Impedance of the LHC recombination chambers

In the design phase of an accelerator like the LHC, the impedance of every single discontinuity of the vacuum chamber (e.g. bellows, beam position monitors, RF cavities, tapers, vacuum valves, etc.) has to be evaluated in order to guarantee that the total contribution of these elements does not exceed the allocated impedance budget. This note deals with the so-called recombination chambers which bring the two beams into a common chamber around the interaction points so as to allow for the collisions. First, the procedure for the estimation of the impedance is presented. In a second section, the presence of a potentially harmful trapped mode is highlighted and the possible consequences are discussed in some detail.