High resolution bead technique measurements and multipole analysis

The bead perturbation of the frequency in a resonant cavity provides accurate mapping of electrical field intensity. The bead diameter is 3 mm, to improve the spatial resolution, which makes the measure extremely sensitive to resonator thermal drifts. A large insulated test chamber was designed and installed in our laboratory to limit the cavity tempera- ture changes to about 0.00001 K/s. The better stability and a more stable electronic equipment allowed to improve the frequency resolution to 1 Hz. The data are analysed to deter- mine the multipole components of the electrical field along the beam axis and to compare them with the results obtained by well-known codes. Practical advantages of the bead tech- nique method, i.e. unbiasness, sensitivity to manufacture er- rors and shape resolution, are outlined.