Third harmonic cavities have been designed and fabricated by FNAL to be used
at the FLASH/XFEL facility at DESY to minimise the energy spread along the
bunches. Modes in these cavities are analysed and the sensitivity to frequency
errors are assessed. A circuit model is employed to model the monopole bands.
The monopole circuit model is enhanced to include successive cell coupling, in
addition to the usual nearest neighbour coupling. A mode matching code is used
to facilitate rapid simulations, incorporating fabrication errors. Curves
surfaces are approximated by a series of abrupt transitions and the validity of
this approach is examinedComment: Proceedings of 14th International Conference on RF Superconductivity
(SRF 2009), 2009, Berlin, German