X-ray free-electron lasers are unique sources of high-brightness coherent radiation. However, existing devices supply only linearly polarized light, precluding studies of chiral dynamics. A device called the Delta undulator has been installed at the Linac Coherent Light Source (LCLS) to provide tunable polarization. With a reverse tapered planar undulator line to pre-microbunch the beam and the novel technique of beam diverting, hundreds of microjoules of circularly polarized X-ray pulses are produced at 500–1,200 eV. These X-ray pulses are tens of femtoseconds long, have a degree of circular polarization of 0.98–0.04+0.02 at 707 eV and may be scanned in energy. We also present a new two-colour X-ray pump–X-ray probe operating mode for the LCLS. Energy differences of ΔE/E = 2.4% are supported, and the second pulse can be adjusted to any elliptical polarization. In this mode, the pointing, timing, intensity and wavelength of the two pulses can be modified.
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The authors thank C.P. O'Grady for the online data-handling system. This work was supported by Department of Energy contract no. DE-AC02-76SF00515. A.O.L. acknowledges funding from the Knut and Alice Wallenberg Foundation through the Max IV synchrotron radiation facility programme. K.H. thanks the AvH Foundation for financial support through the Feodor-Lynen programme. M.I. acknowledges funding from the Volkswagen Foundation within a Peter Paul Ewald-Fellowship.
The authors declare no competing financial interests.
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Lutman, A., MacArthur, J., Ilchen, M. et al. Polarization control in an X-ray free-electron laser. Nature Photon 10, 468–472 (2016). https://doi.org/10.1038/nphoton.2016.79
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