Waveguide mode reflectometry for obstacle detection in the LHC beam pipe including signal attenuation

Unexpected obstacle formation in the LHC beam-pipe during assembly, cool down and operation may lead to major disturbances. Thus a fast, precise, sensitive and reliable means to detect and characterize such a fault is highly desirable, preferably without the need to break the vacuum. Waveguide mode time domain reflectometry using the synthetic pulse technique has been selected for this purpose. The system will use a modern vector network analyzer operating using essentially the fundamental TM mode on the LHC beam-screen. The objective is to measure over a full arc (2.5 km) with access from either side both in reflection (≈1.25 km range) and transmission mode. If the proposed system is implemented a total of 32 couplers will be permanently installed, which may be used in normal operation for beam diagnostics and other applications. The attenuation of several short beam-screen sections has been measured both for TE and TM modes by means of a resonator method and these data are compared with theoretical results. Waveguide calibration on the beam-pipe and digital signal processing to compensate dispersion are studied. Finally, using a 44 m test track the performance of the proposed methods is examined.