Robust linear coupling correction with N-turn maps

The linear one-turn map of a storage ring contains coupling information on which a correction algorithm can be based. In principal, the one-turn matrix can be fitted from turn-by-turn data of beam position monitors. However, the signal-to-noise ratio of the coupling information can be greatly enhanced by fitting maps for larger turn numbers N. Furthermore, by using a number of beam position monitors in a region with only small coupling sources, the determination of the N-turn map can be made robust against failures of individual beam position monitors, and the signal-to-noise ratio of the coupling information is further enhanced. With the so obtained N-turn maps an automated global coupling correction is possible without the need for a tune change. This is demonstrated for the Brookhaven Relativistic Heavy Ion Collider where the implementation of the algorithm allows a global coupling correction within a few seconds at injection.