Comparison of beam sizes at the collimator locations from measured optics and beam-based collimator alignment at the LHC

Abstract

At the Large Hadron Collider (LHC), the collimation hierarchy is defined in units of the betatron beam size using

the sizes at each collimator location. The beam size at a given collimator can be inferred from the gap measurement

during beam-based alignment campaigns, when the collimator touches a reference beam halo defined with the primary collimators. On the other hand, the beta functions at

each collimator are also measured as a part of the standard LHC optics validation. This paper presents a comparison of the beam size measurements at the collimator locations

applying these two techniques for different machine configurations. This work aims at determining which is the

most reliable method for setting the collimator gaps at the LHC.