On-line monitoring of the linear coupling for the LHC

Before correction, the difference coupling coefficient in LHC is expected to reach |c_|≈0.2 at injection and vary by 30 to 40% from injection to top energy. These high values arise mostly from the field imperfections in the superconducting dipoles, the feed-down from the parasitic sextupoles and the strongly-focused low-β insertions. A measurement method which lends itself to continuous monitoring and active feedback is therefore investigated. Our approach is based on the beam response to an excitation in the other plane (coupling BTF). The analytical approach shows that a proper measurement protocol allows extracting the global and local complex coefficients of both the difference and sum coupling resonances. In order to prevent any emittance blow-up, the beam is excited at small amplitude outside its eigen-frequencies with smooth transitions (AC-dipole principle). A first experiment in the SPS confirms the approach and its robustness.