Evidence for a strongly coupled dipole mode with insufficient damping in TTF first accelerating module

A beam experiment has been conducted on the first accelerating module of the TESLA Test Facility (TTF) to investigate transverse higher order modes (HOM) in the superconducting cavities. By injecting the beam with a transverse offset and by modulating the intensity of the 216 MHz bunch train with a tunable frequency in the 0-108 MHz range, transverse HOMs can be excited resonantly. On a resonance the outgoing bunch orbits, measured with a broadband BPM, are transversely modulated at the same frequency. A dipole mode at about 2585 MHz, belonging to the third passband of the TESLA cavities, has been excited and observed in this way, with unexpected low damping in 2 out of the 8 cavities of the module.