Undulator Photon Beams with Orbital Angular Momentum

Abstract Photons carrying an orbital angular momentum (OAM) are present in the off-axis radiation of higher harmonics of helical undulators. Usually, the purity and visibility of OAM carrying photons is blurred by the electron beam emittance. However, high brightness OAM beams are expected in Ultimate Storage Rings (USRs) and FELs, and they may trigger a new class of experiments utilizing the variability of the topological charge, a 3 degree of freedom besides the wavelength and the polarization. In 2013 a 99eV OAM beam from a helical undulator has been observed for the first time at the storage ring BESSY II. We discuss the sensitivity of the conducted interference experiment on the emittance and the settings of the two undulators which were involved. Furthermore, we propose another interference experiment which will be very robust against the undulators field setting.