Impulse of planar and sphere target by nanosecond laser ablation in a large beam spot
Weijing Zhou, Hao Chang, Jifei Ye, Nanlei Li
Abstract
Abstract We investigated an ablation impulse irradiated by a nanosecond laser with an Al planar and sphere target in a large beam spot. A torsion pendulum was developed to investigate single pulsed laser ablation impulses. A beam spot size of about 1 mm diameter was selected as the large spot size because investigations have proved that a beam spot size less than 420 μ m satisfied a one-dimensional plume expansion model. The results show that the minimum laser fluence corresponding to an observable ablation impulse of a planar target is about 3 J cm −2 , and the optimum laser fluence corresponding to the highest impulse coupling coefficient is about 10 J cm −2 for both large and small beam spots. Our investigations prove that these two parameters are independent of the beam spot size. The impulse investigation of a sphere-shaped target reveals that its overall impulse is lower than that of a planar one irradiated by a large laser beam spot in high laser fluence regimes, especially when a strong plasma plume event is happening. This phenomenon may be caused by different plasma plume propagation directions due to a non-flat surface of the sphere targets because the plume expansion by nanosecond laser ablation is mainly normal to the local surface.