Electric fields of a laser spark produced by radiation with various parameters

An experimental investigation was made of the spatiotemporal structure of electric fields of a laser spark produced by radiation of different wavelengths (0.53, 1.06, and 10.6 μm), pulse durations (microsecond, nanosecond, and subnanosecond), and intensities (I = 10⁹ – 10¹³ W cm^–2. The distribution of the fields near a spark corresponded to a quadrupole configuration of charges in a plasma, consisting of two oppositely directed dipoles in the region of the leading and trailing plasma fronts. The relationships governing the spatial evolution of the quadrupole potential structure were established as a function of the pulse duration and radiation intensity. In particular, the conditions for the relative dominance and degeneracy of the component dipoles were found. Typical dipole moments were determined for various radiation parameters. The amplitude of the fields outside the spark was influenced mainly by a geometric factor related to the transverse dimensions of the plasma. Moreover, at high intensities (I ≥ 10¹³ W cm^–2) this influence was supplemented by a contribution of a nonlinear mechanism dependent on the radiation wavelength and causing considerable changes in the amplitude and structure of the fields.