Corrector coils with variable field direction for FLASH 2020

Despite thorough tuning of insertion devices for storage rings or linear accelerators,a (usually gap dependent) kick to the electron beam remains and has to becorrected with the help of steerer coils. These small, resistive coils are placed upstreamand downstream of an undulator and by varying the strength and direction of the generatedmagnetic field, they allow to deflect the beam by a small angle and to consequentlycorrect its trajectory. For the upgrade of the free-electron laser FLASH at DESY Hamburgwe have developed compact and cost-efficient air-cooled steerer coils based on asolid ferrite core around which four sub-coils are wound in a particular geometry. Byenergizing these coils separately, the deflecting field can be tuned and adapted to the gapdependent kick. The ferrite core enhances the coil strength by more than 50% comparedto previous designs enabling them to generate 0.53(0.44)Tmm for horizontal (vertical)field at 1A while still making the steerer suitable for use in a slow feedback system. However,due to the proximity of some steerers to permanent magnet-based phase shifters,significant cross-talk between the two devices is introduced and needs to be mitigated.