X-ray imagery of the solar corona with arcsecond-level angular resolution is a key to understand various activities of hot plasmas in the corona. We report our development activities of precision Wolter mirrors for future soft X-ray imaging observations of the Sun. The Wolter mirrors under development are fabricated by direct polishing of glass-ceramic substrates. We fabricated an engineering Wolter mirror of 3-m focal length whose mirror area was 40 mm x 50 mm (along optical-axis and cylindrical directions, respectively) for each of the parabola and hyperbola sections using EEM (Elastic Emission Machining) precision polishing technology. X-ray imaging performance of the mirror was then evaluated with 8-keV parallel X-rays at SPring-8/BL29XUL. For the mirror area whose tilt up to >6 degrees along the cylindrical direction, the mirror showed imaging performance of ~0.25 arcsec FWHM (~0.55 arcsec HPD) with the scattering level ~1 × 10-4 of the PSF peak at 30-arcsec off-axis position.
Scatterometry has been put into practical use for microstructure measurement of ultra-large-scale integration due to its high process compatibility. On the other hand, its application has been limited to periodic structures. By applying this method to isolated systems and using hard X-rays, it may be possible to significantly exceed a resolution of 10 nm, which is the limit of conventional optical measurement. We demonstrate the feasibility of this measurement by rigorous calculations. For this purpose, we measured the intensity of specular reflection and noise at the beamline of hard X-ray radiation. The virtual target is a 15-nm-wide lattice. The signal-to-noise ratio is low enough for a lattice with a period of 25 nm but 10 times higher for an isolated lattice.
In this study, we manufactured a 460 mm long deformable mirror which has 28 channel-PZTs on the ellipse pre-figured mirror and flat back surfaces. We evaluated the LTP target ellipse shape measurement result and the "junction" effect by applying voltage to each electrode.
At Lawrence Berkeley National Laboratory’s Advanced Light Source, we are developing x-ray wavefront sensors to support the creation and operation of beamlines with diffraction-limited quality. Our new approach to rapid, intermittent wavefront sensing operates in reflection at glancing incidence angles and is compatible with the high-power densities of modern beamlines. For soft x-ray applications especially, the wavefront sensor can operate upstream of the exit slit in a vertically dispersed beam. This single-shot technique supports lateral shearing interferometry and Hartmann wavefront sensing; it can be adapted to speckle-based techniques as well. The reflected beam is directed to an off-axis YAG crystal that produces scintillated visible light. A small mirror reflects the light to a microscope and camera, and the measured wavefront shape information can be used as feedback to adaptive x-ray mirror elements. A compact array of gratings enables measurement across a broad range of photon energies or wavefront curvatures. We describe recent demonstrations at soft x-ray and hard x-ray wavelengths measuring an adaptive x-ray mirror, and a toroidal focusing mirror.
PZT (lead zirconate titanate)-glued bimorph deformable mirrors are widely used in hard X-ray regimes, however, they have not yet been used in soft X-ray regimes because they are less compatible for usage under high vacuum. Therefore, we have developed a glue-free bimorph deformable mirror, in which silver nanoparticles were employed to bond PZT actuators to mirror substrates.
In this study, we manufactured a 400 mm long deformable mirror which has 30 channels on plane mirror surfaces. We evaluated the PZT response by applying voltage to each electrode.
PZT (lead zirconate titanate)-glued bimorph deformable mirrors are widely used in hard x-ray regimes[1], however, they have not yet been used in soft X-ray regimes because they are less compatible for usage under high vacuum. Therefore, we have developed a glue-free bimorph deformable mirror, in which silver nanoparticles were employed to bond PZT actuators to mirror substrates[2]. In this study, we achieved a 2 nm figure error on an elliptical shape of a glue-free deformable mirror. We evaluated the figure change characteristics due to humidity and temperature increasing at the ultrafine figure error condition.
A wavefront measurement method in the microscope (magnifying) geometry can help achieve the required high accuracy for deformable mirrors. This study proposes an image-based wavefront measurement method based on a series of images of a small area near the focus. In this method, phase retrieval calculation using multiple images is performed. A proof-of-concept experiment was performed using multilayer AKB mirrors and an FZP to form the small area. Consequently, wavefront aberration was successfully retrieved using 60 images of a 30-nm-diameter area near the focus.
We have developed an X-ray zoom condenser optical system using deformable mirrors that can adjust the beam size by deformation of their shape. The shapes of deformable mirrors are changed by a combination of mechanical and piezoelectric bending. Large deformations up to third order polynomials are achieved by mechanical bending. More precise shapes are achieved by piezoelectric bimorph mirror. However, because both ends of the mirror are mechanically clamped, capability of deformation by piezoelectric bending is lower than that of free-standing piezoelectric bimorph mirrors. So, we propose a bending method that tunes the mechanical bending conditions to intentionally leave the optimized shape error to be easily compensated by the piezoelectric bending process.
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.
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.
Tight XFEL focusing is very important for significantly enhancing photon flux density, which is highly demanded by users exploring nonlinear X-ray optics. However, focusing XFEL down to 10 nm or less is so difficult from the viewpoints of both optical fabrication and optical alignment. The former can be overcome using techniques of wavefront sensing and fine shape correction. For the latter, techniques for directly measuring beam size on the focus without an influence of vibration of nanobeam are required. We have developed a technique for determining the size of nanobeam on the focus using an intensity interferometer, based on the Hanbury Brown and Twiss effect, of X-ray fluorescence emitted from a thin film inserted into the focus. The spatial coherence of X-ray fluorescence observed far from the focus depends on the distance from the focus and emission region of X-ray fluorescence. Therefore, the measured coherence can determine the size of X-ray nanobeam. This method has advantages that vibration of nanobeam does not affect the result and the setup is so simple.
A demonstration experiment was performed using a 100 nm focusing system based on total reflection KB mirrors at SACLA. X-ray fluorescence (8 keV) emitted from a thin Cu film by irradiation of focused XFEL pulses (12 keV) was detected shot-by-shot with a dual MPCCD. Analyses of approximately 1000 images based on the autocorrelation revealed that the beam size obtained with this method is in good agreement with one obtained with the wire scan method.
Focusing X-ray free-electron lasers (XFELs) is very important for producing ultra-intense X-ray nanoprobes. We have developed a system based on multilayer Kirkpatrick–Baez (KB) mirrors to focus XFELs to 10 nm or less at the SPring-8 Angstrom Compact free-electron LAser (SACLA) facility. The mirror optics in the system are designed with a large NA of greater than 0.01 to produce a diffraction-limited size of 6 nm at 9 keV. We constructed a precise X-ray grating interferometer based on the Talbot effect, and succeeded in fabricating near-perfect focusing mirrors with wavefront aberrations of λ/4.
However, strict error tolerances for mirror alignment can prevent sub-10 nm focusing. Errors of perpendicularity, incident angle, and astigmatism cause aberration on the focusing wavefront and characteristically change the beam shape. In particular, the required accuracy of the incident angle is 500 nrad. Due to shot-by-shot variations in the XFEL beam position and vibration of the optics, a single-shot diagnosis of beam shape is essential to align the mirrors quickly and accurately. By improving the method proposed by Sikorski et al. at the Stanford Linear Accelerator Center (SLAC), National Accelerator Laboratory, we propose a nanobeam diagnosis method based on the speckle pattern observed under coherent scattering. Computer simulation revealed that speckle size and beam size are inversely proportional. Platinum particles with a diameter of 2 nm were prepared and irradiated with X-rays to obtain a speckle pattern. Our experimental results demonstrate the successful estimation of beam shape and the alignment of all mirrors with the required accuracies.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.