Search for trapped modes in TESLA cavities

In resonant cavities which are connected via beam pipes fields usually tend to propagate with shrinking wavelengths. Nevertheless there may also exist some non-propagating resonant modes (“trapped modes”) in the frequency range above the beam-pipe cut-off with relative high shunt impedances which are strongly localised in the inner part of a accelerating structure. Due to their field distributions these trapped modes can hardly be coupled to dampers at the ends of the cavity and therefore could have dangerous effects to beam dynamics. Their detection is very difficult, if one considers that they are embedded in a spectrum of several hundred resonant modes. We therefore started to search trapped modes in a 9-cell copper TESLA cavity. The principles of our method and results from measurements are presented in this paper