Comparison of the current LHC Collimators and the SLAC Phase 2   Collimator Impedances
  One of the key sources of transverse impedance in the LHC has been the secondary graphite collimators that sit close to the beam at all energies. This limits the stable bunch intensity due to transverse coupled-bunch instabilities and transverse mode coupling instability. To counteract this, new secondary collimators have been proposed for the phase II upgrade of the LHC collimation system. A number of designs based on different jaw materials and mechanical designs have been proposed. A comparison of the beam coupling impedance of these different designs derived from simulations are presented, with reference to the existing phase I secondary collimator design. 
Hugo Day and Fritz Caspers and Elias Metral and Benoit Salvant and   Roger Jones
physics.acc-ph