Search

A project is underway that aims to generate attosecond pulses via high-harmonic generation in rare gases, driven by extremely short and highly intense pulses from free-electron-laser oscillators. For this purpose, it has been planned that a new photocathode RF gun, dedicated to high-bunch-charge operation, will be installed at the KU-FEL (Kyoto University Free Electron Laser) oscillator facility. In this study, RF guns with two different structures (1.6-cell and 1.4-cell) were compared, from the perspective of exploring the possibility of introducing bunch-interval modulation, which is important for achieving high extraction efficiency in the FEL oscillator. As a result, it was confirmed that the introduction of bunch-phase modulation would be possible only in the case of the 1.6-cell RF gun. After the structure of the RF gun was decided on, particle-tracking simulations were performed, to study the electron-beam parameters using the 1.6-cell RF gun and 1 nC bunch charge. The results showed that we could obtain the peak current of 1 kA without a large degradation of the other parameters.

A mid-infrared free-electron laser (MIR-FEL) is a synchrotron-radiation-based femto- to pico-second pulse laser. It has unique characteristics such as variable wavelengths in the infrared region and an intense pulse energy. So far, MIR-FELs have been utilized to perform multi-photon absorption reactions against various gas molecules and protein aggregates in physical chemistry and biomedical fields. However, the applicability of MIR-FELs for the structural analysis of solid materials is not well recognized in the analytical field. In the current study, an MIR-FEL is applied for the first time to analyse the internal structure of biological materials by using fossilized inks from cephalopods as the model sample. Two kinds of fossilized inks that were collected from different strata were irradiated at the dry state by tuning the oscillation wavelengths of the MIR-FEL to the phosphoryl stretching mode of hydroxyapatite (9.6 µm) and to the carbonyl stretching mode of melanin (5.8 µm), and the subsequent structural changes in those materials were observed by using infrared microscopy and far-infrared spectroscopy. The structural variation of these biological fossils is discussed based on the infrared-absorption spectral changes that were enhanced by the MIR-FEL irradiation, and the potential use of MIR-FELs for the structural evaluation of biomaterials is suggested.

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.

The isotope selectivity of computed tomography (CT) imaging based on nuclear resonance fluorescence (NRF) transmission method using a quasi-monochromatic laser Compton scattering (LCS) gamma-ray beam in the MeV region was demonstrated at the Ultra Violet Synchrotron Orbital Radiation–III (UVSOR-III) Synchrotron Radiation Facility (Institute of Molecular Science, National Institute of Natural Science) for two enriched lead isotope rods (206Pb and 208Pb) implanted in an aluminum cylinder. Since these two rods show the same gamma-ray attenuation in atomic processes, it is impossible to differentiate between them using a standard Gamma-CT technique based on atomic attenuation of gamma rays. The LCS gamma-ray beam had a maximum energy of 5.528 MeV and an intensity of approximately 5.5 photons/s/eV at the resonance energy ( $J^{\pi } = 1^{-}$ at 5.512 MeV in 208Pb). A lead collimator with a hole diameter of 1 mm was used to define the size of the LCS gamma-ray beam at the CT target. The CT image of the 208Pb rod was selectively obtained with a 2-mm pixel size resolution, which was determined by the horizontal step size of the CT stage.

We introduce a green approach to obtain glucose and low-molecular-weight saccharides directly from the cellulose aggregate by using near- and mid-infrared free electron lasers (IR-FELs). The IR-FEL...

This study reported the longitudinal compression and extension of an electron bunch by free electron laser (FEL) interaction. By observing coherent edge radiation generated by an FEL facility at Kyoto University, which has the highest extraction efficiency of an FEL oscillation, we found that an electron bunch interacting with a cavity-type linac FEL elongated when a detuning length of an optical cavity was positive. The transition from compression to extension of the electron bunch was bounded by a perfect synchronism condition between the electron bunch and the FEL pulse, and the change rate of the root-mean-square bunch length was as high as 14%. Therefore, it was experimentally demonstrated that a slight temporal difference between the electron bunch and the optical pulse dramatically changed not only the FEL power but also the outline of electron distribution in the bunch.

