The Magnetic Model of the LHC at 6.5 TeV

After two years of shutdown, the Large Hadron Collider (LHC) operated in 2015 at an energy of 6.5 TeV. In this paper, we give the first outlook of the behavior of the LHC magnets operating at this field level, corresponding to 8-T peak field in the main dipoles. The main magnetic features are reconstructed through the beam measurements, mainly the tune (quadrupolar components) and the chromaticity (sextupolar components). The decay and snapback at injection and the start of the ramp are expected to increase by 50% due to the higher operational current. The behavior at high field will be affected by the saturation components, which were barely visible in one magnet family in the 4-TeV runs and totally invisible in the rest. Although the initial plan aims at a rather conservative value of β*, during the squeeze, the triplet and matching section quadrupole will become important. Beam measurements will provide interesting information about the magnetic behavior of these quadrupoles; in particular, through beta beating, we will estimate the absolute precision of the knowledge of the quadrupole strength. The statistics of the beam measurements will be rather limited in the first stages of commissioning, but should already reveal the main features of the 6.5-TeV operation.