Magnetic field generation: the Biermann battery in action
Magnetic fields are ubiquitous throughout the universe, but their origin is still an open question. One of the most common explanations to this problem is that weak seed magnetic fields are amplified via turbulent mixing. Misaligned temperature and density gradients generated in shocks can provide this seed magnetic field via the Biermann battery mechanism.
This mechanism of magnetic field generation also plays an important role in laser-solid experiments. Lasers can ionize a solid film of metal or plastic and generate a temperature gradient with a maximum temperature in the center of the beam. The plasma has a density gradient with a decreasing density away from the target. As the newly ionized plasma expands, these perpendicular gradients drive the Biermann battery and can generate magnetic fields as high as megagauss. These fields have been detecte, for example, in experiments using the Omega EP lasers in Rochester, as well as in experiments performed at Rutherford Appleton Lab (RAL).
This movie depicts the expansion of a plasma and the generation of magnetic fields via the Biermann battery. The contours are isosurfaces of the density which is decreasing in the center as the plasma flows outward. Although the plasma does not move significantly, the density gradient downward, and the temperature gradient (not shown) pointing inward generate a clear toroidal magnetic field. It becomes evident that this field reaches an amplitude strong enough to slow down the upper plasma while the plasma closer to the (z=0) surface continues to expand without this constraint.
More information here.