Investigation of a High-q Dipole Mode at the Tesla Cavities

Higher order modes (HOM) can have a destructive influence on the beam emittance in the future linear colliders. Therefore it is important that the quality factor of the high impedance modes be damped under a safe level. Although attentive care was taken in designing the superconducting 9-cell cavities and the HOM couplers for the TESLA collider, a high Q and high impedance mode was discovered in some cavities at the TESLA Test Facility (TTF). In order to discover the reason for which this particular mode is very little damped, a series of investigations have been carried out. The mode frequency is higher for the studied cavities comparatively to others, where this mode seems to be properly damped. Field distribution measurements at the separate cavities show a quasi-trapping of the mode. The simulations and measurements done at individual cavities show that for certain boundary conditions the field is minimum at the location of both HOM couplers in a tube.