Tatsuyuki Sakurai

Reconstruction of complex-valued electron density with x-ray in-line holograms

A reconstruction algorithm is proposed to analyze the complex-valued electron-density distribution in an object with x-ray in-line holograms. The real and imaginary parts of the electron density correspond to local variations of x-ray phase shift and absorption, respectively. In the algorithm, the least-squares error of the holograms is iteratively minimized under sign constraints on the real and imaginary parts. The constraints, which are derived from the physical conditions in the interaction between x rays and materials, facilitate robust reconstruction. The reconstruction was applied to holograms of a biological specimen that caused both phase shift and absorption.

Phase retrieval with two-beam off-axis x-ray holography

A wave front dividing x-ray interferometer with a prism was applied to two-beam off-axis holography. Complex amplitudes at the detector plane, which were derived from the hologram using the fringe scanning method, were inversely transformed to those at the exit surface of the specimen. The x-ray phase shift due to the amorphous carbon particles was quantitatively determined with the background of the phase image being as small as approximately λ∕100 at 12.4keV.

Compact 110 MW modulator for C-band high gradient accelerator

The C-band high gradient accelerator of SACLA is driven by 72 units of 50 MW klystrons and 110 MW modulators. They are tightly placed in each 4 m space intervals in a 400 m-long klystron gallery. Therefore the modulator has to be very compact and reliable. SACLA is the X-ray free electron laser (XFEL) facility, which requests very tight tolerances of 100 ppm on amplitude and 0.2 degree on phase of the accelerating field. Then the voltage fluctuation and timing jitter of the high-voltage pulse on the klystron cathode should be minimized. In order to meet these demands, we developed a compact, reliable, and stable modulator, which is a conventional linetype modulator with a novel architecture design. All the highvoltage components, including a PFN circuit, a thyratron tube, a pulse transformer, and reverse protection circuits, were installed in a compact steel tank, filled with insulation oil. This design provides good EM noise-shield performance, and superior operational stability. Timing jitter of the thyratron is less than 1 ns, and flatness of the high-voltage pulse is 2%, which provides acceptable effect on the RF phase and amplitude fluctuations. In order to stabilize the charging voltage of PFN capacitors, we developed an extremely stable inverter-type high-voltage charger. The pulse-to-pulse stability of the charging voltage is about 10 ppm in standard deviation, which leads sufficient stability on the accelerating field. Since 2011, 72 modulators have been operated in 8000-9000 hours, to provide a stable electron beam for XFEL. Thus the technology of the innovatively compact and stable modulator system was established.

Evaluation of Defects inside Beryllium Foils using X‐ray Computed Tomography and Shearing Interferometry

When beryllium is used in transmission X‐ray optical elements for spatially coherent beams, speckles are usually observed in the transmission images. These speckles seem to be caused by defects either inside or on the surface of beryllium foil. We measured highly polished beryllium foil using two methods, X‐ray computed tomography and X‐ray shearing interferometry. The results indicate that observed speckle pattern is caused by many voids inside beryllium or inner low‐density regions.

Determination of complex transmissivity using x-ray in-line holography and two-beam interferometry

A method for quantitative measurement of the complex transmissivity of samples is presented where in-line holography and additional shearing interferograms, at a fixed sample-detector distance, are used. The wave field at the detector plane is completely determined and the exit-face wave field of the sample is reconstructed by deconvolution. The principle of experiment and the results of imaging of various samples are described.

Commissioning Status of the Extreme-Ultraviolet FEL Facility at SACLA

To equip SACLA with wide ability to provide laser beams in EUV and soft X-ray regions to experimental users, we have constructed a new free electron laser facility for the SACLA beamline-1. Injector components, such as a thermionic electron gun, two buncher cavities, and their RF sources, were relocated from the SCSS test accelerator. At the downstream of a bunch compressor chicane, 3 C-band acceleration units were newly installed to effectively boost a beam energy up to 500 MeV. 3 invacuum undulators with a larger K-value of 2.1 were remodelled for increasing SASE intensity. Beam commissioning was started in autumn 2015. We carefully tuned an electron beam orbit and bunch compression processes to obtain 240 A at the peak along the injector and 2 bunch compressors. The bunch length was successfully compressed from 1 ns to 1 ps. After the tuning, the lasing of the EUV-FEL was realized. So far the FEL radiation with energy of about 25 J and a 30 nm wavelength driven by a 500 MeV electron beams was observed. In this summer, we will install additional 2 C-band accelerator units to raise the maximum beam energy to 750 MeV for providing a laser at 13 nm.