Analysis of the Hom Damping with Modulated Beam in the First Prototype of Superstructure

Superstructures, groups of weakly coupled cavities fed through a single power coupler, are currently investigated as a more cost effective alternative to the 9-cell TESLA cavities. Two Nb prototypes of the superstructure have been built, consisting of two 7-cell cavities, and installed in the TESLA Test Facility at DESY. The HOM damping of these superstructures has been investigated with a modulated beam using the method described in [1]. A charge modulation imposed on the 54 MHz bunch train excites HOM at frequencies n · fb ± fmod, where n is an integer, fb = 54MHz is the bunch frequency and fmod is the charge modulation frequency between 0.5 and 27 MHz. The effects of the excited HOMs on the beam transverse position are observed at a downstream BPM, followed by a direct analysis of the modes at the HOM couplers.