The electron responses of PWO, LaBr3 :Ce, and LYSO:Ce crystal scintillators have been investigated at the Kyoto University Free Electron Laser (KU-FEL) facility, which provides electron beams satisfying a single-electron hit condition. The linearities and scintillation decay times were measured at electron energies between 5 and 40 MeV. The distributions of pulse height were compared with those from Monte Carlo simulations to deduce the deposit energies. All the scintillators are found to show good linearity, and their decay times are roughly constant over the measured energy range.

A mid-infrared Free Electron Laser (MIR-FEL) is a candidate to realize a high-repetition-rate atto-second hard X-ray source based on the high harmonic generation (HHG). A new project to conduct a proof of principle experiment of HHG driven by a MIR-FEL has been started since 2018. For this purpose, upgrade project of MIR-FEL at Kyoto University (KU-FEL) has been started to achieve high peak power and ultrashort pulse operation. As the first step of the upgrade, a photocathode drive laser was improved to enable long macro-pulse photocathode operation. The results of the long macro-pulse photocathode operation of KU-FEL will be presented.

Terahertz parametric oscillator (TPO) is one of the attractive ways to generate THz waves. In this study, we proposed a novel high-conversion-efficiency TPO, which is called as synchronously pumped Terahertz parametric oscillator (sp- TPO) by using a pico-second mode-locked laser pulses as a pump beam. An sp- TPO has been designed and constructied. The system performance was measured. As a result, the conversion efficiency from NIR to THz of 2.4 × 10–5 has been achieved. The maximum micro-pulse energy and the peak power were 1.5 nJ and 200 W, respectively. The spectrum and pulse duration measurements by using a Michelson interferometer are under preparation.

We proposed an isotope-selective computed tomography (CT) imaging based on the Nuclear Resonance Fluorescence (NRF) transmission method using a quasi-monochromatic laser Compton scattering (LCS) gamma-ray beam in the MeV region for nuclear safety applications. As the first step, a two-dimensional (2D) NRF-CT image of 208Pb isotope distribution was selectively obtained for the sample target containing two enriched lead isotope rods (206Pb and 208Pb). We are planning to perform an experiment to obtain a three-dimensional NRF Computed Tomography (3D NRF-CT) image for the specific isotope. An automatic measurement system has been developed. As the result, we obtained an excellent quality of 3D gamma-ray CT image.

The highest extraction efficiency (9.4%) of a free electron laser (FEL) oscillator has been achieved at the midinfrared FEL facility of Kyoto University. Because of the interaction between the electron beam and FEL electromagnetic field, a maximum electron energy decrease of 16% was observed. The measured energy decrease was consistent with the measured FEL spectrum. An FEL micropulse energy of ~100 μJ with the expected few-cycle pulse duration at a wavelength of 11 μm was observed. This result is an important milestone for the high-extraction-efficiency FEL oscillator and will contribute to the strong-field physics of atoms and molecules., 共振器型自由電子レーザの世界最高変換効率を達成 --長波長赤外での強光子場の実現とアト秒X線源の開発に大きく前進--. 京都大学プレスリリース. 2020-10-13.

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.

Spectra of coherent edge radiation (CER) were observed at the S-band linac facility of Kyoto University Free Electron Laser. A local maximum was observed in the CER spectrum on-crest operation of the radio frequency (RF) field. As the phase of the RF field was shifted from the crest, the frequency of the maximum decreased, and the CER spectrum approached a spectrum of Gaussian-distributed electrons in a bunch. It was found that this strange spectrum can be explained by a model in which a satellite pulse exists around a main pulse in the electron bunch. Furthermore, it demonstrated that CER is an effective tool for monitoring the shape of the electron bunch.

Light perception is an intriguing subject, and it has been demonstrated that some animals can perceive wavelengths beyond human vision. However, it is still controversial whether animals can see mid-infrared radiation. A combination of two free-electron laser (FEL) facilities, LEBRA-FEL (Nihon University) and KU-FEL (Kyoto University), can provide light sources with high peak power and spatially coherent monochromatic wavelengths that are continuously tuneable from 400 nm to 20 μm. We show that the crayfish (Procambarus clarkii) compound eye responds to pulsed mid-infrared FELs (1 Hz for KU-FEL; 2 Hz for LEBRA-FEL) from 3 to 17 μm. Our finding provides insight into the animal’s visual sensing of mid-infrared radiation, and that the sensing does not depend on thermal effects. Although the behavioural reason for vision in this wavelength range and the mechanism are still under investigation, understanding this type of visual sensing may lead to applications other than photophysical applications.

