  This study is focused on the development of a HOM-based BPM system for the
ACC39 module currently installed and in operation at FLASH. A similar system is
anticipated to be installed at XFEL. Coupled inter-cavity modes are simulated
together with a limited band of trapped modes. A suite of finite element
computer codes (including HFSS and ACE3P) and globalised scattering matrix
calculations (GSM) are used to investigate the modes in these cavities with a
view to providing guidance on their use as a cavity beam diagnostic.
