We present ongoing efforts on the development of precision Wolter mirrors for the Soft X-ray Imaging Spectrometer (SXIS) aboard PhoENiX mission proposed to JAXA for studying mechanism(s) of particle acceleration and its relationship with magnetic reconnection in solar flares. The Wolter mirrors for PhoENiX/SXIS are made by direct polishing of glass-ceramic substrates. So far, we succeeded in fabricating a small size of high precision Wolter surfaces (e.g., PSF core size of ~0.2 arcsec HPD at 8 keV) as well as good indication of extending the mirror area along the cylindrical direction. Recent status of the mirror development will be reported.
Optical components in optics hutches of a hard x-ray undulator beamline of BL05XU at SPring-8 was restructured for providing a high flux beam at 1% bandwidth in the x-ray energy range from 5 to 100 keV. The so-called pink beam by a double-multilayer monochromator or total reflection mirrors pair with a prism made of glassy carbon as a harmonic separator are prepared in this beamline. The total reflection mirrors have three stripes; rhodium and platinum coated surface and silicon uncoated surface. Additionally, a silicon single crystal monochromator and a silicon channel cut crystal monochromator with liquid nitrogen cooling system are planned to be installed. The installation of these optical components started at January 2020. The commissioning of some components using undulator radiation will be started at April 2020.
Focusing x-ray free-electron lasers (XFEL) allows us to study nonlinear optics within the xray region. Recently, we challenged the focusing XFELs to below 10 nm. However, the conventional multilayer Kirkpatrick-Baez(KB) mirrors require too strict alignment accuracy of the incident angle. To solve this problem, we propose advanced KB (AKB) mirrors, based on Wolter type III geometry. Because the configuration satisfies the Abbe sine condition, AKB mirrors enables a tolerance of incident angle error 1000 times greater than conventional KB mirrors. The remaining problem is how such mirrors are to be fabricated, because required shape accuracy is below 1 nm and the small radius of curvature on the mirrors makes high accuracy shape measurement difficult. In this work, we performed a mirror fabrication procedure based on a combination of a grating interferometer and a differential deposition. Experiment at BL29XUL of SPring-8 demonstrated AKB mirrors with an accuracy of λ/4 fabricated.
Spatial resolution of full-field X-ray microscopes based on total-reflection mirrors was limited by grazing-incidence angle of the mirrors. At practical conditions, achievable spatial resolution is approximately 30 nm. To overcome the limitation, multilayer advanced Kirkpatrick-Baez mirrors and full-field X-ray microscopes with this objective mirrors have been developed in Osaka University and RIGAKU Corp. One of the remarkable points in this design is an achievable spatial resolution of less than 20 nm owing to large grazing-incidence angle and multilayer (Pt/C) with narrow period. Also, the advanced Kirkpatrick-Baez mirrors comprise two mirror pairs based on the Wolter type I and type III optics, respectively, to have sufficiently large magnification even at a compact setup with the whole length of 2 m (between a sample and a camera). The compactness makes it possible to apply the optics to laboratory-based X-ray microscopes, which is another ongoing project.
A performance test using a Siemens star chart at an X-ray energy of 8 keV was performed in SPring-8 BL29XUL. The results showed lines with approximately 30-nm width could be resolved. Also, tests of stability and energy dependence confirmed usability of this system.
High angular-resolution imagery (~1” or better) together with good off-axis scattering performance (<1/1000 of the PSF peak at 10” off-axis position) are essential ingredients for revealing energetic plasma processes ongoing in the solar corona during flares. However, imagery of the corona has never been performed with such performance due to severe technical difficulty in fabricating precision Wolter mirrors with a wide field of view exceeding several 100”.
We are attempting to realize Wolter mirrors with the above-mentioned performance for future X-ray observations of the Sun. The attempt includes fabrication of engineering mirrors of segmented type to which state-of-the-art precision polish and measurements are applied, followed by X-ray evaluation of focusing performance using BL29XUL parallel X-ray beam line at SPring-8 synchrotron facility. Results of the evaluation are then fed-back to polish/measurements for the subsequent mirrors.
Thus far we have successfully fabricated an engineering mirror whose Wolter surfaces 32.5mm x 10mm each for the parabola and hyperbola segments. The mirror focused 8 keV X-rays with the PSF core size ~0.2” HPD (~0.1” FWHM) and with ~3 x 10^(-4) scattering level at 10” off-axis position. Effort has currently been made to increase the area size of the Wolter surfaces towards application to space-borne optics for solar X-ray observations.
Status of the current development on the precision Wolter mirrors will be reported together with some future prospects.
A full-field X-ray microscope utilizing advanced Kirkpatrick–Baez optics, which comprises four concave mirrors, provides high-resolution X-ray images without chromatic aberration. However, a large distance is required between the mirrors and the detector to obtain sufficiently high magnification factor. To achieve reduce this distance, this paper proposes a novel X-ray imaging mirror system consisting of two pairs of concave and convex mirrors, which enables the effective focal length to be decreased by shifting the principal surface. For developing the proposed optics, the mirrors were fabricated with an ion beam figuring system and stitching interferometer, developed by our group, with a peak-to-valley accuracy of ~2 nm. Analysis results indicate that the fabricated mirrors can achieve nearly diffraction-limited imaging performance. We report the mirror fabrication results and the characteristics of the fabricated mirrors.
High resolution imagery of the Sun's X-ray corona provides an essential clue in understanding dynamics and heating processes of plasma particles there. However, X-ray imagery of the Sun with sub-arcsecond resolution has so far never been conducted due to severe technical difficulty in fabricating precision Wolter mirrors. For future X-ray observations of the solar corona, we are attempting to realize precision Wolter mirrors with sub-arcsecond resolution by adopting advanced surface polish and metrology methods to sector mirrors which consist of a portion of an entire annulus, by direct polishing onto the mirror substrate. Based on the knowledge obtained through fabrication of the first (in 2013) and second (in 2014) engineering Wolter mirrors and subsequent evaluations on their X-ray focusing performance, the third engineering mirror was made in 2015−2016. The primary target of improvement over the second mirror was to suppress figure error amplitude especially for spatial frequencies around 1 mm-1 and to suppress the large astigmatism that was present in the second mirror, by introducing improved deterministic polish and smoothing on the precision mirror surfaces (32.5 mm × 10 mm in area for both parabola and hyperbola segments), as well as by careful characterization of the systematic error in the figure measurement system for the precision polish. Measurements on the focusing performance of thus-fabricated third Wolter mirror at SPring-8 synchrotron facility with 8 keV X-rays demonstrated that the mirror attained sub-arcsecond focusing performance with its HPD (half-power diameter) size reaching as small as ~0.2 arcsec for meridional focusing while ~0.1 arcsec for sagittal focusing. The meridional focusing achieved nearly diffraction limited performance (~0.12 arcsec FWHM for the PSF core). We also confirmed that the large astigmatism noted in the second mirror was correctly removed in the third mirror with the correction of the above-mentioned systematic error.