Strong suppression of heat conduction in a laboratory replica of galaxy-cluster turbulent plasmas.

In conventional gases and plasmas, it is known that heat fluxes are proportional to temperature gradients, with collisions between particles mediating energy flow from hotter to colder regions and the coefficient of thermal conduction given by Spitzer's theory. However, this theory breaks down in magnetized, turbulent, weakly collisional plasmas, although modifications are difficult to predict from first principles due to the complex, multiscale nature of the problem. Understanding heat transport is important in astrophysical plasmas such as those in galaxy clusters, where observed temperature profiles are explicable only in the presence of a strong suppression of heat conduction compared to Spitzer's theory. To address this problem, we have created a replica of such a system in a laser laboratory experiment. Our data show a reduction of heat transport by two orders of magnitude or more, leading to large temperature variations on small spatial scales (as is seen in cluster plasmas).

Time-resolved turbulent dynamo in a laser plasma.

Understanding magnetic-field generation and amplification in turbulent plasma is essential to account for observations of magnetic fields in the universe. A theoretical framework attributing the origin and sustainment of these fields to the so-called fluctuation dynamo was recently validated by experiments on laser facilities in low-magnetic-Prandtl-number plasmas ([Formula: see text]). However, the same framework proposes that the fluctuation dynamo should operate differently when [Formula: see text], the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports an experiment that creates a laboratory [Formula: see text] plasma dynamo. We provide a time-resolved characterization of the plasma's evolution, measuring temperatures, densities, flow velocities, and magnetic fields, which allows us to explore various stages of the fluctuation dynamo's operation on seed magnetic fields generated by the action of the Biermann-battery mechanism during the initial drive-laser target interaction. The magnetic energy in structures with characteristic scales close to the driving scale of the stochastic motions is found to increase by almost three orders of magnitude and saturate dynamically. It is shown that the initial growth of these fields occurs at a much greater rate than the turnover rate of the driving-scale stochastic motions. Our results point to the possibility that plasma turbulence produced by strong shear can generate fields more efficiently at the driving scale than anticipated by idealized magnetohydrodynamics (MHD) simulations of the nonhelical fluctuation dynamo; this finding could help explain the large-scale fields inferred from observations of astrophysical systems.

A measurement of the equation of state of carbon envelopes of white dwarfs.

White dwarfs represent the final state of evolution for most stars1-3. Certain classes of white dwarfs pulsate4,5, leading to observable brightness variations, and analysis of these variations with theoretical stellar models probes their internal structure. Modelling of these pulsating stars provides stringent tests of white dwarf models and a detailed picture of the outcome of the late stages of stellar evolution6. However, the high-energy-density states that exist in white dwarfs are extremely difficult to reach and to measure in the laboratory, so theoretical predictions are largely untested at these conditions. Here we report measurements of the relationship between pressure and density along the principal shock Hugoniot (equations describing the state of the sample material before and after the passage of the shock derived from conservation laws) of hydrocarbon to within five per cent. The observed maximum compressibility is consistent with theoretical models that include detailed electronic structure. This is relevant for the equation of state of matter at pressures ranging from 100 million to 450 million atmospheres, where the understanding of white dwarf physics is sensitive to the equation of state and where models differ considerably. The measurements test these equation-of-state relations that are used in the modelling of white dwarfs and inertial confinement fusion experiments7,8, and we predict an increase in compressibility due to ionization of the inner-core orbitals of carbon. We also find that a detailed treatment of the electronic structure and the electron degeneracy pressure is required to capture the measured shape of the pressure-density evolution for hydrocarbon before peak compression. Our results illuminate the equation of state of the white dwarf envelope (the region surrounding the stellar core that contains partially ionized and partially degenerate non-ideal plasmas), which is a weak link in the constitutive physics informing the structure and evolution of white dwarf stars9.

Rayleigh-Taylor instabilities in high-energy density settings on the National Ignition Facility.

The Rayleigh-Taylor (RT) instability occurs at an interface between two fluids of differing density during an acceleration. These instabilities can occur in very diverse settings, from inertial confinement fusion (ICF) implosions over spatial scales of [Formula: see text] cm (10-1,000 µm) to supernova explosions at spatial scales of [Formula: see text] cm and larger. We describe experiments and techniques for reducing ("stabilizing") RT growth in high-energy density (HED) settings on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. Three unique regimes of stabilization are described: (i) at an ablation front, (ii) behind a radiative shock, and (iii) due to material strength. For comparison, we also show results from nonstabilized "classical" RT instability evolution in HED regimes on the NIF. Examples from experiments on the NIF in each regime are given. These phenomena also occur in several astrophysical scenarios and planetary science [Drake R (2005) Plasma Phys Controlled Fusion 47:B419-B440; Dahl TW, Stevenson DJ (2010) Earth Planet Sci Lett 295:177-186].

Generating gradient germanium nanostructures by shock-induced amorphization and crystallization.

Gradient nanostructures are attracting considerable interest due to their potential to obtain superior structural and functional properties of materials. Applying powerful laser-driven shocks (stresses of up to one-third million atmospheres, or 33 gigapascals) to germanium, we report here a complex gradient nanostructure consisting of, near the surface, nanocrystals with high density of nanotwins. Beyond there, the structure exhibits arrays of amorphous bands which are preceded by planar defects such as stacking faults generated by partial dislocations. At a lower shock stress, the surface region of the recovered target is completely amorphous. We propose that germanium undergoes amorphization above a threshold stress and that the deformation-generated heat leads to nanocrystallization. These experiments are corroborated by molecular dynamics simulations which show that supersonic partial dislocation bursts play a role in triggering the crystalline-to-amorphous transition.

