  First-principles kinetic simulations are used to investigate magnetic field
generation processes in expanding ablated plasmas relevant to laser-driven
foils and hohlraums. In addition to Biermann-battery-generated magnetic fields,
strong filamentary magnetic filaments are found to grow in the corona of single
expanding plasma plumes; such filaments are observed to dominate Biermann
fields at sufficiently large focal radius, reaching saturation values of $\sim$
100 T at National Ignition Facility-like drive conditions. The filamentary
fields result from the ion Weibel instability driven by relative
counterstreaming between the ablated ions and a sparse background population,
which could be the result of a gas prefill in a hohlraum or laser pre-pulse.
The ion-Weibel instability is robust with the inclusion of collisions and grows
on a timescale of 100 ps, with a wavelength on the scale of 100-250 $\mu$m,
over a wide range of background population densities; the instability also
gives rise to coherent density oscillations. These results are of particular
interest to inertial confinement fusion experiments, where such field and
density perturbations can modify heat-transport as well as laser propagation
and absorption.
