Biermann battery effect, Weibel instability, and ionization-driven filamentation in picosecond, relativistic laser-produced gas jet plasmas

We have measured the magnitude and morphology of magnetic fields generated by Biermann battery, thermal Weibel instability, and ionization instability mechanisms in a less than 0.1 percent (~10²⁰ cm^-3) solid density plasma produced by a 2 picosecond (FWHM), relativistic intensity (a₀ > 1), CO₂ laser and N₂ gas jet targets. The maximum plasma density is overdense to stop the CO₂ laser, but is low enough to be probed using an 80fs 800nm probe laser pulse. The magnetic fields were diagnosed using a Faraday rotation polarimeter. We find that the Biermann fields arise in the underdense plasma before the CO₂ laser pulse exceeds critical power for self-focusing. Near the relativistic critical density, profile steepening and hole boring due to radiation pressure occur. In this region thermal Weibel instability magnetic fields appear due to the temperature anisotropy of laser-heated electrons. The electric field of the expanding sheath of hot electrons is sufficient to ionize the gas, producing a nearly hemispheric overdense plasma. Any perturbation of the sheath electric field is magnified in the plasma density because of the highly nonlinear dependence of the ionization rate on the electric field. Two-dimensional particle-in-cell simulations show similar density and magnetic field structures that are not observed when the laser pulse propagates in a preformed plasma.