Scheme for correcting coupling variation induced by insertion devices near linear difference resonance

We have developed a scheme for correcting emittance coupling variations resulting from the operation of insertion devices (IDs) in a storage ring operated near a linear difference resonance. The present scheme corrects for weak skew quadrupole error fields and betatron tune shifts induced by the IDs since the sensitivity of the emittance coupling to the skew error field is enhanced near a difference resonance and betatron tune shifts also contribute to the coupling variation. We applied the coupling correction scheme to the operation of the APPLE-II type variable polarization undulator in the SAGA-LS storage ring. Changes in the gap and phase of the APPLE-II undulator significantly alter the emittance coupling, which modulates the photon beam intensities in the experimental stations. The coupling correction system employs a feedforward scheme by using two-dimensional look-up tables. Correction coil currents are determined by measuring the betatron tune and the vertical beam size. Beam test results suggest that the coupling variation can be well explained by the ID skew error field and the tune shift caused by the intrinsic focusing effect of the APPLE-II undulator. They also demonstrate that the correction system sufficiently suppresses the coupling variation.