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

The recombination chamber in 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 on an approximated geometry. We show that in the smooth transitions between pipes of different diameters a trapped mode may exist, both in the real and in the approximated geometry. In the real structure, the mode is weak and not harmful both for the beam stability and power loss.