Shengzhen Yi

Fabrication of nanoscale patterns in lithium fluoride crystal using a 13.5 nm Schwarzschild objective and a laser produced plasma source

Lithium fluoride (LiF) crystal is a radiation sensitive material widely used as EUV and soft x-ray detector. The LiF-based detector has high resolution, in principle limited by the point defect size, large field of view, and wide dynamic range. Using LiF crystal as an imaging detector, a resolution of 900 nm was achieved by a projection imaging of test meshes with a Schwarzschild objective operating at 13.5 nm. In addition, by imaging of a pinhole illuminated by the plasma, an EUV spot of 1.5 μm diameter in the image plane of the objective was generated, which accomplished direct writing of color centers with resolution of 800 nm. In order to avoid sample damage and contamination due to the influence of huge debris flux produced by the plasma source, a spherical normal-incidence condenser was used to collect EUV radiation. Together with a description of experimental results, the development of the Schwarzschild objective, the influence of condenser on energy density and the alignment of the imaging system are also reported.

Eight-channel Kirkpatrick–Baez microscope for multiframe x-ray imaging diagnostics in laser plasma experiments

Because grazing-incidence Kirkpatrick-Baez (KB) microscopes have better resolution and collection efficiency than pinhole cameras, they have been widely used for x-ray imaging diagnostics of laser inertial confinement fusion. The assembly and adjustment of a multichannel KB microscope must meet stringent requirements for image resolution and reproducible alignment. In the present study, an eight-channel KB microscope was developed for diagnostics by imaging self-emission x-rays with a framing camera at the Shenguang-II Update (SGII-Update) laser facility. A consistent object field of view is ensured in the eight channels using an assembly method based on conical reference cones, which also allow the intervals between the eight images to be tuned to couple with the microstrips of the x-ray framing camera. The eight-channel KB microscope was adjusted via real-time x-ray imaging experiments in the laboratory. This paper describes the details of the eight-channel KB microscope, its optical and multilayer design, the assembly and alignment methods, and results of imaging in the laboratory and at the SGII-Update.

Large-field high-resolution Kirkpatrick–Baez amélioré-Kirkpatrick–Baez mixed microscope for multi-keV time-resolved x-ray imaging diagnostics of laser plasma

Abstract. A large-field high-resolution x-ray microscope was developed for multi-keV time-resolved x-ray imaging diagnostics of laser plasma at the Shenguang-III prototype facility. The microscope consists of Kirkpatrick–Baez amélioré (KBA) bimirrors and a KB single mirror corresponding to the imaging and temporal directions of a streak camera, respectively. KBA bimirrors coated with an Ir single layer were used to obtain high spatial resolutions within the millimeter-range field of view, and a KB mirror coated with Cr/C multilayers was used to obtain a specific spectral resolution around 4.3 keV. This study describes details of the microscope with regard to its optical design, mirror coatings, and assembly method. The experimental imaging results of the grid with 3 to 5 μm spatial resolution are also shown.

Measurement of radius of curvature of spherical optical surfaces with small curvature and aperture by optical profiler

Monochromatic energy multilayer Kirkpatrick-Baez microscope is one of key diagnostic tools for researches on inertial confinement fusion. It is composed by two orthogonal concave spherical mirrors with small curvature and aperture, and produce the image of an object by collecting X-rays in each orthogonal direction, independently. Accurate measurement of radius of curvature of concave spherical mirrors is very important to achieve its design optical properties including imaging quality, optical throughput and energy resolution. However, it is difficult to measure the radius of curvature of spherical optical surfaces with small curvature and aperture by conventional methods, for the produced reflective intensity of glass is too low to correctly test. In this paper, we propose an improved measuring method of optical profiler to accomplish accurate measurement of radius of curvature of spherical optical surfaces with small curvature and aperture used in the monochromatic energy multilayer Kirkpatrick-Baez microscope. Firstly, we use a standard super-smooth optical flat to calibrate reference mirror before each experiment. Following, deviation of central position between measurement area and interference pattern is corrected by the theory of Newton’s rings, and the zero-order fringe position is derived from the principle of interference in which surface roughness has minimum values in the position of zero light path difference. Measured results by optical profiler show the low relative errors and high repeatability. Eventually, an imaging experiment of monochromatic energy multilayer Kirkpatrick-Baez microscope determines the measurement accuracy of radius of curvature.

Study of X-ray Kirkpatrick-Baez imaging with single layer

The X-ray Kirkpatrick-Baez (KB) imaging experiment with single layer is implemented. Based on the astigmatism aberration and residual geometric aberration of a single mirror, a KB system with 16X mean magnification and approximately 0.45\circ razing incidence angle is designed. The mirrors are deposited with an Ir layer of 20-nm thickness. Au grids backlit by X-ray tube of 8 keV are imaged via the KB system on scintillator charge-coupled device (CCD). In the \pm80 \mum field, resolutions of less than 5 \mum are measured. The result is in good agreement with the simulated imaging.

