Influence of laser intensity and focal spot size on laser impulse in metal foils and transparent tamper targets

High-power laser ablation is used to study impulse and thermomechanical shock generation. Laser intensity and focal spot size play important roles in determining laser impulse, ablation pressure, and shock generation. Furthermore, confined laser ablation in the presence of transparent tamper on the target surface significantly enhances laser impulse. In this work, the effects of laser intensity and focal spot size on ablation pressure, shock generation, plasma density, and temperature in laser-produced plasmas from different targets are investigated. Plasma plumes were generated by focusing 1064 nm, 10 ns pulses from an Nd:YAG laser onto aluminum (Al) and titanium (Ti) metal foils, as well as transparent fused silica, lithium fluoride, and sapphire. Experiments were conducted over a laser-intensity range of approximately ∼1 × 109 − 1 × 1011 W/cm2, with focal spot sizes of 100 and 400 μm. For a fixed focal spot size and target material, increasing the laser intensity produced 1.5–2× larger plasma expansion and increased plasma density. Electron temperatures measured using EUV spectroscopy also increased with laser intensity and were higher for Ti than for Al. A larger focal spot size led to higher plasma density under otherwise identical laser-irradiation conditions, although the corresponding changes in electron temperature remained within experimental uncertainty. Plasma density and temperature measurements were benchmarked against FLASH radiation-hydrodynamics simulations under similar conditions. The simulated ablation pressure and shock-front depth also showed increasing trends with laser intensity, consistent with the density and temperature results. The electron temperatures and ablation pressures were further compared with predictions from Mora’s model at three intensities.