Abstract
Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. The inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs.
Highlights
In ultra-intense laser-plasma interactions, beams of relativistic ‘hot’ electrons are driven from the interaction region[6] into materials of much higher density
Anisotropies in the speed distributions of both hot and return currents generate polarized X-ray line emission. As such the experimental measurements of the degree of polarization give us a unique ability to model both the non-equilibrium anisotropy in the return current electron distribution function[12,13,26] and in turn allows us to evaluate the resistivity generated in the warm dense matter regime applicable to the study of the viscosity of brown dwarfs
The return current density Jrc is related to the resistivity, Z, by the resistive electric field E 1⁄4 À ZJrc, which converts the energy of the hot electrons into Ohmic heating[16,27]
Summary
In ultra-intense laser-plasma interactions, beams of relativistic ‘hot’ electrons are driven from the interaction region[6] into materials of much higher density. Anisotropies in the speed distributions of both hot and return currents generate polarized X-ray line emission As such the experimental measurements of the degree of polarization give us a unique ability to model both the non-equilibrium anisotropy in the return current electron distribution function[12,13,26] and in turn allows us to evaluate the resistivity generated in the warm dense matter regime applicable to the study of the viscosity of brown dwarfs. It is the nonequilibrium, or anisotropic, parts of the return current distribution that yield information about material resistivity. By matching the measured polarization to detailed modelling we show that the commonly used resistivity models do not adequately describe these amorphous materials
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