HOM (higher-order mode) damper tests of Advanced Photon Source storage ring cavity with a 20-MeV e- beam

A beamline has been assembled with the ANL Chemistry Division linac (20-MeV e- beam with FWHM of 30 ps) to test the effectiveness of damping techniques of the APS storage ring single-cell cavity. The beamline consists of two sections -- the beam collimating section and the cavity measurement section -- separated by two single Al foil windows. The beam diagnostics include a beam position monitor, integrating current transformers, fluorescent screens, and a Faraday cup. The EPICS (Experimental Physics and Industrial Control System) is used for beamline control, monitoring and data acquisition. The cavity was excited by the electron beam to investigate the HOMs. The HOMs were measured with various conditions such as unloaded, critically-coupled, and overcoupled cavity to the waveguide. An rf cavity was also tested with and without various types of dampers. The HOM measurements were made with H-loops and E-probes. The spectral analysis of the HOMs is discussed and compared with both the bead-perturbation method and the computer calculation