High Precision Eccentricity Measurement of Accelerating Cavities by Use of the Small Perturbation Theory

The TESLA-type superconducting accelerating cavities are designed for use in high gradient linear accelerators - operating free electron laser in Hamburg (FLASH) and the future projects X-ray free electron laser (XFEL) and international linear collider (ILC). The cells of these cavities must be precisely aligned to the beam trajectory in order to minimize its unwanted interaction with the acceleration mode as well as beam-excited higher order modes. The eccentricity of cells is measured up to now only mechanically, which has too weak coupling to the electromagnetic field pattern inside of the cavity. In order to increase the precision of measurement and consequently increase the beam stability a contact-less measurement method based on the small perturbation theory was designed and a prototype of measurement setup was built.