Decay and Snapback in Nb3Sn Dipole Magnets

Decay and snapback have been discovered in Tevatron approximately 30 years ago; since then significant advancements have been done in understanding and controlling these effects. The main sources of concern are the dynamic effects in the main dipoles and quadrupoles. For the dipole, the decay of sextupole gives a large chromaticity change during injection and ramp if it is not controlled properly. For the quadrupoles, main concern is the normal quadrupole gradient, giving a change of tune. Both effects can affect beam stability. In Nb3Sn magnets, the experience is less wide and the set of available data is smaller. Here, we review the CERN experience concerning the recently built 11 T dipoles. These studies are relevant to establishing target values for decay and snapback in the future circular collider study, and probe the possibility of injecting below or around the penetration field, thus possibly reducing the injection energy.