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TY - UNPB AU - Wu, Y.K. AU - Benson, S.V. AU - Hao, H. AU - Liu, P. AU - Mikhailov, S.F. AU - Popov, V. AU - Yan, J. ED - Schaa, Volker R.W. ED - Decking, Winfried ED - Sinn, Harald ED - Geloni, Gianluca ED - Schreiber, Siegfried ED - Marx, Michaela TI - Generating Orbital Angular Momentum Beams in an FEL Oscillator J2 - Proc. of FEL2019, Hamburg, Germany, 26-30 August 2019 CY - Hamburg, Germany T2 - Free Electron Laser Conference T3 - 39 LA - english AB - Coherent vortices have been generated with several schemes using a single-pass FEL. This work reports the first experimental demonstration of coherent vortice generation using an oscillator FEL. With the storage ring FEL at Duke University, we have established fundamental harmonic FEL lasing in a variety of coherently mixed orbital angular momentum (OAM) modes, with photons in the |l> and |-l> superposition states (l = 1, 2, 3) and with each eigenstate carrying lh OAM. Spatially organized in Laguerre Gaussian modes, stable FEL lasing of these OAM beams has been achieved with reasonable power. Optical techniques have been developed to characterize these beams. The operation of such an OAM FEL paves the way for the generation of OAM gamma-ray beams via Compton scattering. PB - JACoW Publishing CP - Geneva, Switzerland ER -