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TY - UNPB AU - Holldack, K. AU - Armborst, F. AU - Bahrdt, J. AU - Frentrup, W. AU - Goslawski, P. AU - Pontius, N. AU - Ries, M. AU - Scheer, M. AU - Schälicke, A. AU - Schüssler-Langeheine, Ch. ED - Schaa, Volker RW TI - Alternating the Helicity of X-Ray Photons from an Undulator at Unprecedented Speed J2 - Proc. of IPAC2020, Caen, France, 10-15 May 2020 CY - Caen, France T2 - International Particle Accelerator Conference T3 - 11 LA - english AB - X-ray circular dichroism (XMCD), one of the main tools of modern X-ray physics for studying magnetism, benefits enormously from the capability of a fast alterable helicity of circularly polarized X-ray photons. Here we present a novel method, which allows boosting the alternating frequency between right- and left-handed photons to the MHz regime, more than three orders of magnitude faster than present state-of-the-art technologies. The method is based on a twin elliptical undulator installed in a low emittance electron storage ring being operated in a novel mode. Here, the electron optics is tuned close to a resonance where the electron bunches are spatially separated in so-called transverse resonance island buckets (TRIBs). Propagating through the magnetic structures of the twin undulator, electrons from different islands emit photons of the same wavelength but of opposite helicity. Radiation from these two helicity components can be alternated as fast as 2 ns, given by the radio frequency of the accelerator cavity. In the present proof-of-principle experiment at BESSY II we demonstrate a XMCD measurement at the L2,3 absorption edges of a Ni sample with an 800 ns helicity flip. PB - JACoW Publishing CP - Geneva, Switzerland ER -