In laser inertial confinement fusion, high-precision X-ray imaging diagnostic instrument has become the key to observing the implosion process and quantitatively inverting the implosion parameters. It plays an important role in the research on irradiation uniformity, implosion compression symmetry, hydrodynamic instability, and fuel mixing. Rayleigh-Taylor (RT) instability during implosion is a non-linear high-gain transient process, which requires high spatial resolution, large effective field of view, and high temporal resolution of the diagnostic system. RT instability experiments are typically performed using plan-modulated samples with low amplitude and high spatial frequency sine periods. Higher spatial resolution helps reveal early phenomena of hydrodynamic instability. Currently, diagnostic X-ray imaging equipment widely used in diagnostic science mainly includes pinhole camera, Kirkpatrick-Baez (KB) microscope, Wolter microscope, and spherically bent crystal. Affected by initial configuration and optical processing capabilities, the optimum spatial resolution is 3-5 μm, and the effective field of view is limited to the order of hundreds of microns to millimeters. Improving the spatial resolution of diagnostic equipment at the submicron level is favorable for revealing the phenomena and detailed features that are difficult to observe in implosion diagnostics. In particular, it may enhance the ability to observe low amplitude and high spatial frequency sine samples in the study of RT instability. Wolter microscope is an ideal optical configuration for high-precision X-ray imaging diagnostics due to its high spatial resolution and high optical collection efficiency. However, it is difficult to directly apply the Wolter configuration to laser fusion research. Most of the previous development experience focused on the development of full-aperture Wolter mirrors and imaging systems. It is difficult to obtain the theoretically designed ultra-high spatial resolution since small aperture and closed quadric mirror are hard to be processed. Errors in the form and roughness of the mirror surface directly influence the performance of the Wolter configuration.

A submicron resolution X-ray microscope is designed for high-precision RT instability diagnostics. By improving the Wolter configuration, this paper transforms the closed inner surface that could not be directly processed and tested into an open outer surface that could be directly processed and tested by using part of the sector. The improved Wolter configuration is a double mirror structure based on a rotating hyperboloid mirror and a rotating ellipsoid mirror. It still has the technical features of the original Wolter configuration and can meet the technological requirements of high-precision optical treatment, inspection, and coating. A Wolter microscope system with large grazing angle and high magnification is designed. The main structural parameters of the system, such as object distance, grazing angle, magnification, and mirror size, are optimized by theoretical derivation and ray-tracing simulation. A large grazing angle and high reflectivity at the specific energy point can be achieved by coating periodic Cr/C multilayer films on the mirror surface. The ray-tracing simulation verifies the optical structural parameters and evaluates the imaging performance of the system.

The design and verification of a 2.5 keV submicron resolution modified Wolter microscope has been completed. The system working energy point is designed as 2.5 keV with a grazing angle of 2.0°, and the system magnification factor is 35×. Limited by the angular bandwidth of the multilayer films, the effective field of view is about ±0.35 mm. At the current technical levels, the mirror slope error is 1 μrad, surface shape accuracy is λ/43,and the roughness is 0.3 nm. In this condition, the resolution of the central field of view is about 0.63 μm, and the spatial resolution over the full field of view is better than 1 μm, which satisfies the designed submicron resolution. At the same time, if the accuracy of the surface shape increases to λ/85, the system can achieve imaging ability near the diffraction limit. The system is characterized by high collection efficiency and the geometric solid angle is 3.73×10-5 sr, without considering the reflectivity of multilayer films. While considering it, the response efficiency of the system reaches a peak of 1.52×10-5 sr and is greater than 7.55×10-6 sr in the field of ±0.28 mm.

The design of a submicron resolution X-ray microscope based on an open Wolter configuration is systematically described. The optical structure, design methodology, and performance characteristics of the microscope are presented in detail. A set of 2.5 keV submicron X-ray microscope parameters for RT instability diagnostics is provided. At the same time, it is pointed out that since the open configuration uses a portion of the mirror for imaging, the solid angle is smaller than that of the original configuration, but it is still larger than that of the pinhole camera and KB microscope commonly used in diagnostics. With the improvement in the super smooth rotary quadric mirror processing technology and a further increase in the effective mirror width, the geometric solid angle of the microscope can be greatly raised. This study extends the application of the Wolter configuration to high-precision radiographic imaging diagnostics. An X-ray optical configuration with a large field of view, high spatial resolution, and high collection efficiency is provided, which can effectively compensate for the shortcomings of existing diagnostic equipment. In the future, it is expected to play an important role in studying the growth of disturbance in low amplitude and high spatial frequency planetary modulated targets driven by long laser pulses.