X-ray 1-3 and radio 4-6 observations of the supernova remnant Cassiopeia A reveal the presence of magnetic fields about 100 times stronger than those in the surrounding interstellar medium. Field coincident with the outer shock probably arises through a nonlinear feedback process involving cosmic rays 2,7,8 . The origin of the large magnetic field in the interior of the remnant is less clear but it is presumably stretched and amplified by turbulent motions. Turbulence may be generated by hydrodynamic instability at the contact discontinuity between the supernova ejecta and the circumstellar gas 9 . However, optical observations of Cassiopeia A indicate that the ejecta are interacting with a highly inhomogeneous, dense circumstellar cloud bank formed before the supernova explosion 10-12 . Here we investigate the possibility that turbulent amplification is induced when the outer shock overtakes dense clumps in the ambient medium 13-15 . We report laboratory experiments that indicate the magnetic field is amplified when the shock interacts with a plastic grid. We show that our experimental results can explain the observed synchrotron emission in the interior of the remnant. The experiment also provides a laboratory example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena.High-resolution X-ray images and radio polarization maps of Cassiopeia A show two distinct strong magnetic field regions [3][4][5][6]12 . Narrow X-ray filaments, a fraction of a parsec in width, are observed at the outer shock rim at a radius of about 2.5 pc. These structures are produced by synchrotron radiation from ultrarelativistic electrons (with teraelectronvolt energy) and can be explained by magnetic fields of the order of 100 µG or more 2,3 . The interior of the remnant contains a disordered shell (about 0.5 pc in width at a radius of 1.7 pc) of radio synchrotron emission by gigaelectronvolt electrons 4 . The inferred magnetic field in these radio knots is a few milligauss, about 100 times higher than expected from the standard shock compression of the interstellar medium 15 . Optical observations of Cassiopeia A show the presence of both rapidly moving (5,000-9,000 km s −1 ) and essentially stationary dense knots. Although the moving knots themselves have a high velocity, their overall pattern is nearly stationary 10 . This led to the suggestion 10 that a dense pre-existing inhomogeneous stationary cloud bank could be present. New rapidly moving knots predominantly appear at a position broadly coincident with the shell of bright radio emission 6 . Sizes of the observed small-scale features within the shell range from 0.01 to 0.1 pc arranged in larger patterns extending to 0.5-2 pc (ref. 16). Interaction between the ejecta and the cloud bank may excite the turbulence that amplifies the magnetic field and makes Cassiopeia A an exceptionally bright radio source 4 . The interaction is akin to the Rayleigh-Taylor instability otherwise proposed as a source of turbulenc...

The visible matter in the universe is turbulent and magnetized. Turbulence in galaxy clusters is produced by mergers and by jets of the central galaxies and believed responsible for the amplification of magnetic fields. We report on experiments looking at the collision of two laser-produced plasma clouds, mimicking, in the laboratory, a cluster merger event. By measuring the spectrum of the density fluctuations, we infer developed, Kolmogorov-like turbulence. From spectral line broadening, we estimate a level of turbulence consistent with turbulent heating balancing radiative cooling, as it likely does in galaxy clusters. We show that the magnetic field is amplified by turbulent motions, reaching a nonlinear regime that is a precursor to turbulent dynamo. Thus, our experiment provides a promising platform for understanding the structure of turbulence and the amplification of magnetic fields in the universe.galaxy clusters | laboratory analogues | lasers | magnetic fields | turbulence

By irradiating a thin metal foil with an intense short-pulse laser, we have created a uniform system far from equilibrium. The deposited energy is initially deposited only within the electronic subsystem, and the subsequent evolution of the system is determined by the details of the electron-phonon coupling. Here, we measure the time evolution of the lattice parameter through multilayer Bragg diffraction and compare the result to classical molecular dynamic simulations to determine the lattice temperature. The electron-ion coupling constant for gold is then determined by comparison with the evolution of a two-temperature electron-phonon system.

