Cross talks between storage ring magnets at the Extremely Brilliant Source at the European Synchrotron Radiation Facility

Significant magnetic cross talks were observed between neighboring magnets on the Extremely Brilliant Source (EBS) low emittance storage ring. The main sources for these cross talks are the short longitudinal distances between magnets, which reach 47 mm in some cases. It affects mainly the dipole bending angles and the quadrupole gradients. A 1% bending angle correction was applied to the (permanent) bending magnets before their installation in the storage ring to compensate for the cross talks, while gradient errors as high as 1.8% were observed on quadrupoles. Intensive 3D magnetic simulations gave the longitudinal distribution of the cross talk errors for various magnet pairs and current settings. The error distribution depends on the magnet pairs: cross talks may create errors at magnet edges or all along the magnets, with same or opposite polarity. The simulations were validated by magnetic measurements, the discrepancies between measured and simulated values being in the 10-4 range with respect to the nominal gradients. Cross talk induced gradient errors were added to all quadrupoles and neighboring magnets in the lattice model. Even if it complicated the commissioning, the cross talks have no impact on the final performances of the lattice as the design machine parameters were reached for the most part.