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Self-Consistent Orbits with Beam-Beam Effect in the LHC

Abstract

In part of the straight sections of the LHC the two beams share a common beam tube. Therefore the bunches cross each other not only at the interaction point, but as well at many places on either side, with a typical transverse separation of 10 times the transverse beam size. These parasitic encounters lead to orbit distortions and tune shifts, in addition to higher order effects. Since the string of bunches from the injection machine contains gaps, not all possible 3564 buckets around the machine are filled, but only about 3000. This in turn causes some bunches to not always encounter bunches in the opposite beam at one or several parasitic collision points (so-called pacman bunches), or even at the head-on interaction point (super-pacman bunches). With a special program self-consistent orbits in the LHC have been calculated for the first time with the full beam-beam collision scheme resulting from various injection scenarios. The offsets at the interaction points, and the tune shifts are shown to be small enough to be easily controlled

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