(click on image to enlarge) Lead author Jena Meinecke working in the Vulcan TAW chamber, credit: STFC
A group led by the University of Oxford have used the Vulcan laser to explore turbulent amplification of magnetic fields in laser-produced shocks, believed to be the reason behind synchrotron emission from the remnants of the supernova Cassiopeia A. The paper,
published in Nature Physics (link opens in a new window) last week, describes how the high power laser system was used to mimic supernova blast waves crashing through the clumpy medium surrounding an exploding star.
Supernova explosions, triggered when the fuel within a star reignites or its core collapses, launch a detonation shock wave that sweeps through several light years of space from the exploding star in just a few hundred years. But not all such explosions are alike and some, such as Cassiopeia A which is 11,000 light years from the Earth, show puzzling irregular knots and twists in the radio synchrotron emission by GeV electrons. 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. Motivated by the need to understand the origin of this astrophysical observation the group designed an experiment to “focus on the magnetic field amplification in a clumpy medium by making in situ magnetic field measurements in the turbulent wake of a laser-produced shock in a plasma”.
(click on image to enlarge) Experimental configuration and transverse probe images, credit: Nature Physics 2014
Vulcan (link opens in a new window)’s nanosecond beamlines were used to deliver 300 J over a diameter of 300 um, to drive a shock wave from a carbon rod. Using interferometry and Schlieren probe beamlines they were able to carry out
in situ imaging of the propagating shock front as it expanded into a low pressure argon gas cloud and an induction probe was used to monitor the magnetic field variation with 10 ns temporal resolution. By placing a plastic grid in the propagation path, turbulent flow was induced into the shock front, as observed in the imaging probes, and magnetic field amplification was measured by the induction probe.
Experimental measurements were complimented by HELIOS and FLASH simulations studies, indicating that “a rotational flow, driven by a shock passing through a stationary density perturbation, is necessary to both amplify and sustain strong magnetic fields in an expanding magnetized plasma over distances corresponding to many times the scale of the initial density perturbation.”
The results help piece together a story for the creation and development of magnetic fields in our Universe, and provide the first experimental proof that turbulence amplifies magnetic fields in the tenuous interstellar plasma.
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