Time-resolved turbulent dynamo in a laser plasma

Abstract

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<1). However, the same framework proposes that the fluctuation dynamo should operate differently when Pm≳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≳1 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.

Tearing instability and current-sheet disruption in the turbulent dynamo

“…Thus, we see a degree of large-scale coherence in the amplified field that might help reconcile the view that the saturated dynamo must result in outer-scale magnetic fields [42,[51][52][53][54][55] and some previous indications, numerical and theoretical, that it produces fields whose energy resides on Rm-dependent scales [16-18, 59, 60]. This may also be a step towards simulations becoming more consistent with Faraday-rotation observations of magnetic fields in galaxy clusters suggesting kpc-scale coherence [61,62], as well as recent laboratory laser-plasma experiments exhibiting a Pm 1 fluctuation dynamo [63,64]. Determining whether or not this result holds when using the pressure-anisotropic MHD [39,65] or kinetic [66,67] descriptions more appropriate to the weakly collisional intracluster medium awaits future work.…”

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

“…By recording the wavelength shift of the backscattered light from a probe beam, we infer v turb ≈ 200 km/s. The self-generated magnetic fields were estimated from proton deflectometry to be B RMS ≈ 0.8( B path,∥ /25 kG cm)(𝓁 B /100 μm) 1/2 (𝓁 n /2 mm) 1/2 MG (here B path,∥ is the one component of the magnetic field that we measure), where we took the proton path length 𝓁 n from self-emission x-ray images ( 19 , 20 ), and the magnetic field correlation length, 𝓁 B , corresponds to a wavelength ∼4𝓁 B ≈ 400 μm of the same order as the grid periodicity. Using the same diagnostic, we found the maximum field to be B max ≳ 3 MG.…”

“…1 for details of the experimental setup. The platform is similar to that already used at other laser facilities [e.g., the Omega laser ( 19 , 20 )] but with ∼30 times more energy delivered to the target. A highly turbulent plasma was created by ablating two doped plastic foils, each with 133 kJ of 351 nm light in 15 ns (see Fig.…”

“…To help excite turbulence, plastic mesh grids with 300-μm apertures and 300-μm-diameter wires were placed 2 mm from each foil to disturb the two flows before they collided. Three-dimensional simulations carried out with the FLASH code [validated on previous experiments at smaller laser facilities ( 20 , 21 )] were used to help design the platform and analyze the experimental data.…”

High Repetition Rate Exploration of the Biermann Battery Effect in Laser Produced Plasmas Over Large Spatial Regions

“…Ancillary to future measurements that will explore the mechanisms behind the observed return current, and further interrogate the magnetic field structure in the LPP, we are executing a simulation campaign using the FLASH code 32 . FLASH is a parallel, multi-physics, adaptive-meshrefinement, finite-volume Eulerian hydrodynamics and radiation MHD code, whose capabilities 38 through benchmarks and code-to-code comparisons 39,40 , as well as through direct application to laser-driven laboratory experiments 38,[41][42][43][44][45][46][47][48][49][50][51][52][53] .…”