Evidence for plasma jet formation in long-pulse laser irradiation of thin planar Al targets

Thin planar aluminum targets were irradiated with 8-nsec-duration low-intensity (∼3×1013 W/cm2) glass laser pulses in order to analyze the plasma expansion from both the front and back-target surfaces. An array of vacuum-ion calorimeters and Faraday cup charge collectors was used to determine the conversion from incident laser energy to plasma kinetic energy and to determine the ion mass balance of the laser-produced plasma. Our experimental results do not agree with the ablation structure predicted by simple models or 1-D numerical simulations. The experimental results and 2-D calculations indicate that a jet of plasma forms on the back target surface and ablative acceleration of these thin-foil targets does not occur.