Due to the insufficient understanding of the complex physical processes during fusion, research on inertial confinement fusion (ICF) has reached a plateau after decades of rapid development. Therefore, the development of diagnostic techniques under extreme transient conditions is crucial to advance the research on ICF. Large amounts of plasma X-rays are generated in the fusion process. Both the continuum and the line spectra of the X-rays contain a large amount of information about the plasma parameters and the fusion process, including the plasma temperature, density, spatial scale, etc. The diagnosis of high-temperature plasma X-rays is extremely important in the diagnosis of ICF. X-ray spherically curved crystal imaging with monochromatic, high resolution, and large field of view is one of the key diagnostic techniques. Spherically curved crystal imaging technology can achieve one-dimensional spectral resolution imaging, two-dimensional monochromatic self-illumination imaging, and two-dimensional monochromatic backlight imaging. Compared with self-illumination imaging, spherically curved crystal backlight imaging has lower requirements for backlight radiation power and can achieve higher spatial resolution and narrower spectral resolution. The resolution ability of the spherically curved crystal backlight imaging system is an important indicator of this diagnostic method. However, at present, there are still few studies on the spherically curved crystal backlight imaging system in China, and compared with the international best resolution ability, it also needs to be further improved. Therefore, in this paper, the imaging performance of the spherically curved crystal backlight imaging system was analyzed in detail, and the experimental test was carried out on SG-Ⅱ high-power laser device. It is hoped that the resolution of the system can be further improved by the reasonable design of experimental parameters so that the spherically curved crystal backlight imaging system can be used for the precision diagnosis of laser plasma in ICF.

To explore the imaging performance of the spherically curved crystal backlight imaging system, we focused on both theoretical and experimental aspects. In theory, the influences of system parameters, aberration, small error of the bending crystal radius, and other factors on imaging performance were analyzed. The variation curves of performance parameters with each influencing factor were plotted, which provided theoretical support for the analysis of subsequent experimental results. Based on the theoretical analysis, the specific system parameters were determined. The X-ray source was 360 mm away from the curved crystal, and the imaging object was 230 mm away from the curved crystal; the detector was placed about 3700 mm away from the curved crystal, and the size of the backlight was about 200 μm. Then the imaging performance of the imaging system was tested by the SG-Ⅱ high-power laser device. The Heα line of 1.865 keV excited by laser irradiation of the Si target was selected as the backlight source, with a wavelength of 0.665 nm. The quartz crystal, pressed into a spherically curved crystal with a radius of curvature of 433 mm, was the core imaging element. The Image Plate was chosen as the detector for the backlight imaging and system commissioning phase. The Andor X-ray CCD, with its own higher resolution, was chosen as the detector for the formal imaging phase. In order to check the imaging performance of the imaging system, a Cu grid with 1500 meshes was first imaged, and a grid image with a resolution of approximately 4.8 μm was obtained. In order to further validate the imaging performance of the imaging system, cicada wing specimens were imaged, and clear images of cicada wing specimens were obtained using numerical methods.

Firstly, there were inevitable aberrations in the imaging system which could affect the image quality of the imaging system. Spherical aberration, coma, dispersion, and aberrations in spherically curved crystal backlight imaging systems were theoretically analyzed, and appropriate aberration correction methods were given. The effect of the spherical bend radius error on imaging performance was then investigated, and the equations for the relative rate of change of magnification and spatial resolution caused by the bend radius error were derived. The relative rate of change curves was plotted (Fig. 5). Secondly, experimentally clear grid images were obtained by using a spherically curved crystal imaging system, and the resolution of the grid images was obtained by edge function fitting down to 4.8 μm (Fig. 6). In order to obtain more accurate statistical results, the best imaging position in the grid image was calibrated, and the variation of resolution in the meridional and sagittal directions with deviation from the best imaging position was analyzed (Fig. 7). Within a range of 200 μm from the optimum imaging position, resolutions of down to 4.7 μm in the meridional direction and 4.83 μm in the sagittal direction could be achieved. Even at the edges, where there was a lot of distortion and noise, it was possible to achieve a resolution of 9.95 μm. Finally, in order to validate the imaging performance of the imaging system on biological samples, cicada wing specimens were imaged. However, due to small deviations in the collimation process, the signal-to-noise ratio of the images was relatively low. Data processing of the images using the Retinex algorithm improved the contrast of the images, resulting in clear images of biological specimens (Fig. 9), which proved that the imaging system could also be used for the diagnosis of biological specimens.

We presented a theoretical analysis of the imaging performance of the spherically curved crystal backlight imaging system and discussed the effects of system parameters, aberrations, small errors of the bending crystal radius, and other factors on the imaging performance. A theoretical basis was provided for the subsequent design of experimental parameters and analysis of experimental results. In the experiments, the X-ray spherically curved crystal backlight imaging system was constructed by using a quartz crystal. X-ray CCD was chosen as the detector. The experiments were performed by using SG-Ⅱ high-power laser device. Clear images were obtained for the metal grid and biological samples respectively. Further analysis of the grid images showed that the high spatial resolution of about 4.8 μm could be maintained. Therefore, the spherically curved crystal backlight imaging system is capable of high-resolution imaging over a large field of view and can be used for the precision diagnosis of high-temperature plasma in ICF. In further experiments, we will reduce the effect of noises on an image by adding filters and collimation modules or reducing the size of the backlight to improve the resolution of the spherically curved crystal backlight imaging system.