Polarization control in an X-ray free-electron laser

Abstract

X-ray Free-Electron Lasers (XFELs) are unique sources of high brightness coherent radiation. However, existing XFELs 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 500-1200 eV. These X-ray pulses are tens of femtoseconds long, have a degree of circular polarization of 0.98 +0.02 −0.04 at 707 eV, and may be scanned in energy. We also present a new twocolor X-ray pump X-ray probe operating mode for 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.

Emitter-site-selective photoelectron circular dichroism of trifluoromethyloxirane

“…As the next step, we plan to extend it towards exploration of transient chirality accompanying fragmentation dynamics by inner-shell pump -inner-shell probe experiments at Xray free-electron lasers [63,64].…”

Non-invasive online wavelength measurements at FLASH2 and present benchmark

“…As mentioned in the Introduction, setups employing a number of eTOFs at various angles enables measurements of the polarization Allaria et al, 2014;Lutman et al, 2016). Moreover, temporal information such as the FEL pulse duration and arrival time can be determined with techniques involving streaking of photoelectrons by an external THz field (Grguraš et al, 2012).…”

“…There, it serves as a general diagnostic tool, enabling the simultaneous observation of beam properties such as polarization and photon energy as well as beam position . In analogy to the OPIS, studies using this apparatus at FEL sources have been reported recently (Allaria et al, 2014;Lutman et al, 2016). In order to test the PCA method for OPIS operation we chose a measurement at = 52 nm with argon as the target gas, hence having simple eTOF spectra which show just the Ar 3p photoemission line.…”

Matching-based fresh-slice method for generating two-color x-ray free-electron lasers

“…The electron bunch traverses the dechirper on-axis and no dipole kick is introduced, thus avoiding distortion to the beam orbit in the transport system as well as in the undulator, and allowing the method to be applied in high repetition-rate XFELs. Other advanced applications utilizing such a matching-based method may include the amplification of a seed pulse in a self-seeding scheme [29], x-ray pulse duration control, seeding a helical undulator for polarization control [30] and so on. Experimental studies have been planned at the LCLS.…”