Cold test measurements of a BWO slow-wave structure

Bead-on-string perturbation techniques have been primarily used to study particle accelerator cavities. These techniques are now extended to measure the dispersion relation and axial modes of several transverse magnetic (TM) modes in a slow-wave structure with sinusoidally varying radius. This type of slow-wave structure is commonly used in high power backward wave oscillators (BWO). The dispersion relation is found from the discrete measured resonant frequencies and wave numbers of a cavity containing six periods of the slow-wave structure. The measured axial and radial field profiles are compared with the results of a 2-D code specially developed for calculation of the fields in these generators. For the first three TM modes the frequency error was less than 0.7%