Corrector coils with variable field direction for FLASH 2020

Abstract 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 be corrected with the help of steerer coils. These small, resistive coils are placed upstream and downstream of an undulator and by varying the strength and direction of the generated magnetic field, they allow to deflect the beam by a small angle and to consequently correct its trajectory. For the upgrade of the free-electron laser FLASH at DESY Hamburg we have developed compact and cost-efficient air-cooled steerer coils based on a solid ferrite core around which four sub-coils are wound in a particular geometry. By energizing these coils separately, the deflecting field can be tuned and adapted to the gap dependent kick. The ferrite core enhances the coil strength by more than 50% compared to 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.