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. 2019 Jan 24;9:532. doi: 10.1038/s41598-018-36423-y

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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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(a) The transverse beam envelopes σ_x and σ_y for both accelerated and unaccelerated beams The emittance measurement shown in (b) is used as input for particle tracking, in the (a), (c) IFEL simulated electron energy spectrum. Although the spectrum shows a large population of unaccelerated electrons, the focus into the ICS IP (see (a)) is set to produce a transverse focus only for higher energy electrons. This translates into a simulated X-ray spectrum, shown in (d), that is dominated by 12 keV photons. The inset of (a) is the transverse profile of the electron beam at the ICS IP σ_x × σ_y = 140 × 170(±10) μm; the superimposed circle marks the estimated CO₂ FWHM at the IP ~ 150 μm. The measurement in (b) confirms the preservation of the beam normalized emittance during the IFEL acceleration. Emittance measurement data was taken for both the unaccelerated beam and accelerated beam, yielding normalized emittance values of 2.3 ± 0.1 μm and 2.4 ± 0.15 μm, respectively. The error bars are calculated based on the resolution of the transverse position monitors and the statistics of the measurement.