January 17, 2018
RIKEN
Keio University
A research team from RIKEN and Keio University, including Trainee Amane Kobayashi (a third-year student in the Doctoral Programs at the Graduate School of Science and Technology, Keio University), Visiting Chief Scientist Masayoshi Nakasako (Professor, Department of Physics, Faculty of Science and Technology, Keio University), and Unit Leader Masaki Yamamoto of the Life Science-Use Synchrotron Radiation System Development Unit at the RIKEN SPring-8 Center, has devised and established a theory and measurement method to correctly evaluate the wavefront uniformity (spatial coherence) of each XFEL pulse—at a rate of 30 Hz (30 times per second)—that has been intensity-enhanced by focusing mirrors at the X-ray free-electron laser (XFEL) facility SACLA.
SACLA can generate high-intensity X-ray pulses with uniform wavefronts. For diffraction experiments on samples with poor X-ray scattering capabilities, which require particularly strong XFEL pulses, focusing mirrors are used to reduce the beam size and further increase the intensity. It was previously believed that by appropriately adjusting the position of the focusing mirrors, the wavefront of the XFEL pulses would also be uniform at the sample position. However, in 2014, a joint research group from Germany and SACLA measured a parameter representing the spatial coherence of the XFEL pulses and announced that, with 1.0 representing a perfectly uniform wavefront, the value at SACLA was only about 0.7.
The research team has now discovered a major problem with the analysis method proposed in previous research. To construct a theory for correctly estimating spatial coherence, they decided to incorporate the theory of the dark-field phase retrieval method, developed in 2014 by Visiting Chief Scientist Masayoshi Nakasako and his colleagues, into a physical-mathematical theory published in 2007 for recovering original diffraction patterns from those whose intensity differences were reduced by being recorded on detector pixels. Using these, they reconstructed the data analysis theory, created a program code for practical application, and performed the analysis. As a result, they confirmed that focused XFEL pulses with almost perfect spatial coherence are obtained even after being processed by focusing mirrors. Furthermore, they were able to estimate, for each XFEL pulse, the size of the beam that could contribute to the sample's diffraction and the surrounding area of radiation damage.
With this achievement, a method for confirming the spatial coherence of focused XFEL pulses has been established both theoretically and technologically. It is expected that this method will be widely used at other XFEL facilities.
This research was published in the British online scientific journal Scientific Reports in its January 16 issue.
*Research Team
RIKEN SPring-8 Center, Research and Utilization Division
Beam Line Infrastructure Division, Life Science-Use Synchrotron Radiation System Development Unit
Trainee, Amane Kobayashi
(3rd-year student, Doctoral Programs, Graduate School of Science and Technology, Keio University)
Trainee (at the time of research), Yuki Sekiguchi
Visiting Scientist, Tomotaka Oroguchi
(Full-time Lecturer, Department of Physics, Faculty of Science and Technology, Keio University)
Unit Leader, Masaki Yamamoto
Visiting Chief Scientist, Masayoshi Nakasako
(Professor, Department of Physics, Faculty of Science and Technology, Keio University)
For the full press release, please see the link below.