End field structures for linear/helical insertion devices

The field integral tolerances required on third generation synchrotron sources are of the order of 10-20 Gcm in the whole gap range of an insertion device. This can be achieved without electromagnet correction by a proper magnetic design of the field termination. The paper describes several such end field terminations to be used for planar undulators. A new termination valid for an APPLE II undulator is presented which produces a field integral variation smaller than 20 Gcm for any useful setting of the magnetic gap and phase. It compares quite favorably with other known type of terminations