Dipolar distribution generated by laser-induced plasma (LIP) in air in earliest instants

We present an experimental investigation of the electric field potential and magnetic field generated by a laser-induced plasma in air, for time delays 0 ⩽ t ⩽ 50 ns. The laser used is a Nd : YAG, λ = 1064 nm, 10 ns at FWHM, and the irradiance applied is I = (1011–1012) W cm−2. We find that the collective effect of the charges in the plasma form a dynamic dipole distribution aligned with the laser beam axis. This experimental result is explained based on the mobility of the electric charges detected by mapping near the plasma with a coaxial cable probe. Shadowgraphy and fast photography techniques show that the plasma ionization front advances asymmetrically and mostly toward the lens. The intrinsic dipole moment is estimated by applying an external electric field. The magnetic diagnostic shows the presence of a current aligned with the laser beam that gives rise to an azimuthal magnetic field, corroborating the observed dipolar configuration. (paper)