Investigation of currents accompanying optical breakdown in air near a conducting target

A probe method was used to investigate the spatial and temporal distribution of the electric currents in the vicinity of an air-breakdown plasma produced by CO₂ laser radiation of intensity 10⁷–10⁸ W/cm². In the optical detonation regime a system of currents in the form of a fountain appeared before the plasma became detached from the target surface. Electron currents flowed out of the target in the region of the focal spot and returned to the target outside this spot. The total current to a target having dimensions substantially exceeding those of the illuminated spot was equal to zero at each moment of time. It was discovered that on passing to a predetonation mechanism of the plasma development the current signal in the region of the illuminated spot acquired an oscillatory form, its duration being correlated with that of intense plasma luminescence. A mechanism is proposed for the formation of such signals, resulting from the development of turbulent motion in the laser plasma. Electric current signals were also recorded on arrival of gas shock waves at the probe. The experimental data obtained were explained qualitatively in terms of charge separation in plasma inhomogeneities and at the shock front, where gradients of the electron density and temperature existed.