Measurements of higher order modes in a 30 cm long X-band structure

The use of a cage of metallic wires as a bead is proposed to measure the hi gher order modes (HOMs) in an X-band accelerating structure. These long thi n wires can isolate the longitudinal electric field component from other fi eld components and produce sufficient frequency shift in bead-pull measurem ents. In the setup described in this paper, the bead is made by sputtering silver film onto a thin nylon line through a specially designed fixture. Th e cage has a size of approximately 0.5 mm in diameter, 2 mm in length and m ore than six metallic wires of less than 0.1 mm in width. The fabrication a nd calibration of the cage are described. The longitudinal electric fields of the lowest passband dipole mode TM110 in a 30 cm long X-band structure a re measured by bead-pull measurements. Results are compared with the calcul ated ones obtained from URMELT-code. (C) 2001 Elsevier Science B.V. All rig hts reserved.