Directional amorphization of boron carbide subjected to laser shock compression.

Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laser-driven shock compression, unprecedented high strain rates can be achieved; here we report the directional amorphization in boron carbide polycrystals. At a shock pressure of 45∼50 GPa, multiple planar faults, slightly deviated from maximum shear direction, occur a few hundred nanometers below the shock surface. High-resolution transmission electron microscopy reveals that these planar faults are precursors of directional amorphization. It is proposed that the shear stresses cause the amorphization and that pressure assists the process by ensuring the integrity of the specimen. Thermal energy conversion calculations including heat transfer suggest that amorphization is a solid-state process. Such a phenomenon has significant effect on the ballistic performance of B4C.

Epidemiology of fowl cholera in free range broilers.

Fowl cholera, caused by Pasteurella multocida, remains a major problem of poultry worldwide. In the current report, we describe an outbreak in free range organic broilers. In addition to culturing samples from dead broilers, we attempted to isolate P. multocida from feral cats trapped on the farm. The isolates were identified by PCR as P. multocida and then serotyped using the Heddleston scheme and genotyped using both a multilocus sequence typing (MLST) method and an enterobacterial repetitive intergenic consensus (ERIC)-PCR method. A total of 123 isolates of P. multocida were recovered from 12 broilers. All 123 isolates were examined by ERIC-PCR, and only one pattern was identified. A subset of seven broiler isolates were examined by MLST and all were typed as sequence type (ST) 20. A total of 28 isolates of P. multocida were recovered from 17 cats, and five ERIC-PCR genotypes were identified, with one genotype (E-1, shared by 19 isolates) being the same as the ERIC-PCR pattern associated with the broilers. One representative cat strain for each ERIC-PCR pattern was subjected to MLST. The cat isolate with the same ERIC-PCR genotype as the broiler isolates was confirmed as having the same MLST result, ST 20. The other five cat ERIC-PCR patterns were allocated to four STs: E-2 and E-5 to ST 265, E-3 to ST 30, E-4 to ST 20, and E-6 to ST 264. Both genotyping methods confirmed that isolates of P. multocida were common between the feral cats and the chickens. It was not clear whether the strain was transmitted from the cats to the chicken or whether the cats obtained the strain preying on chicken. The study has shown that cats can harbor P. multocida strains with the same genotype found in chickens affected with fowl cholera.

Studies on the presence and persistence of Pasteurella multocida serovars and genotypes in fowl cholera outbreaks.

Pasteurella multocida was isolated from poultry on six farms (one free-range duck farm, one free-range turkey farm, one conventional enclosed turkey farm, and three free-range layer farms) suffering fowl cholera outbreaks. In addition, historical isolates from previous outbreaks were available for the conventional turkey farm and the three free-range layer farms. The isolates were serotyped using the Heddleston scheme and genotyped using multi-locus sequencing typing. In the current outbreaks, two of the farms had two different sequence types (STs) of P. multocida in the investigated outbreak (the free-range turkey farm and one of the free-range layer farms). The remaining four farms had one ST within the investigated outbreak. In looking at the historical isolates, two of the four farms had multiple genotypes involved. On the four farms with historical isolates from previous outbreaks, at least one new genotype was present in the investigated outbreak as compared with the historical isolates. On one layer farm, one genotype persisted over a 10-year period. Serotyping revealed the presence of multiple serovars in the current and historical outbreaks, with serovars sometimes changing over time. This study has shown that several STs of P. multocida can be present during some outbreaks of fowl cholera, although other outbreaks involve a single ST. Also, the STs present on a property suffering repeated fowl cholera can both persist and change over time.

Nanosecond white-light Laue diffraction measurements of dislocation microstructure in shock-compressed single-crystal copper.

Under uniaxial high-stress shock compression it is believed that crystalline materials undergo complex, rapid, micro-structural changes to relieve the large applied shear stresses. Diagnosing the underlying mechanisms involved remains a significant challenge in the field of shock physics, and is critical for furthering our understanding of the fundamental lattice-level physics, and for the validation of multi-scale models of shock compression. Here we employ white-light X-ray Laue diffraction on a nanosecond timescale to make the first in situ observations of the stress relaxation mechanism in a laser-shocked crystal. The measurements were made on single-crystal copper, shocked along the [001] axis to peak stresses of order 50 GPa. The results demonstrate the presence of stress-dependent lattice rotations along specific crystallographic directions. The orientation of the rotations suggests that there is double slip on conjugate systems. In this model, the rotation magnitudes are consistent with defect densities of order 10(12) cm(-2).

Nanosecond x-ray Laue diffraction apparatus suitable for laser shock compression experiments.

We have used nanosecond bursts of x-rays emitted from a laser-produced plasma, comprised of a mixture of mid-Z elements, to produce a quasiwhite-light spectrum suitable for performing Laue diffraction from single crystals. The laser-produced plasma emits x-rays ranging in energy from 3 to in excess of 10 keV, and is sufficiently bright for single shot nanosecond diffraction patterns to be recorded. The geometry is suitable for the study of laser-shocked crystals, and single-shot diffraction patterns from both unshocked and shocked silicon crystals are presented.

Ultrahigh strength in nanocrystalline materials under shock loading.

Molecular dynamics simulations of nanocrystalline copper under shock loading show an unexpected ultrahigh strength behind the shock front, with values up to twice those at low pressure. Partial and perfect dislocations, twinning, and debris from dislocation interactions are found behind the shock front. Results are interpreted in terms of the pressure dependence of both deformation mechanisms active at these grain sizes, namely dislocation-based plasticity and grain boundary sliding. These simulations, together with new shock experiments on nanocrystalline nickel, raise the possibility of achieving ultrahard materials during and after shock loading.