Measuring magnetic field generation in laser-heated gas jet experiments through the Biermann battery*

Magnetic fields are ubiquitous in our universe and play an important role in fusion plasmas by suppressing heat flow and perturbations. These fields can be generated in an otherwise unmagnetized plasma through the ‘Biermann battery’ mechanism when there is a curl in the electric field produced by the electron pressure gradient. However, the magnitude of Biermann-battery fields is expected to be overpredicted by magnetohydrodynamic (MHD) simulations, in part due to the use of thermal flux limiters, which are required to match experimentally observed temperature profiles but lead to artificially steepened temperature gradients.

Experiments were carried out to measure the magnetic field generated over a nanosecond timescale in a laser-heated gas jet with a controlled density gradient on the Vulcan laser. The magnitude of the self-generated magnetic field was measured by proton radiography. By comparing experimental results to both fluid and kinetic simulations, which more accurately solve for the heat flow, the validity of the use of flux-limited MHD in these regimes can be studied. This allows us to better understand magnetic field generation in fusion plasmas and the wider universe.