On trapped modes in the LHC recombination chambers: numerical and experimental results

The recombination chamber in LHC (Large Hadron Collider) allows the separated proton beams to merge into a common vacuum chamber surrounding the interaction points. It has been subject of thorough studies concerning its interaction with the circulating beam. In this paper we present the numerical and experimental results of our investigation. We show that in the smooth transitions between pipes of different diameters a trapped mode may exist. The mode results to be not harmful for the LHC operation, both for the beam stability and power loss.