Direct observation of turbulent magnetic fields in hot, dense laser produced plasmas

Abstract

Turbulence in fluids is a ubiquitous, fascinating, and complex natural phenomenon that is not yet fully understood. Unraveling turbulence in high density, high temperature plasmas is an even bigger challenge because of the importance of electromagnetic forces and the typically violent environments. Fascinating and novel behavior of hot dense matter has so far been only indirectly inferred because of the enormous difficulties of making observations on such matter. Here, we present direct evidence of turbulence in giant magnetic fields created in an overdense, hot plasma by relativistic intensity (10 18 W∕cm 2 ) femtosecond laser pulses. We have obtained magneto-optic polarigrams at femtosecond time intervals, simultaneously with micrometer spatial resolution. The spatial profiles of the magnetic field show randomness and their k spectra exhibit a power law along with certain well defined peaks at scales shorter than skin depth. Detailed two-dimensional particle-in-cell simulations delineate the underlying interaction between forward currents of relativistic energy "hot" electrons created by the laser pulse and "cold" return currents of thermal electrons induced in the target. Our results are not only fundamentally interesting but should also arouse interest on the role of magnetic turbulence induced resistivity in the context of fast ignition of laser fusion, and the possibility of experimentally simulating such structures with respect to the sun and other stellar environments.intense laser matter interaction | high energy density | astrophysical simulations | filamentary structures T he largest terrestrially available magnetic fields are generated when an intense laser pulse (intensity above 10 18 W∕cm 2 ) irradiates a solid target (1-3). The high energy density produced by laser irradiation generates relativistic electron jets, through the process of wave breaking. These relativistic electron jets carry the laser energy deep into the target ionizing and heating the colder portions behind the laser generated plasma and exciting return shielding currents. In the laboratory, such heating is extremely important for fast ignition of highly compressed targets in laser fusion (4, 5), simulation of intra planetary matter existing at ultrahigh pressure (6), ultrafast X-ray pulses (7), as well as proton and ion acceleration up to the MeV-GeV levels (3). It also serves as an excellent tool for modeling astrophysical systems (8-10). The transport of relativistic electrons through hot dense matter is very complex and is barely understood (11,12). Simulations have shown that relativistic electron transport in plasma media is fraught with severe plasma instabilities particularly the Weibel instability (13), which leads to spatial separation of forward and backward currents and eventually to the emergence of turbulent structures (14) and rapid energy dissipation. A major physical parameter that mirrors this complex physics is the giant magnetic field-as high as hundreds of megagauss-generated in this interaction. In earlier st...

Radiation from particles moving in small-scale magnetic fields created in solid-density laser-plasma laboratory experiments

“…5, conducted at the Tata Institute of Fundamental Research (TIFR), an aluminum coated, BK-7 glass target was irradiated by a 10 18 W/cm 2 (800 nm, 30 fs duration) laser pump beamthereby creating a plasma in the aluminum layer (with thickness several times larger than the electron skin-depth) of the target. A low-intensity probe beam (400 nm, 80 fs) was then introduced at a delay to the initial pump beam.…”

“…We will focus our attention upon the experiment discussed in Ref. 5. This experiment provides a concrete example of an applicable laser plasma.…”

“…The current filaments may further evolve, via coalescence/tearing/screw instabilities, into current channels, [15][16][17] which further initiate filamentary magnetic structures. 5 Additionally, Weibel-like electromagnetic fields have been implicated in the mediation of astrophysical collisionless shocks in (initially) unmagnetized plasma media. [18][19][20][21][22][23] It is strongly believed that presently existing laser facilities, such as OMEGA/OMEGA EP (extended performance) and NIF, will eventually observe these Weibel-mediated shocks in the laboratory, i.e., to make a "gamma-ray burst in a lab."…”

“…[1][2][3][4][5] Understanding and controlling electromagnetic (EM) turbulence in these environments is critical to the studies in the fusion energy sciences, and for the inertial confinement concept, 6,7 in particular. Additionally, electromagnetic turbulence is a crucial aspect of numerous astrophysical systems such as gamma-ray bursts and supernova shocks.…”

Diffusion and radiation in magnetized collisionless plasmas with small-scale Whistler turbulence

“…Such turbulence is a common feature of astrophysical and space plasmas, e.g. in high-Mach-number collisionless shocks and in reconnection regions in weakly magnetized plasmas (Nishikawa et al 2003;Frederiksen et al 2004;Spitkovsky 2008;Medvedev 2009a;Sironi & Spitkovsky 2011Sironi, Spitkovsky & Arons 2013). Additionally, turbulent magnetic fields existing on 'sub-Larmor scales' play a critical role in laboratory plasmas; especially in high-intensity laser plasmas, as observed in experiments at the National Ignition Facility (NIF), OmegaEP, Hercules, Trident and others (Tatarakis et al 2003;Ren et al 2004;Mondal et al 2012;Huntington et al 2015).…”