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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 FLASH, 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. The impact of these errors on dispersion generation depends on the actual operating conditions of the accelerator, so the dispersion must be measured and controlled frequently. In this paper we present numerical studies of spurious dispersion generation, first dispersion measurements and correction results.
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