Yasushi Hayakawa

Compact laboratory-scale x-ray sources still rely on the same fundamental principles as did the first x-ray tubes developed more than a century ago. In recent years, significant research and development has focused on large-scale x-ray sources such as synchrotrons and free-electron lasers, leading to the generation of high-brightness coherent x-rays. However, the large size and high costs of such sources prevent their widespread use. The quest for a compact and coherent x-ray source has long been a critical objective in modern physics, gaining further importance in recent years for industrial applications and fundamental scientific research. Here, we review the physical mechanisms governing compact coherent x-ray generation. Of current interest are coherent periodic interactions of free electrons in crystalline materials, creating hard x-rays via a mechanism known as parametric x-ray radiation (PXR). Over the past decade, x-ray sources leveraging this mechanism have demonstrated state-of-the-art tunability, directionality, and broad spatial coherence, enabling x-ray phase-contrast imaging on a compact scale. The coming years are expected to show substantial miniaturization of compact x-ray sources, facilitated by progress in electron beam technologies. This review compares the most promising mechanisms used for hard x-ray generation, contrasting parametric x-ray radiation with inverse Compton scattering and characteristic radiation from a liquid-jet anode. We cover the most recent advancements, including the development of new materials, innovative geometrical designs, and specialized optimization techniques, aiming toward x-ray flux levels suitable for medical imaging and x-ray spectroscopy at compact scales.

To determine the structure of carbohydrate polymers using conventional analytical technology, several complicated steps are required. We instead adopted a direct approach without the need for pretreatments, using an intense infrared (IR) laser for carbohydrate analysis. IR free-electron lasers (FELs) driven by a linear accelerator possess unique spectroscopic features, including extensive wavelength tunability and high laser energy in the IR region from 1000 cm⁻¹ (10 μm) to 4000 cm⁻¹ (2.5 μm). FELs can induce IR multiphoton dissociation reactions against various molecules by supplying vibrational excitation energy to the corresponding chemical bonds. Chitin from crayfish and cellulose fiber were irradiated by FELs tuned to νC–O (9.1–9.8 μm), νC–H (3.5 μm), and δH–C–O (7.2 μm) in glycosidic bonds, and their low-molecular-weight sugars were separated, which were revealed by combining synchrotron radiation IR spectroscopy and electrospray ionization mass spectrometry. An intense IR laser can be viewed as a “molecular scalpel” for dissecting and directly analyzing the internal components in rigid biopolymers. This method is simple and rapid compared with general analytical techniques.

Application of intense infrared (IR) laser to analyze carbohydrate polymers is shown. IR free electron laser (FEL) driven by a linear accelerator possesses unique spectroscopic features in terms of extensive wavelength tunability and high laser energy in the IR region from 1000 cm^-1 (10 mm) to 4000 cm^-1 (2.5 mm). The FEL can induce IR multiphoton dissociation reaction against various molecules by giving the vibrational excitation energy to the corresponding chemical bonds. Chitin from crayfish and cellulose fiber were irradiated by the FELs that are tuned to nC-O (9.1-9.8 mm), nC-H (3.5 mm), and dH-C-O (7.2 mm) in glycoside bonds, and their low-molecular weight sugars were separated, which were revealed by combining synchrotron-radiation IR spectroscopy and electrospray ionization mass spectrometry. The intense IR laser can be proposed as “molecular scalpel” for dissecting and direct analyzing internal components in rigid biopolymers.

Lignin monomers have attracted attention as functional materials for various industrial uses. However, it is challenging to obtain these monomers by degrading polymerized lignin due to the rigid ether linkage between the aromatic rings. Here, we propose a novel approach based on molecular vibrational excitation using infrared free electron laser (IR-FEL) for the degradation of lignin. The IR-FEL is an accelerator-based pico-second pulse laser, and commercially available powdered lignin was irradiated by the IR-FEL under atmospheric conditions. Synchrotron-radiation infrared microspectroscopy analysis showed that the absorption intensities at 1050 cm⁻¹, 1140 cm⁻¹, and 3400 cm⁻¹ were largely decreased alongside decolorization. Electrospray ionization mass chromatography analysis showed that coumaryl alcohol was more abundant and a mass peak corresponding to hydrated coniferyl alcohol was detected after irradiation at 2.9 μm (νO-H) compared to the original lignin. Interestingly, a mass peak corresponding to vanillic acid appeared after irradiation at 7.1 μm (νC=C and νC-C), which was supported by our two-dimensional nuclear magnetic resonance spectroscopy analysis. Therefore, it seems that partial depolymerization of lignin can be induced by IR-FEL irradiation in a wavelength-dependent manner.

We investigated the transmission characteristics of formalin fixed human liver samples in which normal liver tissue and malignant liver tumor were mixed using terahertz (THz) coherent synchrotron radiation at an infrared free-electron laser (FEL) facility at Nihon University. Infrared-FEL imaging has indicated that the amount of water molecules in the tumor tissue is not different from that in the normal tissue. However, the transmission of the incipient tumor tissue was lower than that of the normal tissue in THz imaging because the tumor tissue contained more water molecular clusters than the normal tissue. The tumor tissue became more permeable owing to the development of fibrous tissue around it. THz imaging will be more useful for discriminating liver tissues by increasing the spatial resolution.

This study presents a novel technology to measure electron bunch length with a high time resolution by measuring coherent synchrotron radiation using a narrow-band detector at Laboratory for Electron Beam Research and Application (LEBRA)—an S-band linear accelerator facility for free-electron lasers. The form factor was observed to decrease exponentially with charge—in concordance with the relationship between the intensity of the coherent synchrotron radiation and the magnitude of electron bunch charge—in the region in which the effect of electron bunch charge on bunch length is negligible. The calculated root-mean-square bunch length was observed to agree well with the value determined from the spectral shape obtained. The aforementioned results are expected to be useful in real-time observation of small changes in electron bunches in advanced accelerators.

A research program towards an attosecond photon source based on high-harmonic generation driven by mid-infrared FEL oscillators has been established. We present the scope and the current status of the research program.

Nihon University and National Institute of Advanced Industrial Science and Technology have planned to develop an intense terahertz-wave (THz-wave) source based on coherent Cherenkov radiation (CCR) at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University. By passing an electron beam through a hollow conical dielectric with apex facing the incident electron beam, the wave front of CCR generated on the inner surface of the hollow conical dielectric is matched to the basal plane. The CCR matched to a circle plane wave is more powerful than conventional THz-wave sources based on electron accelerators. A hollow conical high-resistance silicon with a diameter of the basal plane of 40 mm and an inner diameter of 6 mm will emit the THz-wave beam with a macropulse energy of 14 mJ at LEBRA.