Vertical Crossing Angle in IR8

The operation of the LHCb spectrometer dipole has a considerably larger and more challenging impact on the geometry of the LHC beams than the magnets in the high luminosity regions [1]. The integrated dipole field of 4 Tm deflects the beams in the horizontal plane, and using a set of three dipole magnets, called "compensators" a closed horizontal orbit bump is created. This paper summarizes the basic layout of the beam geometry in IR8 under the influence of the LHCb dipole and its compensators and shows the theoretically expected beam orbits, envelopes and aperture needs in the originally designed version. LHCb operation with both field polarities leads to unequal net crossing angles between the two beams and affects the experiment acceptance. It had been proposed therefore to establish a LHC operation mode where the originally designed horizontal crossing angle is shifted at high energy into the vertical plane leading to a vertical crossing scheme at luminosity operation. The new scheme has been successfully implemented in the LHC operation in 2012 together with the so-called diagonal luminosity leveling procedure [2]. In view of the future 25 ns bunch spacing for LHC operation after the long shut down LS1 however, the present injection scheme of LHC in IR8 will have to be modified. A vertical crossing angle at injection therefore is proposed to avoid parasitic bunch crossings during injection and acceleration mode to overcome the problem. In this case additional aperture is required in the vertical direction. Accordingly this scenario relies either on small beam emittances and orbit tolerances or on a rotated beams screen inside the IR8 triplet magnets, if the LHCb dipole magnet cannot be ramped down for beam injection.