Hirokatsu Yumoto

Focusing mirror for x-ray free-electron lasers

We present the design, fabrication, and evaluation of a large total-reflection mirror for focusing x-ray free-electron laser beams to nanometer dimensions. We used an elliptical focusing mirror made of silicon that was 400 mm long and had a focal length of 550 mm. Electrolytic in-process dressing grinding was used for initial-step figuring and elastic emission machining was employed for final figuring and surface smoothing. A figure accuracy with a peak-to-valley height of 2 nm was achieved across the entire area. Characterization of the focused beam was performed at BL29XUL of SPring-8. The focused beam size was 75 nm at 15 keV, which is almost equal to the theoretical size.

New micro-beam beamline at SPring-8, targeting at protein micro-crystallography

A new protein micro‐crystallography beamline BL32XU at SPring‐8 is under construction and scheduled to start operation in 2010. The beamline is designed to provide the stabilized and brilliant micro‐beam to collect high‐quality data from micro‐crystals. The beamline consists of a hybrid in‐vacuum undulator, a liquid‐nitrogen cooled double crystal monochromator, and K‐B focusing mirrors with large magnification factor. Development of data acquisition system and end station consists of high‐precision diffractometer, high‐efficiency area detector, sample auto‐changer etc. are also in progress.

Damage threshold investigation using grazing incidence irradiation by hard x-ray free electron laser

X-ray free electron lasers (XFELs) with intense and ultra-short pulse X-rays possibly induce damage to optical elements. We investigated the damage thresholds of optical materials by using focusing XFEL beams with sufficient power density for studying ablation phenomena. 1-μm focusing beams with 10 keV photon energy were produced at the XFEL facility SACLA (SPring-8 Angstrom Compact free electron LAser). The focusing beams irradiated samples of rhodium-coated substrate, which is used in X-ray mirror optics, under grazing incident condition.

An adaptive optical system for sub-10nm hard x-ray focusing

In the hard X-ray region, to obtain the theoretical resolution or diffraction limited focusing size in an imaging optical system, both ultraprecise optics and highly accurate alignment are necessary. An adaptive optical system is used for the compensation of aberrations in various optical systems, such as optical microscopes and space telescopes. In situ wavefront control of hard X-rays is also effective for realizing ideal performance. The aim of this paper is to develop an adaptive optical system for sub-10nm hard X-ray focusing. The adaptive optical system performs the wavefront measurement using a phase retrieval algorithm and wavefront control using grazing incidence deformable mirrors. Several results of experiments using the developed system are reported.

Wave-optical assessment of alignment tolerances in nano-focusing with ellipsoidal mirror

High-precision ellipsoidal mirrors, which can efficiently focus X-rays to the nanometer dimension with a mirror, have not been realized because of the difficulties in the fabrication process. The purpose of our study was to develop nano-focusing ellipsoidal mirrors in the hard X-ray region. We developed a wave-optical focusing simulator for investigating alignment tolerances in nano-focusing with a designed ellipsoidal mirror, which produce a diffraction-limited focus size of 30 × 35 nm2 in full width at half maximum at an X-ray energy of 7 keV. The simulator can calculate focusing intensity distributions around the focal point under conditions of misalignment. The wave-optical simulator enabled the calculation of interference intensity distributions, which cannot be predicted by the conventional ray-trace method. The alignment conditions with a focal length error of ≲ ±10 µm, incident angle error of ≲ ±0.5 µrad, and in-plane rotation angle error of ≲ ±0.25 µrad must be satisfied for nano-focusing.

Stable delivery of nano-beams for advanced nano-scale analyses

Advanced nano-scale analyses such as x-ray absorption fine structure (XAFS), x-ray magnetic circular dichroism (XMCD) spectroscopy and x-ray fluorescence (XRF) with a ~100 nm spatial resolution have been routinely conducted by upgrading two x-ray beamlines BL37XU and BL39XU at SPring-8. The focusing beam size can be varied from 50 nm to 2 μm and high flux achieved at 1012 photons/s/0.01% b.w. with the beam size of 300 × 300 nm2 (full width at half maximum) at 12 keV.

Development of contamination-free x-ray optics for next-generation light sources

We studied typical forms of contamination on X-ray mirrors that cause degradation of beam quality, investigated techniques to remove the contaminants, and propose methods to eliminate the sources of the contamination. The total amount of carbon-containing substances on various materials in the vicinity of a mirror was measured by thermal desorption-gas chromatography/mass spectrometry and thermal desorption spectroscopy. It was found that cleanliness and ultra-high vacuum techniques are required to produce the contamination-free surfaces that are essential for the propagation of high-quality X-ray beams. The reduction of carbonaceous residue adsorbed on the surfaces, and absorbed into the bulk, of the materials in the vicinity of the mirrors is a key step toward achieving contamination-free X-ray optics.

Current status and future plan of the soft x-ray beamline at SACLA (Conference Presentation)

SACLA was inaugurated in March 2012 with two beamlines: BL3 for hard X-ray FEL and BL1 for wide range spontaneous emission. To enhance the research opportunities in soft X-ray region, the SCSS test accelerator, which was a prototype linac of SACLA and decommissioned in 2013, was upgraded, relocated to the SACLA undulator hall, and connected to BL1. The commissioning of this upgraded BL1 had been started from September in 2015, and user operation was started from June 2016. Currently, SASE-FEL pulses in the photon energy range of 20 to 150 eV are available and average pulse energy is about 70 μJ at 100 eV. We are developing beam diagnostic systems such as an arrival timing diagnostics between the SXFEL and the synchronized optical laser. We have further upgrade plans of the accelerator and the beamline. In this presentation, I will report the latest status and future upgrade plans of this beamline.

X-ray microfocusing with off-axis ellipsoidal mirror

High-precision ellipsoidal mirrors for two-dimensionally focusing X-rays to nanometer sizes have not been realized because of technical problems in their fabrication processes. The objective of the present study is to develop fabrication techniques for ellipsoidal focusing mirrors in the hard-X-ray region. We design an off-axis ellipsoidal mirror for use under total reflection conditions up to the X-ray energy of 8 keV. We fabricate an ellipsoidal mirror with a surface roughness of 0.3 nm RMS (root-mean-square) and a surface figure error height of 3.0 nm RMS by utilizing a surface profiler and surface finishing method developed by us. The focusing properties of the mirror are evaluated at the BL29XUL beamline in SPring-8. A focusing beam size of 270 nm × 360 nm FWHM (full width at half maximum) at an X-ray energy of 7 keV is observed with the use of the knife-edge scanning method. We expect to apply the developed fabrication techniques to construct ellipsoidal nanofocusing mirrors.

Focusing mirror for coherent hard X-rays

The development of high-precision x-ray mirrors is described. Atomistic surface polishing technique, elastic emission machining (EEM), was developed more than 30 years ago. This chapter describes how the polishing technique was improved to successfully apply to the synchrotron radiation and XFEL mirrors. The key was actually the figure metrology. The metrology was dramatically advanced by using coherent x-rays at the 1-km beamline of SPring-8, where classical ray-tracing approach was no more valid so that wave-optical approach was developed. Various optical references were developed to improve the global surface figures of mirrors. Successful development of elliptical mirrors helped attaining less than 100 nm focal spot in the conventional beamlines using Kirkpatrick-Baez focusing configuration, reaching 7 nm at the 1-km beamline. The speckle-free mirrors and focusing mirrors were used in XFEL facilities. These mirrors have been applied for various types of scanning nano-probes, imaging system, and generation of highintensity photon fields for nonlinear x-ray optical experiments.