Local correction schemes to counteract insertion device effects

Insertion devices (IDs) of various types provide light of high brilliance to experimenters at the Swiss Light Source (SLS) beamlines. However, changes in the photon energy and polarization by movement of the ID gap and phase shift cause orbit distortions that result in a displacement of the photon beam in both angle and position at the beamline. A feed-forward correction scheme has been developed to quantify and precisely correct these effects using designated correctors local to the photon source. The settings for these correctors are determined using an orbit configuration consisting of 73 digital beam position monitors (DBPMs) and associated correctors; recently commissioned X-ray beam position monitors (XBPMs) located at the beamline front-end are also included in the correction algorithm to further constrain the photon beam to its specified position. The feed-forward correction procedure is finally implemented at the local processor level and applied at a rate of 10 Hz. A photon pointing stability at the sub-microradian level is achieved. The entire gap scan, feed-forward generation and subsequent verification can be completed from within a few minutes to several hours depending on the complexity of the ID. The methodology of the procedure is described and the results from several ID analyses are presented. The effect of the ID on the betatron tune is also discussed. Both feed-forward and feedback procedures have been implemented to maintain the horizontal and vertical components of the tune at the desired working points.