Ionization-seeded current filamentation in expanding plasma sheaths

We report a seeding mechanism for the current filamentation instability that was identified using an experimental platform where a relativistically intense 2-ps (full width at half maximum) long-wavelength infrared pump laser both produces and interacts with a plasma that is overdense to the pump yet transparent to a near-infrared probe laser. This platform enables simultaneous high spatial resolution measurements of the self-generated magnetic filaments using Faraday rotation polarimetry and density filaments using interferometry. Supporting simulations show that the highly nonlinear dependence of the ionization rate on the local sheath electric field produced by pump-laser-heated hot electrons gives rise to 20-µm-scale filaments of cold electrons that form a return current, generating local azimuthal magnetic fields that can last for over 100 picoseconds.