In recent years, significant progress has been made in the study of the dynamic effects of backward-ablation plumes generated by pulsed laser ablation of target materials, with applications emerging in fields such as laser manufacturing, laser-induced plasma spectroscopy, spacecraft attitude control, and active space debris removal. This technology utilizes high-energy laser pulses to irradiate the target surface, inducing the formation of a high-speed plasma plume. The interaction between the plume and the target surface generates a recoil force, enabling directional propulsion of the target material. Compared to continuous lasers, pulsed lasers, with their high power density, are more effective in exciting plasma shockwave effects, making them the core light source for laser ablation propulsion systems. The ablation efficiency of combined laser irradiation (using both pulsed and continuous lasers) is higher than that achieved by using either pulsed or continuous lasers alone. Current research on combined laser ablation primarily focuses on thermal effects, such as improving ablation efficiency and the removal rate of molten materials, while studies on the dynamic effects are relatively limited. Therefore, building on the findings of thermal effect studies on combined laser ablation of targets, this research applies the combined laser ablation method to investigate the dynamic effects, aiming to explore the underlying principles of the dynamic behavior in solid targets ablated by combined lasers. This paper takes aluminum alloy as the research subject and conducts simulation studies on the dynamic effects of combined pulsed/continuous laser ablation. It analyzes the relationship between the backward-ablation plume velocity generated by pulsed laser ablation and the laser energy, as well as the influence of parameters such as the time delay between the pulsed and continuous lasers and the spot size ratio on the dynamic effects of the target material. The study examines the behavior of the target material after ablation by nanosecond pulsed lasers and combined lasers, aiming to elucidate the underlying mechanisms.

The research focused on an aluminum alloy target with a side length of 10 mm and a thickness of 1 mm. A simulation model was established using a multiphysics simulation tool, wherein a two-dimensional model of the target was created, dividing the computational domain into two parts: the metal target and the air region above it. To investigate the enhancement effect and underlying mechanisms of combined pulsed/continuous laser ablation on the dynamics of the target, the study first examined the velocity variation of the backward-ablation plume generated by single-mode laser ablation. The target was irradiated with lasers of different energies (5 J to 8 J) to analyze the changes in backward-ablation plume velocity and its evolution process under different energy densities. Subsequently, the target was preheated by a continuous laser with a power of 1000 W and a spot radius of 1 cm, while simultaneously being ablated by pulsed lasers with energies ranging from 5 J to 8 J. The observations focused on comparing the improvement in the backward-ablation plume velocity and the evolution of the plume under the combined laser action. Finally, the study explored the effect of varying the spot size of the continuous laser on the preheating of the target and the enhancement of the backward-ablation plume.

The calculation results of the backward-ablation plume generated by pulsed laser ablation of aluminum alloy showed that after the laser acted on the target material, the surface material underwent phase change and generated a backward-ablation plume. During the pulsed laser action time, the backward-ablation plume velocity gradually increased, reaching the maximum at 10 ns. When the laser energy was 5 J, the maximum plume velocity was 1.15×10⁴ m/s (Fig.4). As the laser energy increased from 5 J to 8 J, the backward-ablation plume velocity also increased, showing a positive relationship between the laser energy and the plume velocity. When a continuous laser with a power of 1000 W and a spot radius of 1 cm was used to preheat the target material for 500 ms to 2000 ms, and then pulsed laser ablation was performed, the temperature changes of the target material under different action times were observed. It was observed that the longer the continuous laser acted, the more obvious the temperature rise of the target material (Fig.9). Under the combined laser action, the relationship between the peak velocity of the backward-ablation plume and the action time showed that compared with the single-pulse laser ablation, the peak velocity of the backward-ablation plume increased when the continuous laser acted for 500 ms to 2000 ms. From the velocity results, the maximum velocity increased by 13.7 %, 27.5 %, 39.7 %, and 46.7 % respectively (Table 3). When studying the ablation of target materials by continuous lasers with different power densities, when the spot radius of the continuous laser was 0.5 cm, the velocity increased the fastest and was the largest, with the maximum plume velocity being 17.29 km/s; while when the spot radius of the continuous laser was 1.5 cm, the maximum plume velocity was the smallest, being 12.07 km/s. When the spot radius of the continuous laser changed from 0.5 cm to 1.5 cm, the combination with a spot radius of 0.5 cm had the best effect on increasing the maximum backward-ablation plume velocity, and was significantly better than the other two groups. When the pulsed laser energy was 5 J, the continuous laser with a spot radius of 0.5 cm achieved improvements of 32 % and 43 % compared with spot radii of 1 cm and 1.5 cm, respectively.

In this study, a model of the dynamic effects of single-pulse and combined laser ablation of target materials is established. The velocity values of the backward-ablation plume and the ejection process are obtained through numerical simulation. The calculation results of the backward-ablation plume of aluminum alloy by pulsed laser show that the surface material of the target undergoes phase change to produce the backward-ablation plume after laser irradiation. During the pulsed laser irradiation time, the backward-ablation plume velocity gradually increases and reaches the maximum at 10 ns. When ablating aluminum alloy target with combined laser, the backward-ablation plume velocity is increased by 46.7 % compared with that by single-pulse laser when the continuous laser acts for 2000 ms. The synergy of continuous laser and pulsed laser is essentially the precise coupling of “stable thermal effect” and “instantaneous high-energy impact” in time. When ablating aluminum alloy target with combined laser, the continuous laser acts first for pre-ablation, preheating the target surface, increasing the target temperature and pre-treating the surface material. When the pulsed laser acts on the target, the target can reach the phase change condition faster, and vaporization and plasma generation occur. A larger proportion of the pulsed laser energy is used for vaporization and plasma generation of the target, and the backward-ablation plume velocity is also higher, achieving a stronger target recoil force through the law of conservation of momentum. Compared with the use of pulsed laser alone, the backward-ablation plume velocity produced by combined laser ablation of the target is higher.