Experimental set-up-angular streaking spectroscopy. Previous streaking measurements of XFEL pulses 37-39 used a linearly polarized streaking field to encode their temporal profile onto the kinetic energy of photoelectrons. Depending on the amplitude and phase of

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.