  The purpose of this paper is to calculate the longitudinal and transverse
wakefields of the FCC collimators by using the electromagnetic codes ECHO3D and
IW2D. We cross-checked our results using CST particle studio for long bunches,
and found them to be in good agreement. The obtained results show that the
collimators give one of the highest contributions to the overall FCC-ee wake
potentials. Using the code PyHEADTAIL, we have found that the presence of the
geometric wakefield of the collimators leads to the occurrence of transverse
mode coupling instability (TMCI) at a significantly lower bunch population as
compared to that of all other contributions and solutions to reduce this
geometric term must be found.
