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. 2019 Jul 17;9:10391. doi: 10.1038/s41598-019-45024-2

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Global strategy of the experiment (a), and picture of the KARA storage ring (b). Interaction of a relativistic electron bunch with its own emitted coherent radiation leads to the so-called microbunching instability, and formation of a pattern with few millimeter period in the longitudinal direction. For monitoring the longitudinal electron bunch shape, we record the electric field evolution in its vicinity (at few millimeters), using a specially designed picosecond-speed photonic-time-stretch analog-to-digital converter. The digitization is made in two steps: (i) laser pulses are modulated by the electric field using an electro-optic crystal, and (ii) the modulated pulses are analyzed in single-shot, picosecond resolution, and multi-MHz acquisition rate. Note that the crystal is actually placed above the electron beam (the whole photonic time-stretch digitizer is detailed in Fig. 3). The electron bunch microstructure is also emitting intense coherent synchrotron radiation (CSR), which is simultaneously recorded. KARA photograph by Carina Franck (licensed under CC BY).