Ken Hayakawa

Characteristics of the fundamental FEL and the higher harmonic generation at LEBRA

Abstract The FEL system of Laboratory for Electron Beam Research and Application (LEBRA) at Nihon University has provided relatively high FEL gain around 10% at the wavelength of 1.5 μm . The numerical estimation of the FEL gain suggests that LEBRA-FEL system has a sufficient performance on the electron beam bunching. In addition, the amplification of the third harmonics accompanying the intense fundamental FEL has been observed and the spectra of which have been measured. The experimental result indicates that the phenomenon is a kind of nonlinear harmonic generation.

Advanced applications of PXR at LEBRA, Nihon University

The monochromatic X-ray source based on parametric X-ray radiation (PXR) was developed by using Si(111) perfect crystals and the electron beam from the 125-MeV linac at Nihon University. Since the X-ray beam from the PXR system has a large exposure area with uniform flux density, the PXR-based source is suited for X-ray radiography. In addition to ordinary radiography, X-ray absorption spectroscopy and phase-contrast imaging have been developed as advanced applications of PXR. The absorption spectra of several samples were obtained using the energy dispersion of PXR, and the X-ray absorption fine structures (XAFS) were actually found in the spectra. With respect to phase-contrast imaging, refraction-contrast images have been obtained by using the X-ray diffraction in the (+, −, +) arrangement of perfect crystals. The high-contrast and the phase-reversal of the images taken in the experiment suggest that LEBRA-PXR has a high spatial coherence sufficient for phase-contrast imaging.

X-ray imaging using a tunable coherent X-ray source based on parametric X-ray radiation

A novel X-ray source based on parametric X-ray radiation (PXR) has been employed for X-ray imaging at the Laboratory for Electron Beam Research and Application (LEBRA), Nihon University. Notable features of PXR are tunable energy, monochromaticity with spatial chirp, narrow local bandwidth and spatial coherence. Since the X-ray beam from the PXR system has a large irradiation area with uniform flux density, the PXR-based source is suited for X-ray imaging, especially for application to phase-contrast imaging. Despite the cone-like X-ray beam, diffraction-enhanced imaging (DEI) can be employed as a phase contrast imaging technique. DEI experiments were performed using 14- to 34-keV X-rays and the phase-gradient images were obtained. The results demonstrated the capability of PXR as an X-ray source for phase-contrast imaging with a large irradiation field attributed to the cone-beam effect. Given the significant properties of the LEBRA-PXR source, the result suggests the possible construction of a compact linac-driven PXR-Imaging instrument and its application to medical diagnoses.

Wavelength Dispersive X-ray Absorption Fine Structure Imaging by Parametric X-ray Radiation

The parametric X-ray radiation (PXR) generator system at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University is a monochromatic and coherent X-ray source with horizontal wavelength dispersion. The energy definition of the X-rays, which depends on the horizontal size of the incident electron beam on the generator target crystal, has been investigated experimentally by measuring the X-ray absorption near edge structure (XANES) spectra on Cu and CuO associated with conventional X-ray absorption imaging technique. The result demonstrated the controllability of the spectrum resolution of XANES by adjusting of the horizontal electron beam size on the target crystal. The XANES spectra were obtained with energy resolution of several eV at the narrowest case, which is in qualitative agreement with the energy definition of the PXR X-rays evaluated from geometrical consideration. The result also suggested that the wavelength dispersive X-ray absorption fine structure measurement associated with imaging technique is one of the promising applications of PXR.

Observation of intense terahertz-wave coherent synchrotron radiation at LEBRA

We observed intense coherent synchrotron radiation (CSR) in the terahertz region using an S-band linac at the Laboratory for Electron Beam Research and Application at Nihon University. The evolution of the CSR power was measured, and the CSR reflected in the vacuum chamber of the bending magnet could be extracted through the quartz window for a few tens of picoseconds. The long wave packet of the delayed CSR in the autocorrelation suggests that the delayed CSR was the non-resonant ring-down of the vacuum chamber of the bending magnet. To design a high-energy accelerator, it is necessary to decrease high-energy photons resulting from Compton backscattering with intense CSR.

