STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
STFC/DL, Daresbury, Warrington, Cheshire
The choice of undulator design and minimum magnet gap is crucial in the definition of every short wavelength FEL and is ultimately a cost driver for that project. The magnet gap selection is a compromise between wanting to minimise harmful wakefield effects whilst at the same time generating high magnetic fields with short periods. The NLS project has tried to take a holistic approach in the definition of the undulators. This has been carried out by first assessing the impact of resistive wall wakefields in general on the FEL performance and then selecting the maximum level of wakefield which has a just tolerable impact on the FEL. This wakefield is then translated into equivalent circular and elliptical vessel geometries. Suitable vessel thickness and mechanical tolerances are then added to define the undulator magnet gap for the case of a circular vessel (Delta undulator) and an elliptical vessel (APPLE-2 undulator). Finally, the two types of undulator have been modelled, their parameters compared, and a selection made. This paper summarises this global self-consistent approach to undulator definition and reports on the result for the NLS.
