Session NO6: Laboratory Plasma Astrophysics III

Laser driven high energy density radiative blast waves launched in clustered gases

Intense lasers deposit energy efficiently in clustered gases creating hot plasma with low density, conditions ideal for launching radiative blast waves (BWs) of interest for laboratory astrophysics (LA). We report measurements in a range of gases irradiated by the Astra-Gemini laser with energies $>$10J. Optical imaging, self emission and temporally resolved x-ray spectra are used to characterise BW evolution. The high repetition rate of the laser allows us to explore the influence of atomic number and density on the BW dynamics. Altering the emitted radiation and opacity of the medium has a strong effect on the BW profile and energy loss. Strongly radiative BWs exhibit shell thinning, increasing their susceptibility to instabilities. We have demonstrated the onset of a velocity instability, driven by the exchange of energy between the shock and precursor in krypton BWs. We discuss the threshold conditions for this behaviour and the potential to study spatial shock front instabilities. Our results will be compared to simulations and analytical calculations with a view to designing scalable LA experiments.   

Grid-induced turbulence in laser driven shock wave experiments and magnetic field generation

Although magnetic fields exist throughout the Universe, their origin is still uncertain. They are seen in galaxy clusters, filaments and voids, with intensities ranging from a few $\mu$G to a fraction of a fG. Thanks to the development of high power laser facilities it is now possible to study such astrophysical systems in the laboratory using simple scaling laws. We have developed a new experimental platform where we investigate the generation and amplification of tiny seed magnetic fields through induced turbulence. This was achieved by focusing the Vulcan laser ($\sim$300 J, 527 nm, 1 ns) onto a graphite rod, with the resultant blast wave propagating through ambient argon gas at 1 mbar pressure. A broad range of diagnostics including interferometric, schlieren and spectroscopic self-emission imaging along with temporally resolved induction coils were used to characterise the magnetic field and shock evolution. Homogeneous turbulence was generated by placing a wire mesh array in the path of the shock. Comparison with current models of turbulent amplification of magnetic fields are discussed.   

Richtmyer-Meshkov instability induced by strong shocks

We systematically study the Richtmyer-Meshkov instability (RMI) induced by strong shocks for fluids with contrasting densities and with small and large amplitude initial perturbations imposed at the fluid interface. The Smoothed particle hydrodynamics code (SPHC) is employed to ensure accurate shock capturing, interface tracking, and accounting for the dissipation processes. Simulations results achieve good agreement with existing experiments and with the theoretical analyses including zero-order theory describing the post-shock background motion of the fluids, linear theory providing RMI growth-rate in a broad range of the Mach and Atwood numbers, weakly nonlinear theory accounting for the effect of the initial perturbation amplitude on RMI growth-rate, and highly nonlinear theory describing evolution of RM bubble front. We find that for strong-shock-driven RMI the background motion is supersonic, and the interfacial mixing can be sub-sonic or supersonic. Significant part of the shock energy goes into compression and background motion of the fluids, and only a small portion remains for interfacial mixing. The initial perturbation amplitude appears a key factor of RMI evolution. It strongly influences the dynamics of the interface, in the fluid bulk, and the transmitted shock.   

Instabilities in counter-propagating ion beams and plasmas

Collisionless shocks are frequent events in the interstellar medium, they can also take place in inertial fusion targets where high energy ion beams interact with target plume plasma. The understanding of the processes is consequently important from a theoretical point of view and for laboratory laser--plasma interaction experiments. In this paper, we consider interaction of two counter-propagating homogeneous plasma beams with sub-relativistic velocities and no external magnetic field applied. In numerical simulations performed with a particle-in-cell code three stages of evolution can be identified. The shock formation is initiated with development of the electron-ion Weibel-like micro-instabilities, followed by fast electron heating and ion de-acceleration and heating. We present a theoretical analysis of the instabilities development and nonlinear saturation to explore the origins of the heating and the magnetic field generation. The analysis is done in the center of mass frame, considering the Lorentz transformation for each beam from its own reference frame. From the dispersion relation, instability is characterized and dependence on the electron temperature and ion velocity is studied.The growth rate and characteristic scales of instability are compared to simulations.   