Note: Tandem Kirkpatrick–Baez microscope with sixteen channels for high-resolution laser-plasma diagnostics

Multi-channel Kirkpatrick-Baez (KB) microscopes, which have better resolution and collection efficiency than pinhole cameras, have been widely used in laser inertial confinement fusion to diagnose time evolution of the target implosion. In this study, a tandem multi-channel KB microscope was developed to have sixteen imaging channels with the precise control of spatial resolution and image intervals. This precise control was created using a coarse assembly of mirror pairs with high-accuracy optical prisms, followed by precise adjustment in real-time x-ray imaging experiments. The multilayers coated on the KB mirrors were designed to have substantially the same reflectivity to obtain a uniform brightness of different images for laser-plasma temperature analysis. The study provides a practicable method to achieve the optimum performance of the microscope for future high-resolution applications in inertial confinement fusion experiments.

A novel extreme ultraviolet four channels normal incidence imaging system for plasma diagnostics of Z-pinch facility

A novel EUV four channels normal incidence imaging system for plasma diagnostics of Z-pinch facility was presented in this paper, which consists of four concave mirrors and one convex mirror used for focusing an object onto four different positions with about 30 μm resolution on the same image plane. In addition, this imaging system can work at the energies of 50 eV, 95 eV, 150 eV, and broadband of 50-100 eV by using different multilayer films deposited on the concave and convex mirrors. This instrument, combined with framing camera, can achieve the power of two-dimensional spatial and temporal resolution, as well as the ability to imaging the plasma at the specific temperature. In the paper, the four channels microscope centering at multi-energies was developed.

Angle alignment method for soft x-ray using double-periodic multilayer

A double-periodic multilayer method was proposed to test KBA system of 4.75keV using 8keV source. Alignment of angle is the key for most of grazing incidence systems in x-ray range. But for soft x-ray, strong absorption makes the alignment have to be operated in vacuum, which is difficult enough. A double-periodic multilayer was used to experiment at 8keV in air replacing 4.75keV in vacuum. This multilayer includes two parts, the top and the bottom. The top is W/B4C multilayer with four bilayers and 6.93nm periods. The bottom is W/B4C multilayer with 10 bilayers and 3.95nm periods. For 8keV energy, x-ray will penetrate through the top and reflected by the bottom. While for 4.75keV, x-ray will be reflected by the top directly. The full width of half maximum is 0.1° at 8keV and 0.3° at 4.75keV, so it is accurate enough for 4.75keV to experiment at 8keV, which was also verified by the 1-D KBA experiment. This double-periodic multilayer provides a valid solution for alignment in soft x-ray range.

1D-KBA microscope using double-periodic multilayer

To study the action of shock wave in CH target, one-dimensional grazing incidence KBA microscope for 4.75keV energy was set up. Because of strong absorption in air, 4.75keV energy microscope can just work in vacuum. Accordingly, the alignment and assemblage will be very complicated and difficult. A special multilayer method, using double periodic multilayer, was proposed to solve this problem. This multilayer has high reflectivity not only for 4.75keV x-rays but also for 8keV x-rays at the same grazing incidence angle. It means 1D-KBA microscope has the same light trace for different working energies. Therefore, we can implement the alignment and assembly of 4.75keV system by the help of 8keV x-rays. Because 8keV x-rays is very easy produced by x-ray tube and has strong transmittability in air, the alignment and assemblage process became relatively easy. By now, we have finished the alignment experiment at 8keV and obtained imaging results. The performance is about 2-3μm resolution in 250μm field of view. It is coincide with the calculation.

Design and fabrication of x-ray Kirkpatrick-Baez microscope for ICF

A hard x-ray (8 keV, Kα line of Cu) Kirkpatrick-Baez (KB) microscope was designed for the diagnostics of inertial confinement fusion (ICF). Three main parts including optical design, fabrication of multilayers, and alignment method were discussed in this paper. According to the deduced equation of aberration in whole field, an optical system was designed, which gives attention to not only spatial resolution but also the collection efficiency. Tungsten (W) and boron carbide (B4C) were chosen as multilayer materials and the non-periodic multilayer with 40 layers was deposited. The measured reflectivity by XRD is better than 18% in the bandwidth range of about 0.3%. Super accurately alignment is another difficulty in the application of KB microscope. To meet the requirements of pointing and co-focusing, a binocular laser pointer which is flexible enough was designed. Finally, an 8keV x-ray tube was used as source in x-ray imaging experiment and images with magnification of 2× were obtained.