The Thomson scattering cross section in a magnetized, high density plasma

We calculate the Thomson scattering cross section in a non-relativistic, magnetized, high density plasma -- in a regime where collective excitations can be described by magnetohydrodynamics. We show that, in addition to cyclotron resonances and an elastic peak, the cross section exhibits two pairs of peaks associated with slow and fast magnetosonic waves; by contrast, the cross section arising in pure hydrodynamics possesses just a single pair of Brillouin peaks. Both the position and the width of these magnetosonic-wave peaks depend on the ambient magnetic field and temperature, as well as transport and thermodynamic coefficients, and so can therefore serve as a diagnostic tool for plasma properties that are otherwise challenging to measure.Comment: Main paper: pp 1-8. Appendix: pp 8-10. 2 figure

Kinetic stability of Chapman-Enskog plasmas

In this paper, we investigate the kinetic stability of classical, collisional plasma - that is, plasma in which the mean-free-path $\lambda$ of constituent particles is short compared to the length scale $L$ over which fields and bulk motions in the plasma vary macroscopically, and the collision time is short compared to the evolution time. Fluid equations are typically used to describe such plasmas, since their distribution functions are close to being Maxwellian. The small deviations from the Maxwellian distribution are calculated via the Chapman-Enskog (CE) expansion in $\lambda/L \ll 1$, and determine macroscopic momentum and heat fluxes in the plasma. Such a calculation is only valid if the underlying CE distribution function is stable at collisionless length scales and/or time scales. We find that at sufficiently high plasma $\beta$, the CE distribution function can be subject to numerous microinstabilities across a wide range of scales. For a particular form of the CE distribution function arising in magnetised plasma, we provide a detailed analytic characterisation of all significant microinstabilities, including peak growth rates and their associated wavenumbers. Of specific note is the discovery of several new microinstabilities, including one at sub-electron-Larmor scales (the 'whisper instability') whose growth rate in some parameter regimes is large compared to other instabilities. Our approach enables us to construct the kinetic stability maps of classical, two-species collisional plasma in terms of $\lambda$, the electron inertial scale $d_e$ and $\beta$. This work is of general consequence in emphasising the fact that high-$\beta$ collisional plasmas can be kinetically unstable; for strongly magnetised CE plasmas, the condition for instability is $\beta > L/\lambda$. In this situation, the determination of transport coefficients via the standard CE approach is not valid.Comment: 182 pages total (99 main text, remaining appendices), 31 figure

Efficient micromirror confinement of sub-TeV cosmic rays in galaxy clusters

Recent observations suggest a stronger confinement of cosmic rays (CRs) in certain astrophysical systems than predicted by current CR-transport theories. We posit that the incorporation of microscale physics into CR-transport models can account for this enhanced CR confinement. We develop a theoretical description of the effect of magnetic microscale fluctuations originating from the mirror instability on macroscopic CR diffusion. We confirm our theory with large-dynamical-range simulations of CR transport in the intracluster medium (ICM) of galaxy clusters and kinetic simulations of CR transport in micromirror fields. We conclude that sub-TeV CR confinement in the ICM is far more effective than previously anticipated on the basis of Galactic-transport extrapolations.Comment: Utilizes PIC and MHD simulations, complemented by deep learning for data analysis. Currently under journal review. Comments welcome

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 turbulent laser-plasmas

Understanding magnetic-field generation in turbulent plasma is essential for explain- ing the presence of dynamically significant magnetic fields in astrophysical environments such as the intracluster medium. Seemingly plausible theoretical frameworks attributing the origin and sustainment of these fields to amplification by the so-called fluctuation dynamo are somewhat hampered by conceptual uncertainties concerning the validity of the models in which these frameworks are formulated. A recent experiment on the OMEGA laser facility attempted to overcome some of these uncertainties by demonstrating the feasibility of magnetic-field amplification by stochastic motions up to dynamical strengths in actual turbulent plasma. In order to realise the scientific goals of this experiment, accurate measurements of stochastic magnetic fields arising in turbulent laser-plasmas were required. This thesis reports on the development of an analysis technique which meets this requirement by recover- ing the magnetic-energy spectrum from proton imaging data, as well as the mean magnetic-energy density and characteristic structure sizes. The general applicability and reliability of the technique is considered in depth. On application to data derived from the OMEGA experiment, the magnetic-energy density is found to increase over five hundred times in the experiment from its initial value; in addition, estimates of the maximum magnetic field strength indicate that the field is likely to be dynamically significant. The experiment therefore constitutes the first demonstration in the laboratory of the fluctuation dynamo. The results of a second experiment on the OMEGA laser facility â in which a remodelled variant of the previously employed experimental platform is used to provide a time-resolved characterisation of a plasma dynamoâs evolution, measuring temperatures, densities, flow velocities and magnetic fields â are also described. It is shown that the initial growth of the dynamo-generated fields occurs exponentially at a rate which significantly exceeds the turnover rate of the driving-scale stochastic motions in the plasma. Both experiments validate the claim that the fluctuation dynamo is indeed capable of amplifying magnetic fields significantly.</p

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)