Collisional Particle-In-Cell simulation of collision-less counter-streaming plasmas

Experimental measurements [Phys. Plasmas 19, 056501 (2012)] done at the Omega laser facility under the ACSEL collaboration have shown a strong increase of both electron and ion temperature when two counter streaming plasmas interact, while density measurements show no evidence of stagnation. Collisional PIC simulations show that the 2-stream-ion-acoustic instability can efficiently couple the ion temperature to the electron, which are heated by resistive effects. Details of the numerical difficulties encountered to obtain the correct physics will be described. In particular spurious transverse diffusion has to be controlled to avoid numerical transfer of the very large kinetic energy into thermal energy while time scales over 6 orders of magnitude have to be resolved (fs-ns).   

Particle-in-cell simulations of particle energization from low Mach number fast mode shocks

Low Mach number, high plasma beta, fast mode shocks likely occur in the outflows from reconnection sites associated with solar flares. These shocks are sites of particle energization with observable consequences, but there has been much less work on understanding the underlying physics compared to that of Mach number shocks. To make progress, we have simulated a low Mach number/high beta shock using 2D particle-in-cell simulations with a ``moving wall'' method and studied the shock structure and particle acceleration processes therein [Park et. al (2012), Phys. Plasmas, 19, 062904]. The moving wall method can control the shock speed in the simulation frame to allow smaller simulation boxes and longer simulation times. We found that the modified two-stream instability in the shock transition region is responsible for shock sustenance via turbulent dissipation and entropy creation throughout the downstream region long after the initial shock formation. Particle tracking and the particle energy distributions show that both electrons and ions participate in shock-drift-acceleration (SDA). The simulation combined with a theoretical analysis reveals a two-temperature Maxwellian distribution for the electron energy distribution via SDA.   

Modeling High-Energy Density Astrophysical Shock Environments using the Hybrid Code LSP

Collisionless shocks are believed to play an important role in numerous high-energy astrophysical scenarios, from gamma-ray bursts to the generation of cosmic rays. The ACSEL collaboration has performed a series of Omega experiments producing and characterizing high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks [1]. Using proton radiography, large, stable electromagnetic field structures have been observed that extend for much larger distances than the intrinsic plasma spatial scales, and persist for much longer than the plasma kinetic time scales. These results suggest that large-scale plasma self-organization can occur within astrophysically relevant plasma flows in the laboratory [2]. Modeling this through cm-scale 1-D fluid-electron kinetic-ion simulations, we observe a correlation with the formation of strong off-center temperature gradients. In this presentation we describe further one- and two-dimensional simulation results relevant to counter-streaming plasma flows using the hybrid code LSP. Both collisional and collisionless scenarios are examined with special emphasis placed on multi-species effects.\\[4pt] [1] J.S. Ross et al, Phys. Plas., 19, 056501 (2012).\\[0pt] [2] N.L. Kugland et al, submitted to Nature-Physics (2012).   

Experimental study of formation of differentially rotated supersonic plasma flows

We will present experiments designed to form a differentially rotating supersonic plasma flow with dimensionless parameters relevant to modeling physics of astrophysical discs. The set-up is based on a modification of cylindrical wire array z-pinch, combined with a cusp magnetic field. The azimuthal component of the JxB force introduces an angular momentum into the ablation flow converging on the array axis, leading to formation of rotating disc supported in equilibrium by the ram pressure of the flow. The level of radiative cooling can be controlled by variation of the wire material. Plasma parameters of the formed disc were measured with laser probing (interferometry and shadowgraphy) and Thomson scattering. The disc rotates with velocity of $\sim $30km/s, has Mach number of $\sim $4 and Reynolds number $>$10$^{5}$. Development of hydrodynamic instabilities in the rotating plasma will be investigated and discussed.   

The impact of Hall physics on magnetized plasma jets produced by radial foil configurations

Although no one argues that plasma resistivity is important to include in the astrophysical simulations, based upon experiments with magnetized jets on pulsed power machines in the laboratory, we believe it may also be important to include the Hall term in the generalized Ohm's law in astrophysics simulation codes. In this talk, experiments carried out at Cornell University and at Imperial College on 1 to 1.5 MA pulsed power generators feature a plasma disk and a collimated, axial plasma jet with large Re (10$^5$) and Rem (10$^3$). The plasma jet is generated by ablation from electrical currents, which flow in a thin aluminum foil and converge to a central multi-pin cathode located under the foil. A twist in the pins produce the axial magnetic field necessary to magnetized the jet. It was observed that changing the polarity of the current alters drastically the plasma dynamics, an indication of the importance of the Hall effect in plasmas produced by radial foils. The overall agreement between experimental results and numerical simulations indicates that PERSEUS accounts properly for Hall physics in this geometry and plasma parameter range. Scaling to astrophysical occurrences via numerical simulations should highlight how Hall physics affects the dynamics of larger accretion disks.   

