Turbulent amplification of magnetic fields in laboratory laser-produced shock waves

Abstract

X-ray 1-3 and radio 4-6 observations of the supernova remnant Cassiopeia A reveal the presence of magnetic fields about 100 times stronger than those in the surrounding interstellar medium. Field coincident with the outer shock probably arises through a nonlinear feedback process involving cosmic rays 2,7,8 . The origin of the large magnetic field in the interior of the remnant is less clear but it is presumably stretched and amplified by turbulent motions. Turbulence may be generated by hydrodynamic instability at the contact discontinuity between the supernova ejecta and the circumstellar gas 9 . However, optical observations of Cassiopeia A indicate that the ejecta are interacting with a highly inhomogeneous, dense circumstellar cloud bank formed before the supernova explosion 10-12 . Here we investigate the possibility that turbulent amplification is induced when the outer shock overtakes dense clumps in the ambient medium 13-15 . We report laboratory experiments that indicate the magnetic field is amplified when the shock interacts with a plastic grid. We show that our experimental results can explain the observed synchrotron emission in the interior of the remnant. The experiment also provides a laboratory example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena.High-resolution X-ray images and radio polarization maps of Cassiopeia A show two distinct strong magnetic field regions [3][4][5][6]12 . Narrow X-ray filaments, a fraction of a parsec in width, are observed at the outer shock rim at a radius of about 2.5 pc. These structures are produced by synchrotron radiation from ultrarelativistic electrons (with teraelectronvolt energy) and can be explained by magnetic fields of the order of 100 µG or more 2,3 . The interior of the remnant contains a disordered shell (about 0.5 pc in width at a radius of 1.7 pc) of radio synchrotron emission by gigaelectronvolt electrons 4 . The inferred magnetic field in these radio knots is a few milligauss, about 100 times higher than expected from the standard shock compression of the interstellar medium 15 . Optical observations of Cassiopeia A show the presence of both rapidly moving (5,000-9,000 km s −1 ) and essentially stationary dense knots. Although the moving knots themselves have a high velocity, their overall pattern is nearly stationary 10 . This led to the suggestion 10 that a dense pre-existing inhomogeneous stationary cloud bank could be present. New rapidly moving knots predominantly appear at a position broadly coincident with the shell of bright radio emission 6 . Sizes of the observed small-scale features within the shell range from 0.01 to 0.1 pc arranged in larger patterns extending to 0.5-2 pc (ref. 16). Interaction between the ejecta and the cloud bank may excite the turbulence that amplifies the magnetic field and makes Cassiopeia A an exceptionally bright radio source 4 . The interaction is akin to the Rayleigh-Taylor instability otherwise proposed as a source of turbulenc...

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

“…3B. Translated into wavenumber spectrum, this spectrum is MðkÞ ≈ k −17=9 (see Supporting Information), substantially shallower than the low-Rm Golitsyn spectrum k −11=3 (29), which we observe in the case of no jet collision, so both less turbulence and lower Rm (10). The emergence of progressively shallower magnetic spectra is a sign of nonlinear field amplification, which is a precursor to turbulent dynamo (31).…”

“…Details are given in ref. 10. The initial (t < 100 ns) high-frequency noise due to the laser-plasma interaction with the target has been filtered.…”

Pressure-anisotropy-driven microturbulence and magnetic-field evolution in shearing, collisionless plasma

“…Besides being of astrophysical (origin of observed fields) and fundamental (what happens?) physical interest, it is likely soon to be attacked not just theoretically, but also experimentally, with the advent of laboratory experiments aiming to reproduce the plasma dynamo process (Spence et al 2009;Meinecke et al 2014Meinecke et al , 2015Plihon et al 2015;Forest et al 2015). The first demonstration of this process in a 3D kinetic numerical simulation by Rincon et al (2016) has indeed confirmed the ubiquitous appearance of mirror and firehose fluctuations, although a detailed investigation of the full multiscale problem remains computationally too intensive to be affordable.…”

Precision Mapping of Laser-Driven Magnetic Fields and Their Evolution in High-Energy-Density Plasmas

“…Much progress has been made in recent years on understanding the generation and transport of these fields in high-energy-density plasmas [12], motivated by problems in laboratory astrophysics [13], magnetic reconnection [14][15][16][17][18], hydrodynamic-instability growth [19], shock-wave dynamics [20][21][22], and inertial confinement fusion [23]. In these conditions, much attention has been given to understanding the Biermann battery mechanism [24] and the magnetic fields that can be generated at the surface of a laser-irradiated solid target [25].…”