An experimental investigation of refraction-induced distortions in harmonic-light images of laser-produced plasmas

In this paper the results of a simple experiment designed to study refraction-induced distortion of images of narrow-band-light-emitting regions in a laser-produced plasma are described. Source regions were formed by relaying the image of a backlit pinhole array into the plasma (25 μm-diam. pinhole; 30-ps, 0.25-μm backlighter beam). Images of these source regions were recorded on film and compared with and without plasma present. Refraction effects, evidenced by translation of the apparent pinhole locations and blurring of the pinhole spots, are compared for different plasma sizes and for different delays of the backlighter pulse with respect to the plasma-production laser pulse. These results were then compared with expectations based on numerical raytraces through a plasma-density profile derived from a hydrodynamic simulation of our exploding-foil plasma. The significance of these results in the imaging of harmonic and half-harmonic light as a plasma-density diagnostic is discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48