With the goal of achieving laser inertial confinement fusion (ICF), countries around the world have established a series of high-power laser facilities, among which typical representatives include the National Ignition Facility (NIF) in the United States, the Laser Megajoule (LMJ) Facility in France, and the ShenGuang (SG) Facility in China, etc. The NIF achieved its first successful ignition in December 2022, obtaining a target gain of 1.5 times, marking a significant advancement in the application of fusion energy. Due to the thermal effect of large-aperture chip amplifiers, the repetition rate of such devices is extremely low, with intervals between each shot being several hours, which cannot meet the demands of future fusion power stations. To solve this issue and increase the repetition rate of large-scale high-power laser devices, researchers have proposed various configurations of high-power lasers devices including gas-cooled multi-slabs designs and activated mirror designs, and have built multiple systems, making continuous new progress in output power and repetition rate. Shortening the thermal recovery time of the SG-II upgrade device and developing a high-repetition-rate high-energy laser amplifier have great significance of achieving inertial fusion energy. In this paper, a new upgrade design of SG-II is proposed. With the new design, the thermal recovery time is greatly reduced and the laser gain remains the same. Another advantage of the new design is the need of nitrogen for cooling is decreased as well.

The basic idea of the new design of the SG-II main laser amplifier is combining the gas-cooled multi-slab configuration with the existing chip amplifiers in the SG-II. Without changing the structure of the SG-II upgrade device and utilizing the existing nitrogen purging system of the main amplifier, a group of thinner neodymium glass slabs is set to replace the existing thicker single neodymium glass. Nitrogen is used for purging and cooling between the slabs. The structure is showed in Fig. 7. When performing energy storage extraction and laser amplification, the laser beam passes through the multiple neodymium glass slab groups. While achieving the same gain, the multi-slab gas-cooled configuration adopted by the slab groups effectively improves the cooling efficiency and reduces the thermal recovery time. Based on the configuration of the amplifier, an ANSYS instantaneous analysis model is established to simulate and analyze the cooling process of the neodymium glass in the amplifier. To achieve the simulation analysis of the thermal recovery process, the required boundary conditions include the internal heat source of neodymium glass (thermal deposition distribution), heat dissipation conditions (surface heat exchange coefficient), and solid mechanical constraints (side fixation). The finite element analysis software can be used to obtain the temperature, stress, and deformation distributions inside the neodymium glass. Then, through calculation, the thermal wavefront of the neodymium glass can be obtained. The calculation model is shown in Fig. 2.

To obtain the most efficient design, different slab group structures, different purging time, and different nitrogen flow rates under the same configuration are compared with those in the existing main amplifier in the simulation experiment. From the comparison in Figs. 8(a) and (b), the slab group with 4 pieces of 10 mm thick neodymium glass has the highest efficiency. The thermal recovery time of temperature and wavefront is 20.7% and 17.1% of those of the existing structure, respectively. From the comparison in Figs. 9(a) and (b), when the slab group has a thickness of 10 mm and 4 neodymium glasses, the thermal recovery time for the two methods of continuous cooling and intermittent purging is basically the same. This result indicates that the internal thermal recovery is nearly achieved within two cooling cycles due to the advantage of the short heat conduction path. After ensuring the structure of slab group, the other key factor influencing the thermal recovery time is the flow of nitrogen. The results from Fig. 10, Fig. 11(a), and Fig. 11(b) show that increasing the nitrogen flow rate has a marginal effect on the improvement of cooling efficiency. Further increasing the nitrogen flow rate on the existing basis of 230 L/min has a relatively small effect on shortening the thermal recovery time. Based on the analysis above, the total nitrogen consumption is approximately 42.5% of that of the existing structure.

A new optimized design for the main amplifiers in the SG-II is proposed and its effectiveness is verified through simulation experiments. By replacing the existing thicker neodymium glass slabs by slab groups containing multiple thinner neodymium glass slabs, the thermal recovery time is reduced significantly. Through simulation analysis, the influence of slab group design, purging time and nitrogen flow rate on the thermal recovery time is analyzed and compared. The results show that for a group of 4 neodymium glass slabs with a thickness of 10 mm, the design in which the nitrogen flow rate in the internal gap of the slab group remains consistent with that (230 L/min) on the outside has obvious advantages. When this slab group is adopted, the internal temperature difference and the thermal recovery time of the wavefront are reduced to 20.7% and 17.1% of those of the existing structure, respectively, and within a single thermal recovery cycle, the nitrogen usage is reduced to approximately 42.5% of the existing structure. This method provides a guiding approach for improving the repetition rate of the existing large-scale laser drivers and offers an optimization idea for the thermal recovery time for the subsequent construction of new devices.