A Thomson scattering diagnostic has been used to measure the parameters of cylindrical wire array Z pinch plasmas during the ablation phase. The scattering operates in the collective regime (α>1) allowing spatially localised measurements of the ion or electron plasma temperatures and of the plasma bulk velocity. The ablation flow is found to accelerate towards the axis reaching peak velocities of 1.2-1.3×10 7 cm/s in aluminium and ∼1×10 7 cm/s in tungsten arrays. Precursor ion temperature measurements made shortly after formation are found to correspond to the kinetic energy of the converging ablation flow.Wire array Z pinch implosions can efficiently convert stored electrical energy into powerful bursts of soft xrays. Experiments on the 20 MA Z pulsed power generator [1] have achieved peak powers of 280-300 TW with a >2 MJ yield at 20% efficiency [2] by imploding cylindrical arrays consisting of hundreds of fine metallic wires. It was found [3][4][5] that in wire array Z pinches the wires remain stationary for the first 60-80% of the implosion time and steadily ablate plasma which is accelerated by the j×B force. This ablation flow distributes mass in the array interior which sets the initial conditions for the implosion phase. The ablation mass distribution depends on the ablation rate, dm/dt, and on the flow velocity, which are related via force -momentum balance. However, the flow velocity was not measured in past experiments apart from the velocity of the initial part of the flow which was inferred from the time of plasma appearance on the array axis and from end-on interferometry measurements, giving V∼1.5×10 7 cm/s [6] [7]. The velocity of the ablation flow in the later stages, when the majority of the ablated material is moving into the array interior, is not known. Instead, the ablation rate and mass redistribution is often discussed using the so-called 'ablation velocity', introduced in the rocket model [3] via momentum balance arguments, which is inferred from the implosion dynamics or from unfolds of x-ray radiography measurements of mass distribution in the array interiorNumerical simulations [10][11] show that velocity is changing both with time and with the radial position of the flow, indicating that the flow is accelerated by the j×B force acting on the plasma in the array interior. However, these simulations do not model the initial heating of the wires and the process of conversion of initially cold metallic wires into a plasma. Instead, simulations start from wire material already converted into plasma at some initial temperature and density, and these initial conditions are adjusted to produce agreement of the simulated implosion dynamics with experimental observations. Direct measurements of the ablation flow velocity and density are needed to allow verification of these numerical models. Knowledge of the flow velocity is also important for other applications of different wire array Z pinch configurations, used e.g. in laboratory astrophysics [12] and other HEDP basic science research.In th...

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We report on experimental investigations into strong, laser-driven, radiative shocks in noble-gas cluster media. Cylindrical shocks launched with several J exhibit strong radiative effects such as increased deceleration and radiative preheat. Using time-resolved propagation data from single-shot streaked Schlieren measurements we observe temporal modulations on shock position and velocity, which we attribute to the thermal cooling instability, an instability which until now has not been observed experimentally. PACS numbers: Valid PACS appear hereShocks are a common phenomenon in astrophysics and high-energy-density (HED) environments in general. A shock forms when material expands with supersonic speed into an ambient medium, faster than the surrounding material can adapt to the expansion. If the energy deposition initially launching the shock is limited in time, the shock is followed by a rarefaction which eventually catches up with the shock front and a blast wave is formed, often consisting of a thin shell containing much of the swept-up material [1].An understanding of shocks and the dynamics of thermal and dynamical instabilities in HED plasmas is vital for numerical models of complex plasma systems. In such environments, radiation can lead to fundamental structural and dynamical changes in the system evolution. A shock becomes radiative if the post-shock conditions lead to an efficient cooling rate through radiative energy losses. The radiation is transmitted through the shock shell and, in an optically thin case, is lost from the system. In contrast, if the upstream material ahead of the shock front is optically thick to parts of the emission spectrum, radiation can be reabsorbed leading to preheat and ionization of the material ahead of the shock front. This modifies the shock propagation dynamics and can lead to growth of instabilities [2,3].The temporal expansion of a shock radius is often described as a power-law type function of the formwhere E 0 denotes the deposited energy per unit length (in cylindrical geometry) and ρ is the mass density. The parameter α is the deceleration parameter determined by the geometry and the energy dissipation in the system, which for cylindrical, adiabatic blast waves is α = 0.5 [4]. Dissipative processes such as radiation or ionization necessarily reduce the polytropic index, γ, of the system, * Electronic address: M.Hohenberger@Imperial.ac.uk and therefore α, to a value below the adiabatic solution and the blast wave decelerates more quickly. In case where radiative losses in the shell are sufficiently large such that the shell cannot support itself any longer, it is pushed by the low-density but high-pressure interior of the shock and collapses to high densities. Specifically the transition to this pressure driven snowplow regime and the associated shell-thinning is thought to make the shock more susceptible to radiation-driven instabilities, one of which we address in detail in this paper. Radiative shocks can be studied experimentally by utilizing the efficient a...

Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy—gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100–1,000 km above the surface of the white dwarf, far too small to be resolved with current telescopes. Here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. We find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to AM Herculis star systems, with theoretical predictions.

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