INTRODUCTION

Undulators and wigglers exhibit small variations of the field integral and thus a finite residual deflection angle, which is depending on the ID operating parameters. This dipole kick causes orbit distortions whenever the ID parameters are altered. The fast orbit feedback (FOFB) [1] is able to decouple different beamlines, i.e. an orbit distortion induced by moving the ID of one beamline is not observable by the others under the condition that the frequency is of the order of a few Hz. However, since the orbit distortion is a predictable and unique function of the ID gap and, in some cases, can also be quite large, it is advantageous to take steps to cancel the predicted distortion in advance, by means of a hardwired ID feed forward (IDFF) table, in order to ease the burden of the FOFB.