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Wavefront sensing of individual XFEL pulses using ptychography
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology, Molecular biophysics.ORCID iD: 0000-0002-1887-7551
University College London.
Oxford University.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Cell and Molecular Biology, Molecular biophysics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The characterization of the wavefront dynamics is important for many X-ray free-electron laser (XFEL) experiments, in particular for coherent diffractive imaging (CDI), as the reconstructed image is always the product of the incoming wavefront with the object. An accurate understanding of the wavefront is also important for any experiment wishing to achieve peak power densities, making use of the tightest possible focal spots. With the use of ptychography we demonstrate high-resolution imaging of the Linac Coherent Light Source (LCLS) beam focused at the endstation for Atomic, Molecular and Optical (AMO) experiments, including its phase and intensity at every plane along its propagation axis, for each individual pulse. Using a mixed-state approach, we have reconstructed the most dominant beam components that constitute an ensemble of pulses, and from the reconstructed components determined their respective contribution in each of the individual pulses. This enabled us to obtain complete wavefront information about each individual pulse. We hope that our findings aid interpretation of data from past and future LCLS experiments and we propose this method to be used routinely for XFEL beam diagnostics. 

National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:uu:diva-329011OAI: oai:DiVA.org:uu-329011DiVA: diva2:1152295
Available from: 2017-10-24 Created: 2017-10-24 Last updated: 2017-10-24
In thesis
1. Algorithms for Coherent Diffractive Imaging with X-ray Lasers
Open this publication in new window or tab >>Algorithms for Coherent Diffractive Imaging with X-ray Lasers
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Coherent diffractive imaging (CDI) has become a very popular technique over the past two decades. CDI is a "lensless" imaging method which replaces the objective lens of a conventional microscope by a computational image reconstruction procedure. Its increase in popularity came together with the development of X-ray free-electron lasers (XFELs) which produce extremely bright and coherent X-rays. By facilitating these unique properties, CDI enables structure determination of non-crystalline samples at nanometre resolution and has many applications in structural biology, material science and X-ray optics among others. This work focuses on two specific CDI techniques, flash X-ray diffractive imaging (FXI) on biological samples and X-ray ptychography.

While the first FXI demonstrations using soft X-rays have been quite promising, they also revealed remaining technical challenges. FXI becomes even more demanding when approaching shorter wavelengths to allow subnanometre resolution imaging. We described one of the first FXI experiments using hard X-rays and characterized the most critical components of such an experiment, namely the properties of X-ray focus, sample delivery and detectors. Based on our findings, we discussed experimental and computational strategies for FXI to overcome its current difficulties and reach its full potential. We deposited the data in the Coherent X-ray Database (CXIDB) and made our data analysis code available in a public repository. We developed algorithms targeted towards the needs of FXI experiments and implemented a software package which enables the analysis of diffraction data in real time.

X-ray ptychography has developed into a very useful tool for quantitative imaging of complex materials and has found applications in many areas. However, it involves a computational reconstruction step which can be slow. Therefore, we developed a fast GPU-based ptychographic solver and combined it with a framework for real-time data processing which already starts the ptychographic reconstruction process while data is still being collected. This provides immediate feedback to the user and allows high-throughput ptychographic imaging.

Finally, we have used ptychographic imaging as a method to study the wavefront of a focused XFEL beam under typical FXI conditions. 

We are convinced that this work on developing strategies and algorithms for FXI and ptychography is a valuable contribution to the development of coherent diffractive imaging. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 64 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1589
Keyword
X-ray lasers, coherent diffractive imaging, algorithms, lensless imaging, flash diffractive imaging, flash X-ray imaging, aerosol injection, FEL, XFEL, CXI, CDI, FXI
National Category
Biophysics Atom and Molecular Physics and Optics
Research subject
Physics with specialization in Biophysics
Identifiers
urn:nbn:se:uu:diva-329012 (URN)978-91-513-0129-7 (ISBN)
Public defence
2017-12-15, Room B7:101a, Biomedicinska Centrum (BMC), Husargatan 3, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2017-11-21 Created: 2017-10-24 Last updated: 2017-11-21

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