Study of preheat effects in ND-laser irradiated finite Al disk targets

Energy streaming around and through an Al disk target irradiated with a Nd-glass laser delivering a 0.8-ns pulse is discussed as a function of target thickness, disk diameter, and laser irradiance from 5 x 1012 to 5 x 1014 W cm-2. Front and rear surface behaviors of the disk are studied by means of frame and streak shadowgraphy, short-time exposure interferometry, ion calorimetry, and charge collectors. Experimental data clearly show that preheat of the rear side of a 500-μm-diam disk is due to plasma encompassing the target and occurs even at laser irradiance as low as 3 x 1013 W cm-2. Moreover, it is seen on time-resolved photography of double-disk targets, that the second foil expands before being struck by the first disk (irradiated disk) at laser irradiance as low as 1013 W cm-2. This effect is ascribed to preheating by particle bombardment. Numerical simulations do not match the experimentally observed temporal behaviors for the 500-μm-diam disks even with some preheat included in the hydrodynamic code, but they agree quite closely with the experimental results when the irradiated disk diameter is much larger than the characteristic density gradient length