Interferometric investigation of sharp directional jets in an expanding laser plasma

The interaction of high-intensity picosecond lasers with matter has attracted great interest because of its potential as a source of picosecond x-ray pulses, an application for acceleration of relativistic particles by laser light, as well as for laser fusion. Despite a relatively large amount of investigations of laser-plasma interaction there is a lack of data on the spatial and temporal charge density distribution at focus of an intense laser pulse during its propagation in plasma. In this paper we report direct measurement of density profiles in the focal region of the ps-laser beam by means of transverse holographic interferometry and transverse absorption photography. A high intensity IR pulse (60 mJ, 100 ps, 1064 nm) from Nd:YAG laser was focused by a 25 cm focal length lens to a maximum intensity of 5·1013 W/cm2. The laser beam (s-polarized) was incident normal on the planar Al target in air. The target was placed in one arm of Michelson interferometer. The probing laser beam at second harmonic (Λ = 532 nm) was optically delayed with respect to the ignition beam from - 1.3 ns to 10 ns. Target imaging and the focal region of the laser pulse were achieved with a magnification optics (x125) and a 10 run interference filter centered at Λ = 532 run to reduce the light emitted by the laser-produced plasmas. The charge-coupled-device camera (CCD) images were digitised with 8 bit depth with a 512x512 pixel frame. The distributions of interference phase was determinate by linear FFT algorithm. When using only one mirror this arrangement was used for producing transverse absorption pictures on a CCD camera