レーザによる光絶縁破壊プラズマの光学的測定

抄録

Experiments on induction and triggering of a long-gap discharge by a laser have attracted much attention in a fundamental study of the laser-induced lightning. A plasma channel generated by the optical breakdown of the atmosphere using a high power laser is often used as a guiding path of the discharge. However, many discrete plasma beads produced near the focal point become obstacles from the viewpoint of making a long, uniform and highly conductive plasma channel. The mechanism of generation of plasma beads and an interaction between incident laser light and plasma beads have not yet been well understood. It is important to clarify these phenomena in order to make a more suitable plasma channel for the laser-induced discharge.
To investigate the microscopic structure and the characteristics of the plasma beads produced optically by a YAG laser or a KrF excimer laser, we used a CCD camera to observe the scattering images of the incident laser due to plasma beads, and we measured the electron number density in the plasma using the Mach-Zehnder interferometer. It was found that incident laser light was strongly scattered by the plasma produced by itself, and the scattering process was dependent on the wavelength of the laser. An interferometer measurement showed that electron density in the plasma was larger than 10²⁴m^-3 at its early phase.