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. A charge modulation imposed on the 54 MHz bunch train excites HOM at frequencies n · f_b ± f_mod, where n is an integer, f_b = 54 MHz is the bunch frequency and f_mod 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.