Structure and dynamics of supersonic plasma jets, jet collisions, and their spontaneous fields

Understanding the spatial structure and temporal evolution of plasma jets, and the interactions between colliding jets, is important for frontier astrophysics and for the basic science of high-energy-density physics. Scaled laboratory experiments have now been used to explain and quantify several important properties of supersonic astrophysical jets and their response to self-generated fields. We report the first observations of spontaneously-generated fields in jets and the effects of those fields on plasma jet propagation. Subsequent to collision of two jets with each other, low-Mach-number plasma shocks (a nonlinear consequence of plasma instabilities and fields) are observed by imaging electric fields associated with shock fronts. The shocked downstream regions lack collisional field dissipation, indicating these shocks are essentially collisionless. This work was supported in part by the U.S. DOE, LLNL, GA and LLE.   

Launching High Density, High Velocity Plasma Jet with A Ring of Laser Beams

We propose a novel way of producing high Mach number, highly collimated plasma jets with multiple intense laser beams irradiating a planar plastic target. Our high resolution radiative hydrodynamics simulations show that these supersonic jets can be formed when the focal spots of those beams are somewhat separated from each other instead of all focusing on the center of the target. We carry out runs with various degrees of beam separation and study their effects on the properties of produced jets. Relevance to astrophysical jets and implication for laboratory collisionless shock experiment are also discussed.   

Modeling Laboratory Astrophysics Experiments in the High-Energy-Density Regime Using the CRASH Radiation-Hydrodynamics Model

The radiation hydrodynamics code developed by the Center for Radiative Shock Hydrodynamics (CRASH) at the University of Michigan has been used to model experimental designs for high-energy-density physics campaigns on OMEGA and other high-energy laser facilities. This code is an Eulerian, block-adaptive AMR hydrodynamics code with implicit multigroup radiation transport and electron heat conduction. CRASH model results have shown good agreement with a experimental results from a variety of applications, including: radiative shock, Kelvin-Helmholtz and Rayleigh-Taylor experiments on the OMEGA laser; as well as laser-driven ablative plumes in experiments by the Astrophysical Collisionless Shocks Experiments with Lasers (ACSEL), collaboration. We report a series of results with the CRASH code in support of design work for upcoming high-energy-density physics experiments, as well as comparison between existing experimental data and simulation results. This work is funded by the Predictive Sciences Academic Alliances Program in NNSA-ASC via grant DEFC52- 08NA28616, by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-FG52-09NA29548, and by the National Laser User Facility Program, grant number~DE-NA0000850.   

Laboratory Experimental Design of Molecular Cloud Implosions

The interaction of ionizing radiation with its surrounding medium is a ubiquitous issue in astrophysics. Although the interaction can occur in many environments, the interaction of an ionization front with a molecular cloud is of particular interest. Material ablated form the cloud can form turbulent structure [Peters \textit{et al}, 2008] and coupled with the radiatively-driven implosion of the cloud can have important consequences in stellar formation. Our understanding of stellar formation is based on computer simulations and models. To improve our understanding of these models, data is required. We present the design of an experiment to study the interaction of an ionization front with a high density sphere, which acts as a surrogate for the molecular cloud. Irradiating a high-Z foil with laser beams generates the ionization front. The ionization front will propagate in a low density medium before interacting with the sphere. We will present our experimental design along with initial simulations. This work is funded by the~NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-FG52-09NA29548.   

Laboratory photoionized plasma experiments at Z relevant to astrophysics

Photoionized plasmas are present in many astrophysical environments, such as accretion disks and radiatively-driven winds of x-ray binaries and active galactic nuclei. We discuss an experimental and modeling effort in which the intense x-ray flux emitted at the collapse of a z-pinch is employed to produce and backlight a neon photoionized plasma to study the atomic kinetics through K-shell line absorption spectroscopy. The plasma is contained in a cm-scale gas cell filled with neon and placed at various distances from the z-pinch. The filling pressure is monitored in situ thus providing the particle number density of the plasma. High-resolution spectra are recorded with a TREX spectrometer with two elliptically-bent KAP crystals and a set of slits to record up to six spectra per crystal per shot. The transmission data shows line absorption transitions in several ionization stages of neon including Be-, Li-, He- and H-like ions. Analysis of the transmission spectra yields the charge state distribution and ion areal-densities used to benchmark atomic kinetics calculations. In addition, the electron temperature extracted from a level population ratio is used to test heating models of the photoionized plasma.