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Increase in transverse beam size in the undulator caused by dispersive effects is one of the major limitations for the operation of the VUV-FEL at DESY. Sources of the (spurious) dispersion are field errors and stray magnet fields in the undulator beam line as well as spurious dispersion created upstream of the undulator by, for instance, rf coupler kicks, magnet misalignments and field errors. In this contribution, a procedure for dispersion measurement and correction is described and simulations and first experimental results for the VUV-FEL are presented.
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