• P. Zhang, R.M. Jones, I.R.R. Shinton
  UMAN, Manchester
• N. Baboi, B. Lorbeer, P. Zhang
  DESY, Hamburg
• T. Flisgen, H.-W. Glock, U. van Rienen
  Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock

Third harmonic superconducting cavities have been designed and fabricated by FNAL to minimise the energy spread along bunches in the FLASH facility at DESY. A module, consisting of four nine-cell 3.9 GHz cavities, has been installed in FLASH. The first measurement with beam excitation is presented, and the comparisons to transmission measurement without beam and simulations are made. Higher order modes (HOMs) are able to propagate to adjacent cavities through attached beam tubes. Modes from 1.3 GHz cavities in the module nearby also propagate into this module.

*Work supported by European Commission under the FP7 Research Infrastructures grant agreement No.227579.

• R.M. Jones, I.R.R. Shinton, P. Zhang
  UMAN, Manchester
• N. Baboi
  DESY, Hamburg
• T. Flisgen, H.-W. Glock, U. van Rienen
  Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock

Transverse modes in the 3.9 GHz cavities designed and fabricated by FNAL are reported on. These modes have the potential to cause significant emittance dilution if they not sufficiently suppressed. Recent experiments, both probe-based and beam-excited, have indicated significant discrepancies between modes predicted in stand-alone 9-cell cavities compared to those in 4-cavity modules. We employ a suite of computer codes and circuit models to analyze these modes, coupled through beam tubes whose cut-off is above that of the first dipole band. We also report on preparations to instrument the higher order mode couplers with electronics suitable for diagnosing both the beam and cavity position, based on modes with sufficient R/Q values.