Active HOMs excitation in the first prototypes of superstructure

An alternative layout of the TESLA collider, based on superstructures, reduces investment cost for the main accelerator due to a much lower number of input couplers and resulting from that, a simplification in the RF distribution system. In May 2002, two first Nb prototypes of the superstructure (P1, P2) were installed next to the injector in the TESLA Test Facility linac (TTF) at DESY to verify experimentally the predicted RF-properties. Part of the experimental program was devoted to the damping of Higher Order Modes (HOM). Good suppression of HOMs in both prototypes has been achieved with HOM couplers based on the coaxial line technique. A several methods have been applied to measure impedances Z=(R/Q)·Q_ext of parasitic modes. We report here mainly on results of the active mode excitation method and the measurements of HOMs´ parameters by means of a Network Analyzer, which we performed on cold- and on warm prototypes. The other method, HOMs excitation by charge modulation, is reported in more detail in Castro, et al., (2003).