  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.
