Deep-incident-angle performance of pulsed laser propulsion on aluminum

• The effects of pulsed laser ablation incident angle on impulse were investigated. • Pulsed laser ablation experiments were performed with various incident angles. • The correlation between surface morphology and impulse vector was also determined. In practical applications of laser ablation propulsion in space, the laser beam is expected to be at an oblique angle relative to the surface of the irradiated object. Several studies have investigated the tendency of the impulse vector generated by repeated oblique laser pulse irradiation at the same point to shift from the direction perpendicular to the irradiated surface toward the beam incident direction. Previous studies suggested that this phenomenon is related to surface structures in the order of tens of micrometers, caused by repeated irradiation at the same spot. However, the details of the dependence of the impulse-vector characteristics on the incidence angle and surface structure remain unclear. In this study, the ablation of aluminum was performed using an Nd:YAG laser with a wavelength of 1064 nm and pulse width of 9 ± 2 ns under fluence conditions of 10 J/cm 2 . The impulse vector was measured with respect to variations in the beam incident angle (up to 70°) and cumulative number of irradiated laser pulses (up to 1000). Additionally, the surface shape of the ablation crater formed under the same conditions was measured. In the impulse measurement, shifts in the impulse vector that increased with the number of irradiations were observed at incident angles between 50° and 70°, with a maximum shift of approximately 20° observed at an incident angle of 70° with 1000 pulses. A periodic structure with directional alignment perpendicular to the beam incident surface developed on the crater surface after a cumulative irradiation of 100 to 200 pulses when ablation was performed with beam incident angles between 55° and 70°. In this structure, an approximately constant relationship was observed in the ratio between height and width. When analyzing the relationship between the impulse and structure, a correlation was found between the amount of impulse-vector directional change and the scale length of the structure. These findings provide insights into impulse-vector control in laser ablation propulsion, with potential expansion to the field of material processing.