Objective Fusion energy based on inertial confinement fusion (ICF) is both efficient and environment friendly. It is necessary for ICF research to diagnose weak X-ray signals by focusing on imaging. Because the crystal has a periodic and regular arrangement of internal atoms and the lattice spacing is close to the order of X-ray wavelength, the X-ray diffraction spectrometer can obtain relevant information about the X-ray source using the crystal as the spectroscopic element; thus, the crystal spectrometer is an important part of the spectrum diagnostic instrument. Several types of crystals that are currently being developed have some issues. Planar crystals, for example, do not have the ability to focus rays. Likewise, the cylindrically bend crystal spectrometers are not suitable for diagnostic experiments using coupled fringe cameras. The application of spherically bend crystals for self-luminous imaging requires Bragg angle close to 90°, limiting the imaging energy spectrum selection range. As a result, the development of a crystal spectrometer for diagnosing X-rays with both strong-focus performance and high-resolution is an urgent need for plasma X-ray diagnosing technology for the current ICF devices with ever-increasing performance. This paper proposes a toroidal quartz crystal that can perform focused imaging on Ti-target X-ray at 4.75 keV. The results of simulations and experiments show that after the Ti-target X-rays are diffracted by the toroidal quartz crystal, they have good imaging focusing performance on the sagittal and meridional planes and can achieve high spatial resolution imaging results.
Methods A quartz crystal with a toroidal structure was proposed in this paper. First, the imaging principles and properties of spherical and toroidal crystals were investigated and analysed. Then, to demonstrate that the toroidal crystal had high-resolution properties, the X-ray diffraction of spherical and toroidal crystals was simulated in this paper using the X-ray diffraction tracing principle. The diffraction results of various crystal structures were compared and analysed while the radiation source and imaging object remained constant. Then, the imaging spatial resolution of spherical and toroidal crystals in the sagittal plane was calculated. In addition, a Ti-target laser device, an IP plate, and a toroidal quartz crystal were used to complete the X-ray backlight imaging experiment. The imaging results of the toroidal crystal were obtained through actual experiments and the actual resolution was calculated. Finally, the differences between the actual and simulated results were analysed. The related influencing factors were discussed in this paper.
Results and Discussions The toroidal crystal’s X-rays have good imaging spatial resolution in the meridional and sagittal planes. The imaging grid’s length in both directions is 500 μm, which meets the imaging magnification relationship, and the shape of the imaging grid has almost no deformation [Fig. 4(b)]. The toroidal crystal has a spatial resolution of aproximately 5 μm in the meridional and sagittal planes [Fig. 5(b) and Fig.5(d)]. The simulation of focusing imaging on spherical and toroidal crystals indicates that the circular spot can be better focused and imaged after being diffracted from the toroidal crystal. The size of the imaging spot is roughly equal to the size of the original source. In addition, the sizes of the imaging spots on the meridional and sagittal planes are nearly equal (Fig. 6). From the simulation of the focused imaging of the source, it can be concluded that the toroidal quartz crystal proposed in this paper has strong-focus characteristics and can effectively focus the source, thereby improving the intensity of rays, which is useful for subsequent data processing and analysis. The actual experiment of diffraction focusing imaging on a Ti-target laser device with a pulse width of 920 ps and energy of 1137.34 J can achieve the resolution of 10 μm.
Conclusions We propose a toroidal quartz crystal and conduct a simulation study based on X-ray diffraction tracing using a toroidal crystal and a spherical crystal of the same material under the same conditions. The focused imaging image of X-rays diffracted by toroidal crystals has a high spatial resolution in the meridional and sagittal planes, as determined by comparing and analysing the imaging results of X-rays diffracted by two crystals of different structures. Metal grids in both planes can be clearly distinguished, and the imaged metal grids show almost no deformation. The imaging result data is used to calculate the simulated imaging spatial resolution, which is approximately 5 μm. The Ti-target laser device is used as the source in the experiment to test the effect of diffraction imaging of the quartz crystal’s toroidal structure. The backlight imaging experiment yields a focused imaging image with a magnification of five on both the meridional and sagittal planes. The spatial resolution of X-ray diffraction imaging of toroidal quartz crystal is estimated to be 10 μm. The imaging spatial resolution obtained by the simulation and imaging spatial resolution measured by the experiment differs slightly. The difference depends on the actual source’s size, geometric aberrations, and crystal defects. In future, we intend to discuss the aforementioned factors that influence resolution. In conclusion, the quartz crystal with the toroidal structure proposed in this paper has the ability of strong-focus and high-resolution, which can meet the requirements for the spectral diagnosis of high energy density plasma.
