Correction of linear coupling on the basis of response matrix modelling and X-ray pinhole measurement

A new attempt is made to model a linearly coupled machine using the measured coupled-orbit response. Rather than seeking for the skew errors of the existing magnets, the aim is to obtain an effective skew error distribution that consistently describes the measured coupling characteristics. Such a solution is actually found, which is assured by the simulation that the measured orbit cross-talk provides sufficiently accurate information on the locally integrated skew errors in the ESRF configuration. To pursue the limit of correction, an optimisation of the skew corrector position is then made with respect to the obtained error distribution, with which a large reduction by nearly an order of magnitude is achieved on the model as compared to the standard correction. The actual steps in realising the lowest coupling values on the real machine with additional correctors are described. In the regime where the magnitude of the orbit cross-talk is already too small to be measured, the X-ray pinholes are utilised, which are capable of measuring the vertical emittance down to several picometres.