Coupler Structures for the LHC Beam Pipe Waveguide Mode Reflectometer

The LHC reflectometer will be used to detect and localise obstacles and other kinds of discontinuities in the LHC beam screen. An important part of this device is the RF coupler element, which provides the interface between the beam screen and the measurement equipment. Two different scenarios of operation are considered. The first option consists in carrying out measurements during assembly by directly branching a coupler to the end of the beam screen. The second one is a permanent installation to be used in situ requiring a different kind of coupler to keep the aperture free. The goal is to achieve a reasonably well matched spurious mode-free excitation over a 25% bandwidth for the TM01 and the TEc11 mode1, respectively. The fulfillment of the required features is severely complicated by space and material restrictions arising mainly from vacuum and installation constraints.