Characterization of the RF Fingers Contact Force for the LHC Warm Vacuum Bellow Modules

Along the 27 Km of LHC beam pipe, various types of vacuum bellow modules are needed to compensate the mechanical misalignments of the vacuum chambers during installation and to absorb their thermal expansion during the bake-out. In order to reduce the beam impedance during operation with beams these modules are equipped with RF bridges to carry the image current. They are usually made out of a copper tube insert at one side and Cu-Be RF fingers at the other end of the module. A spring is used to keep the contact between the RF fingers and the tube insert. The geometry and the choice of this spring become critical to ensure a good electrical contact. In this paper, a description of the test bench used to measure the contact force together with the procedure applied and the measurements performed are given. A summary of the maximum radial and axial offsets between the RF fingers and the insert tube while keeping a good electrical contact is presented.