Use of the wave-front reversal effect to investigate a jet formed by focusing laser radiation on a plane target

Wave-front reversal produced by the Brillouin scattering was used to measure the intensity distribution in the focal spot of a neodymium laser (τ_L ~2.5 nsec, E_L ~10 J) beam sharply focused on a plane target in vacuum. It was found that 50% of the energy was contained in a circle of diameter ~30 μ. Photographs of the laser jet were obtained in the light of individual spectral lines of multiply charged ions of C, Fe, Ni, and Cu. The region where the ions existed near the target had a much smaller diameter than the transverse spherical aberration of the focusing lens. There was emission from CuXXIII ions over a region of diameter ~30 μ. A strong increase was observed in the angle at which the ions were ejected compared with the case of a normal focusing lens without wavefront reversal. Compensation of the aberrations of the focusing lens was observed.