Dipolar field and plasma expansion at the onset of laser-induced breakdown in a uniform dc field

An experimental investigation into the initial phases of laser-induced breakdown in air, with and without an external field, is presented. The plasma is produced by focusing the light of a Nd : YAG laser between two parallel plates connected to a dc bias supply. The diagnostics employed included fast photography and electric field measurements with a D-dot field probe. It is found that the laser power threshold required to initiate breakdown increases when an external field transverse to the laser beam is applied; the effect is not observed when the field is parallel to the beam. Measurements performed with a field probe show that the plasma produces a dipolar electric field that is proportional to the strength of the bias. The dipole is caused by charge redistribution over the plasma surface rather than by charge creation at separate points. The estimated plasma-polarizability coefficient associated with the dipole is a function of the beam energy and focal lengths employed only.