Structure of a spark channel formed by optical breakdown of gases at atmospheric pressure in the caustic of an axicon

The structure of a laser spark and its dynamics during the early stage of formation of a continuous channel were investigated by recording the spatial, temporal, and spectral characteristics of the scattered heating radiation. A plasma formed in the spark channel had a quasiperiodic (in the longitudinal direction) structure with characteristic dimensions 0.1–0.15 mm. An analysis was made of various possible factors which could be responsible for such a structure. A model of the Fresnel reflection of light from a plasma boundary was used to estimate the electron density immediately after optical breakdown: This density was (0.7–1.4) X 10²⁰ cm ^– 3 in air, (4–9) X 10¹⁹ cm ^– 3 in argon and neon, and (5–8) X 10¹⁸ cm ^– 3 in helium.