Simulations of higher order modes in the ACC39 module of FLASH
  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. 
I. R. R. Shinton, R. M. Jones, Z. Li, P. Zhang
physics.acc-ph