Measuring simultaneous Biermann battery and Weibel instability generated magnetic fields*

The origin of magnetic fields in initially unmagnetized plasmas is critical to our understanding of both astrophysical and laboratory plasmas. We have performed an experiment at the Brookhaven National Laboratory Accelerator Test Facility to probe the magnetic field structure in the plasma formed by a relativistic short pulse CO₂ laser ionizing a gas jet target with densities exceeding the critical density. This interaction creates large orthogonal density and temperature gradients that generate Biermann battery magnetic fields, while also developing anisotropy along the temperature gradient that allows the Weibel instability to grow. These two mechanisms of self-generating magnetic fields could provide the seed fields that are then amplified by turbulent dynamo effect to create the magnetic fields observed in galaxies. We used an ultrashort Ti:Sapphire laser probe traveling perpendicular to CO₂ to measure azimuthal magnetic fields via Faraday rotation using an imaging polarimeter to distinguish the filamentary Weibel magnetic fields from macroscopic Biermann fields, exploring the regimes in which either mechanism dominates.