Phase Contrast Imaging of Biological Materials using LEBRA‐PXR

Phase contrast x‐ray imaging is an important technique for investigation of materials consisted of light atoms, such as soft biological tissues. The tunable monochromatic x‐ray source based on Parametric X‐ray Radiation (PXR), which was developed at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University, provides x‐rays with a high spatial coherence which is an essential property required for phase contrast imaging. In preliminary experiment, refraction contrast images for leaf tissues of a tree and animal specimen have been obtained successfully with the LEBRA‐PXR x‐rays. In the imaging system, the x‐ray that passed through the sample once reflects off the silicon perfect‐crystal x‐ray analyzer at the Bragg angle, and then enters the imaging plate. The bright‐field and the dark‐field phase contrast images have been obtained by infinitesimal rotations of the analyzer, showing the evidence of contrast reversal. Although the conventional radiograph by absorption contrast was also taken with the LEBRA‐PXR, significant differences are found between the radiograph and the phase contrast images.Phase contrast x‐ray imaging is an important technique for investigation of materials consisted of light atoms, such as soft biological tissues. The tunable monochromatic x‐ray source based on Parametric X‐ray Radiation (PXR), which was developed at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University, provides x‐rays with a high spatial coherence which is an essential property required for phase contrast imaging. In preliminary experiment, refraction contrast images for leaf tissues of a tree and animal specimen have been obtained successfully with the LEBRA‐PXR x‐rays. In the imaging system, the x‐ray that passed through the sample once reflects off the silicon perfect‐crystal x‐ray analyzer at the Bragg angle, and then enters the imaging plate. The bright‐field and the dark‐field phase contrast images have been obtained by infinitesimal rotations of the analyzer, showing the evidence of contrast reversal. Although the conventional radiograph by absorption contrast was also ...

Phase‐contrast imaging with a novel X‐ray source

A novel X‐ray source based on Parametric X‐ray radiation (PXR) has been employed for phase‐contrast imaging at the Laboratory for Electron Beam Research and Application (LEBRA), Nihon University, Japan. The PXR X‐rays were generated by the 100 MeV electron beam passing through a Si single crystal. The X‐rays in the 16∼34 keV range were chosen for imaging of biological samples. The quasi‐monochromatic, tunable, and coherent X‐ray source is appropriate for this application. In addition, the large X‐ray beam irradiation field of approximately 100 mm in diameter, which was achieved without special optics, suggests that the PXR is applicable to imaging for medical diagnostics.

GEOMETRICAL EFFECT OF TARGET CRYSTAL ON PXR GENERATION AS A COHERENT X-RAY SOURCE

The experiments of the PXR performance carried out for the target crystals with different cutting planes have shown significant difference in the PXR property, which suggests another way to increase the PXR intensity than by increasing the electron beam current. In order to investigate the effect of the geometrical condition of the crystal surface on the PXR property, the experiments have been carried out for the target crystal with a knife-edge-shaped cut surface. For the case with symmetric Bragg geometry on the front surface and asymmetric condition on the rear surface, the rather low intensity X-ray beam has shown considerably good spatial coherence. The X-ray beam with narrow line width has made it possible to obtain X-ray absorption spectra with a high resolution. In contrast, relatively high intensity, which enabled taking an absorption image with the exposure for several tens of seconds, has been obtained for the geometry with asymmetric front surface and symmetric rear surface. This configuration, however, has raised a problem of degradation in the spatial coherence of the X-rays due to the superposition of two different X-ray beams.

Suppression of Energy Fluctuation for the Free Electron Laser at LEBRA.

The first lasing of the free electron laser (FEL) was achieved in May 2001 at Nihon University. Problems occurred regarding the energy stability of the electron beam in the pulse duration and the long operation time for the FEL. The energy stability has been improved by compensation of the RF phase drift with a function generator and a feedback circuit.

Construction and development of an UV free electron laser under the cooperation of Nihon U., KEK, PNC, ETL and Tohoku U.

Abstract The construction and the development of an UV free electron laser at Narashino Campus, Nihon U. have been started under the cooperation of Nihon U., KEK 1 , PNC 2 , ETL 3 and Tohoku U. The project requires a 125 MeV S-band electron linear accelerator to expand the oscillation of FEL to the UV region using fundamental mode. The injection system consists of a thermionic RF-gun with a LaB 6 cathode and a magnet for magnetic bunching. We are studying to reduce the back-bombarding electrons to realize the macropulse length of 20 μs. Electron beams, up to the energy of 125 MeV, are injected into the optical cavity. Changing the accelerating energy and/or undulator parameters, this system will cover the range from infrared to ultraviolet for the applications in the various fields.