An inertial confinement fusion (ICF) high-power laser driver with a high-quality near-field distribution is key to enhancing the output performance of a facility. Meanwhile, a high-quality beam with a uniform intensity distribution is beneficial for the protection of the subsequent optics and reduces the operating costs of the facility. Furthermore, it can satisfy the long-term high-fluence operation requirements of a system and improve the frequency-conversion efficiency and beam focus. Therefore, assessment methods should be used to evaluate the quality of near-field beams to provide early warning to operators. However, the current methods for evaluating near-field beam quality are incomplete and cannot analyze status changes and the details of near-field beam quality under the normal operating conditions of a facility. Additionally, the multiple outputs of high-power laser facilities place stringent requirements on real-time analysis; therefore, a multidirectional assessment of the facility operating status must be provided to the operators. This study may provide a theoretical basis and experimental support for the safe operation of the SG-II UP grade facility under high-load irradiation, thus facilitating timely maintenance and adjustments to the operating strategy.

Domain evaluation and distribution statistics were proposed to analyze the fundamental frequency near-field status of the SG-Ⅱ upgrade laser facility in our study. First, the probability density function of the near-field fluence distribution (Rician distribution) and its approximate expression (Gaussian distribution) were obtained using statistical optics theory, which was used to analyze the effects of high and low fluences in the near-field distribution on beam quality. We solved for the noise fluence, signal fluence, and near-field distribution signal to noise ratio, which were used to reflect the magnitude of and variation in random perturbations in the near-field distribution. Subsequently, local contrast and local modulation were proposed to further analyze the local features and variations in the near-field distribution using image-segmentation processing algorithms. Next, statistical methods and threshold labelling were used to analyze the accumulation and evolution of high-fluence points in the local regions. Finally, an exponential model was used to describe the statistical relationship between the high-fluence points and the contrast of the near-field beam. Additionally, we discussed the possible reasons for the anomalous near-field distribution and provided recommendations for the operation and maintenance of the facility.

Signal and noise fluences can be used to describe the ideal beam distribution and perturbation [Figs. 4(a) and 4(b)]. The signal to noise ratio (SNR) of the near-field distribution reflects the degree to which the beam is affected by perturbations [Fig. 4(c)]. The tail dragging and bias of the probability density function of the near-field fluence distribution reflects its overall distribution characteristics and the reasons for the changes observed (Fig. 6). It provides early warning to the operator such that the operator would verify the effectiveness of gain-inhomogeneity compensation in the pre-amplifier and main-amplifier system, the pointing stability of the pre-amplifier system, and the optics damage. When the local areas of the near-field beam are abnormal, the local contrast and local modulation are significantly higher than those in other cases, thus enabling the identification of anomalous regions (Figs. 8 and 9). However, the status evolution of the near-field can be analyzed during the operation of the facility and operators can perform the necessary pre-shielding measures in a timely manner (Fig. 13). Additionally, we investigate the evolution of high-fluence points using fluence statistics from the near-field beam to identify regions where damage is suspected to occur (Fig. 14). An exponential model between the high-fluence points and SNR suggests a control requirement for contrast; when the number of high-fluence pixel points increases as the SNR decreases, the contrast C is approximately 0.16 (Fig. 16). Using the historical near-field data of the SG-II UP grade facility, this study provides a theoretical basis and experimental support for the safe operation of high-power laser drivers under high-load irradiation, expands the dimensions of near-field status evaluation, and provides guidance for laser-facility maintenance.

In the present study, based on the existing near-field beam-quality analysis methods, domain evaluation, and distribution statistics, we evaluated the near-field beam quality of each shot in terms of multiple aspects during the operation of a high-power laser facility to understand the possible issues of a facility and provide the corresponding maintenance suggestions. Contrast and modulation can be used to evaluate the safety of facility operations at a global level. Based on an image-segmentation processing algorithm, we used the local contrast for near-field beam uniformity analysis, whereas local modulation was used for emphasis and optics damage analysis, which revealed the occurrence of abnormalities and areas in the operation, thus facilitating the timely implementation of maintenance measures. Meanwhile, the fluence-point statistics was used to comprehensively monitor the accumulation of abnormal regions in the near-field beam and any changes. For the design and operation of a facility, high-order contrast and near-field intensity entropy can be used to define the emergence of small-scale self-focusing and provide the B-integral criteria. The PSD curve was used to analyze the development of near-field modulation, optics damage, and small-scale self-focusing in the frequency domain. Based on the theory of statistical optics, the signal fluence, noise fluence, and signal to noise ratio were used to present the deterioration of the near-field distribution more intuitively. The probability-distribution-function model was used to analyze the ratio of high and low fluences in the near field to provide maximum support for usable data. Our study show that these methods can provide a theoretical basis and experimental support for the safe operation of the SG-II UP grade facility under high-load irradiation, expand the dimensions of near-field status evaluation, and guide facility maintenance and operation strategy adjustments.