holes. We will discuss magnetic field generation, particle energization and radiation processes, and their observational consequences.   

Magnetorotational Dynamo Action in the Shearing Box

Magnetic dynamo action caused by the magnetorotational instability is studied in the shearing-box approximation with no imposed net magnetic flux. Consistent with recent studies, the dynamo action is found to be sensitive to the aspect ratio of the box: it is much easier to obtain in tall boxes (stretched in the direction normal to the disk plane) than in long boxes (stretched in the radial direction). Our direct numerical simulations indicate that the dynamo is possible in both cases, given a large enough magnetic Reynolds number. To explain the relatively larger effort required to obtain the dynamo action in a long box, we propose that the turbulent eddies caused by the instability most efficiently fold and mix the magnetic field lines in the radial direction. As a result, in the long box the scale of the generated strong azimuthal (stream-wise directed) magnetic field is always comparable to the scale of the turbulent eddies. In contrast, in the tall box the azimuthal magnetic flux spreads in the vertical direction over a distance exceeding the scale of the turbulent eddies. As a result, different vertical sections of the tall box are permeated by large-scale nonzero azimuthal magnetic fluxes, facilitating the instability.   

Transverse Cascade and Sustenance of Turbulence in Keplerian Disks with an Azimuthal Magnetic Field

The magnetorotational instability (MRI) in the sheared rotational Keplerian explains fundamental problems for both astrophysics and toroidal laboratory plasmas. The turbulence occurs before the threshold for the linear eigen modes. The work shows the turbulence occurs in nonzero toroidal magnetic field with a sheared toroidal flow velocity. We analyze the turbulence in Fourier k-space and x-space each time step to clarify the nonlinear energy-momentum transfers that produce the sustenance in the linearly stable plasma. The nonlinear process is a type 3D angular redistribution of modes in Fourier space – a transverse cascade – rather than the direct/inverse cascades. The turbulence is sustained an interplay of the linear transient growth from the radial gradient of the toroidal velocity (which is the only energy supply for the turbulence) and the transverse cascade. There is a relatively small “vital area in Fourier space” is crucial for the sustenance. Outside the vital area the direct cascade dominates. The interplay of the linear and nonlinear processes is generally too intertwined in k-space for a classical turbulence characterization. Subcycles occur from the interactions that maintain self-organization nonlinear turbulence. The spectral characteristics in four simulations are similar showing the universality of the sustenance mechanism of the shear flow driven MHDs-turbulence.   

Astrophysical ZeV acceleration in the jets from an accreting blackhole

An accreting blackhole produces extreme amplitude Alfven waves whose wavelength (wave packet) size is characterized by its clumsiness. The ponderomotive force driven by the bow wake of these Alfven waves propagates along the AGN (blazar) jet, and accelerates protons/nuclei to extreme energies beyond Zetta-electron volt (ZeV =$10^{21}$ eV)[Ebisuzaki,Tajima (2014)]. Such acceleration is linear and does not suffer from the multiple scattering/bending involved in the Fermi acceleration that causes excessive synchrotron radiation loss beyond $10^{19}$ eV. This bow wake acceleration was confirmed one-dimensional particle-in-cell simulations[Lau et al (2015)]. General relativistic Magneto-hydrodynamics simulations also show the intermittent eruptions of electro-magnetic waves from the innermost region of the accretion disk around a black hole [Mizuta et al (2017) priv. comm.]. The production rate of ultra-high energy cosmic rays in M82 starburst galaxy is estimated from its gamma-ray luminosity and is found to be consistent with the observed flux of the northern hot spot by Telescope Array[Abbasi et al (2014)]. We will discuss the possible acceleration in an intermediate mass black hole candidate M82 X-1 and the magnetic bending in the cosmological filaments in the local super cluster.   

Observational Signatures Of The Gamma Rays From Bright Blazars And Wakefield Theory

Gamma-ray observations have detected a strong variability in blazar luminosity in the gamma ray over time scales as short as several minutes. We show, for the first time, that the correlation of spectrum with intensity is consistent with the behavior with luminosity of blazar SEDs along a blazar sequence for low synchrotron peak blazars. We show that the observational signatures of variability with ux are consistent with wakefield acceleration of electrons initiated by instabilities in the blazar accretion disk. This mechanism produces time variations as short as intervals of 100 seconds. The wakefield mechanism also predicts a reduction of electron spectral index with an increase in gamma-ray luminosity, which could be detected in higher energy observations well above the inverse Compton peak.