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)

Magnetic field amplification in laser-produced plasmas

The universe abounds with shock waves, from those arising during structure formation, to those driving supernova explosions that create the elements of which life is made and can even trigger star formation. In the early universe, matter was nearly homogeneously distributed; today, as a result of gravitational instabilities, it forms a web-like structure of clusters, filaments, and voids. Radio-Synchrotron emission and Faraday Rotation measurements have revealed that clusters, filaments, and voids are all magnetised from a few nG to tens of μG. When integrated over the whole universe, this magnetic energy represents a sizeable component of the cosmic energy budget, making magnetic fields essential players in the dynamics of luminous matter in the universe. At present, the origin and distribution of magnetic fields are far from understood. The standard model for the origin of galactic and intergalactic magnetic fields is through the generation of small seed fields by some mechanism (e.g. Biermann Battery) and the amplification of these seed fields via dynamo or turbulent processes to the level consistent with current observations. Due to the advent of high-powered lasers, scaled astrophysical phenomena can be created in the laboratory - a supernova several parsecs in diameter can be scaled down to the size of a baseball. These laboratory plasmas are similar to plasmas in the universe in terms of localisation, heat conduction, viscosity, and radiation. Here we report on experimental measurements of magnetic field amplification by turbulent motions in both laser-produced shock waves scaled to supernova remnants and laser-produced jets analogous to cluster merger events. These measurements of turbulent magnetic field amplification in a laser-produced plasma are a precursor to turbulent dynamo [11] in which amplification is no longer limited by diffusion, and a necessary component in explaining the magnetisation of luminous matter in the universe.</p

Magnetic field amplification in laser-produced plasmas

The universe abounds with shock waves, from those arising during structure formation, to those driving supernova explosions that create the elements of which life is made and can even trigger star formation. In the early universe, matter was nearly homogeneously distributed; today, as a result of gravitational instabilities, it forms a web-like structure of clusters, filaments, and voids. Radio-Synchrotron emission and Faraday Rotation measurements have revealed that clusters, filaments, and voids are all magnetised from a few nG to tens of &mu;G. When integrated over the whole universe, this magnetic energy represents a sizeable component of the cosmic energy budget, making magnetic fields essential players in the dynamics of luminous matter in the universe. At present, the origin and distribution of magnetic fields are far from understood. The standard model for the origin of galactic and intergalactic magnetic fields is through the generation of small seed fields by some mechanism (e.g. Biermann Battery) and the amplification of these seed fields via dynamo or turbulent processes to the level consistent with current observations. Due to the advent of high-powered lasers, scaled astrophysical phenomena can be created in the laboratory - a supernova several parsecs in diameter can be scaled down to the size of a baseball. These laboratory plasmas are similar to plasmas in the universe in terms of localisation, heat conduction, viscosity, and radiation. Here we report on experimental measurements of magnetic field amplification by turbulent motions in both laser-produced shock waves scaled to supernova remnants and laser-produced jets analogous to cluster merger events. These measurements of turbulent magnetic field amplification in a laser-produced plasma are a precursor to turbulent dynamo [11] in which amplification is no longer limited by diffusion, and a necessary component in explaining the magnetisation of luminous matter in the universe.</p

Proton imaging of an electrostatic field structure formed in laser-produced counter-streaming plasmas

We report the measurements of electrostatic field structures associated with an electrostatic shock formed in laser-produced counter-streaming plasmas with proton imaging. The thickness of the electrostatic structure is estimated from proton images with different proton kinetic energies from 4.7 MeV to 10.7 MeV. The width of the transition region is characterized by electron scale length in the laser-produced plasma, suggesting that the field structure is formed due to a collisionless electrostatic shock.ISSN:1742-6588ISSN:1742-659

Developed turbulence and nonlinear amplification of magnetic fields in laboratory and astrophysical plasmas

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

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 (Pm &lt; 1). However, the same framework proposes that the fluctuation dynamo should operate differently when Pm &amp; 1, the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports an experiment that creates a laboratory Pm &amp; 1 plasma dynamo. We provide a time-resolved characterization of the plasma&apos;s evolution, measuring temperatures, densities, flow velocities, and magnetic fields, which allows us to explore various stages of the fluctuation dynamo&apos;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

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&apos;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&apos;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)