Inertial confinement fusion (ICF) is the primary approach for achieving controlled nuclear fusion. To enhance the efficiency of laser fusion energy and attain higher yields, Tabak proposed a fast-ignition scheme for ICF. A critical challenge in fast-ignition fusion schemes is precisely guiding multiple high-energy picosecond laser beams onto the target. Because the bottom circle of the target cone measures only tens of micrometers, an error of a few micrometers or preheating the cryogenic target will result in a failed ignition experiment. Consequently, the precision of beam?target coupling directly determines the result of the ICF experiment. This study presents a novel design for a target-alignment system for PW laser beams. The laser beams and target image are separated by a beam splitter; therefore, this design inhibits the direct laser irradiation of the cryogenic target. However, the alignment system enables high-precision beam–target coupling without direct laser irradiation, which benefits fast-ignition-scheme experiments.

Based on the laser-beam layout and the requirement of beam?target coupling in a fast-ignition scheme, the structure of the target-alignment system of picosecond PW laser beams was designed in this study; a schematic diagram of the system is shown in Figure 1. Based on the actual situation of beam?target coupling, the target-aiming system was constructed on an offline experimental platform; subsequently, the resolution test target was used to verify the imaging resolution of the system. A beam?target coupling experiment was conducted, and the beam–target coupling accuracy of this system was measured through multiple experiments. The Zemax software was used to optimize the imaging simulation of the target, and the resolution experiment was simulated and analyzed. Subsequently, the factor contributing to the low resolution was determined, and the imaging mirror set was designed and optimized to further improve the imaging quality. Finally, a program was designed to identify the positions of the target and focal spot in real time, thus improving the accuracy and efficiency of the beam?target coupling.

Results and discussions The resolution-verification experiment performed on this system shows that the initial plain plate splitter introduces severe aberration [Figs. 4(a) and (b)], which was solved after performing optimization using a cube plate splitter [Figs. 4(c) and (d)]. Further use of aspheric mirror improves the resolution, and the final resolution of the target in the experiment is 90.5 lp/mm (11.04 μm) in a 5 mm×5 mm field-of-view [Figs. 4(e) and (f)]. Figure 5 shows the beam?target coupling process using this system. The results of multiple experiments show that the error in the x-direction is slightly larger than that in the y-direction, which may be caused by the system asymmetry. Finally, the average position error of the beam?target coupling system is 4.32 μm (Table 1), which is lower than that of the existing beam?target coupling scheme and satisfies the high-precision beam?target coupling requirements. Zemax was used to verify the experimental results of target imaging, and the simulation analysis shows that the lens can be further optimized. Therefore, the lens was optimized, and the results are shown in Figure 7. The MTF (modulation transfer function) curve and simulated imaging results indicate that the resolution is greater than 5 μm in a 10 mm×10 mm field-of-view. Finally, a real-time recognition program was designed to identify the positions of the focal spot and target (Fig. 2), thus reducing the error of subjective judgment and improving the efficiency of beam?target coupling. Currently, this system can be further optimized. The mirror set optimized using Zemax has neither been processed nor adjusted, and further measurements of the target-imaging effect will be performed in subsequent studies.

This paper introduces an innovative target-alignment system for picosecond PW laser beams. To satisfy the requirements of cryogenic target coupling in an ICF fast-ignition experiment, beam?target coupling was realized via non-laser direct irradiation, which offers high levels of resolution, precision, and efficiency. An image-recognition program based on beam?target coupling was developed to avoid errors caused by manual judgment and to improve the efficiency of beam?target coupling. An offline verification experiment was performed, where the target resolution is 90.5 lp/mm (11.04 μm) in a 5 mm×5 mm field-of-view. Multiple experiments show that the average position error of beam?target coupling is 4.32 μm, which is lower than that of the existing remote-observation beam?target coupling scheme, which features tens of microns of error. Optical optimization was performed via simulation. After the optimization, the imaging resolution of the target is expected to be greater than 5 μm in a 5 mm×5 mm field-of-view. This system provides a high-precision solution for the picosecond laser-beam coupling of a cryogenic target and presents wide application prospects in future large-scale picosecond laser-beam experiments.