Cavity Tilt Measurement in a 1.3 GHz Superconducting Cryo-Module at FLASH

Abstract

TESLA superconducting (SC) cavities are used for the acceleration of electron bunches at FLASH. The Higher Order Modes (HOMs) excited by the beam in these cavities may cause emittance growth. The misalignment of the cavities in a cryo-module is one of the essential factors which enhance the coupling of the HOMs to the beam. The cavity offset and tilt are the two most relevant misalignments. These can be measured by help of dipole modes, based on their linear dependence on the beam offset. The cavity offset has been measured before in several modules at FLASH. However, the cavity tilt has so far proved to be difficult to be measured, because the angular dependence of the dipole mode is much weaker. By carefully targeting the beam through the middle of a cavity, the strong offset contribution to the dipole fields could be reduced. Careful data analysis based on a fitting method enabled us then to extract the information on the cavity tilt. This measurement has been implemented in the cavities in one cryo-module at FLASH. First results of the ongoing measurements from several cavities are presented in this paper. It is for the first time that the cavity tilt in several cavities has been measured.