Structure and properties of an optical breakdown plasma in a CO2 laser beam near a target surface

An experimental investigation was made of a surface optical discharge plasma in a CO₂ laser beam. The spatial temperature distributions were measured in the optical discharge plasma and on the surface of the irradiated target in the presence of the optical discharge. It was found that the optical discharge in the target vapor was localized in the caustic of the focused beam, while that in the gas surrounding the target moved a considerable distance from the target. This was due to the difference between their energy balance conditions. An optical discharge in the target vapor can increase substantially the power flux absorbed by the target at the center of the focal spot, while one in the gas transfers the power absorbed from the laser beam to an area which is considerably larger than that of the spot. The influence of the laser beam focusing conditions on the efficiency of the localized interaction with the target is indicated and methods are proposed for increasing this efficiency in the presence of plasma.