Digital Signal Processing Techniques to Monitor Bunch-by-Bunch Beam Positions in the LHC for Machine Protection Purposes

This paper presents the development of an upgrade to the beam position interlock system for the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN). The beam orbit at the beam dump kicker is continuously monitored by 16 beam position monitors that are part of the machine protection system. In case of unacceptable orbit movement the system has to trigger the beam abort immediately to prevent damage to the machine. An upgrade of the present system is underway with the aim of handling a larger dynamic range of bunch intensities, and coping with different bunch time structures (both the standard bunch spacing of 25 ns and special doublet bunches spaced by 5 ns). The proposed architecture combines the analogue signals from opposite pickup electrodes on a single read-out channel, and stretches it with a delay-line based comb-filter. The resulting signal, covering a dynamic range of 60 dB, is digitised at 3.2 GSPS and processed inside a Field-programmable Gate Array (FPGA) to extract a position value. Different signal processing techniques are compared on simulated ideal beam signals, and preliminary results of a prototype installation is presented.