Intergalactic plasmas in the lab

Magnetic fields in intergalactic space are stronger than one might expect. At up to tens of microgauss, they carry a significant fraction of the energy of the rarefied plasmas they pervade. (See the article by Philipp Kronberg, Physics Today, December 2002, page 40 .) The origin of that strength is largely a mystery. Now, capitalizing on the scale invariance of the relevant hydrodynamic equations, Jena Meinecke , Gianluca Gregori , and their collaborators have designed a lab experiment to mimic a collision of two galaxy clusters and the magnetic fields that the collision generates. Working at the Rutherford Appleton Laboratory’s Central Laser Facility , the researchers focused 240 J of laser energy (represented by the green triangles in the figure) onto each of two carbon foils to create a pair of opposing plasma jets. The jets collide 800 ns after the laser shot, as shown in the top panel; within 700 ns after that—equivalent to about 350 million years in an astrophysical system—a region of turbulence develops, as shown in the bottom panel. Prior to the jet collision, magnetic fields arise via the Biermann battery mechanism: Temperature and pressure gradients pointing in different directions generate an electric current, which creates a magnetic field. The turbulence then amplifies those fields in a possible precursor to a so-called turbulent dynamo—one favorite mechanism for modeling the growth of intergalactic fields. (J. Meinecke et al., Proc. Natl. Acad. Sci. USA 112, 8211, 2015 .)