A Neutron/Gamma-ray combined inspection system for hidden special nuclear materials (SNMs) in cargo containers has been developed under a program of Japan Science and Technology Agency in Japan. This inspection system consists of an active neutron-detection system for fast screening and a laser Compton backscattering gamma-ray source in coupling with nuclear resonance fluorescence (NRF) method for precise inspection. The inertial electrostatic confinement fusion device has been adopted as a neutron source and two neutron-detection methods, delayed neutron noise analysis method and high-energy neutron-detection method, have been developed to realize the fast screening system. The prototype system has been constructed and tested in the Reactor Research Institute, Kyoto University. For the generation of the laser Compton backscattering gamma-ray beam, a race track microtron accelerator has been used to reduce the size of the system. For the NRF measurement, an array of LaBr3(Ce) scintillation detectors has been adopted to realize a low-cost detection system. The prototype of the gamma-ray system has been demonstrated in the Kansai Photon Science Institute, National Institutes for Quantum and Radiological Science and Technology. By using numerical simulations based on the data taken from these prototype systems and the inspection-flow, the system designed by this program can detect 1 kg of highly enriched 235U (HEU) hidden in an empty 20-ft container within several minutes.

We developed the isotope-specific Computed Tomography imaging technique based on the Nuclear Resonance Fluorescence absorption method at BL1U of UVSOR-III by using high flux and quasi-monochromatic gamma-ray beam for a sample target containing two enriched lead isotope rods (206Pb and 208Pb). The maximum Laser Compton Scattering gamma-ray beam energy was 5528 keV and the gamma-rays flux was greater than 107 photons/s after collimation. A two-dimensional image of 208Pb isotope distribution was selectively obtained using an exited state in 208Pb at 5512 keV.

Infrared free electron laser (IR-FEL) enables us to investigate novel laser-driven phenomena in various research fields, due to its high photon density in a picosecond time scale as well as wide wavelength tunability in the infrared region. As an application of IR-FEL to biomedical field, we show here efficient dissociation of amyloid fibrils that are cause of serious amyloidosis by using the FEL tuned to amide band.

A mid-infrared Free Electron Laser (MIR-FEL) is a candidate to realize a high-repetition-rate attosecond hard X-ray source based on the high harmonic generation (HHG). A new project to conduct proof of principle experiment of HHG driven by a MIR-FEL has been started from November 2018. For this purpose, upgrade projects of MIR-FEL at Kyoto University (KU-FEL) has been started to achieve high peak power and ultra-short pulse operation.

Photocurrent of CeB6 was measured over wide range of temperature. The photocurrent at high temperature exceeds the value expected by the Fowler-DuBridge theory. A transmission coefficient was introduced to the photoemission equation in order to account for electron escape probability. The fitting of measured data reveals that the transmission coefficient can not explain the rise in photocurrent at high temperatures for different photonenergies.

Melanin is rigidly constructed by several nitrogen-containing aromatic rings, and its excess accumulation in skin tissue is closely associated with melanosis. Although visible lasers (wavelength: 600-1000 nm) are conventionally used for the photo-thermolysis of melanocyte, several pigmented nevi are difficult to be treated. Here, we propose an alternate method for targeting the molecular structure of melanin using an infrared free-electron laser (FEL) tuned to 5.8 μm that corresponds to the stretching vibrational mode of carboxylate group. A drastic morphological change on the black-colored surface of melanin powder was observed after the pulse irradiation with power energy of 500 mJ cm-2 , and the minimum irradiation time for damage to the morphology was 1.4 s. Analyses by mass spectroscopy, infrared spectroscopy, and 13 C-nuclear magnetic resonance implied that a pyrrole group was removed by the FEL irradiation. In addition, the FEL irradiation dispersed almost all of the melanoma cells from a culture solution without any influence on other ingredients in the medium, and one-cell analysis by infrared microscopy showed that the structure of melanoma could be substantially damaged by the irradiation. This study proposes the potency of intense mid-infrared laser as novel alternative way to reduce melanin.