Cavity tilt measurement using beam excited higher order mode signals at the Free Electron Laser FLASH

Abstract

At the Free Electron Laser in Hamburg (FLASH), TESLA-type superconducting (SC) cavities are used for the acceleration of electron bunches. Higher Order Modes (HOMs) excited by electron bunches may cause emittance growth and decrease the FEL performance. The misalignment of the SC cavities housed inside cryo-modules is one of the main factors which enhance the coupling of HOMs to the bunches. We consider the cavity offset and tilt as the two most relevant misalignments. These can be measured by utilizing the beam excited HOM signals. The cavity offset has been measured before in several modules at FLASH, while the tilt has so far proved to be much more difficult to be measured. This is because the beam trajectory tilt excited modes are much weaker than the ones excited by offset. We used a method for measuring cavity tilt by carefully targeting the beam through the middle of a cavity. An efficient data processing routine based on signal fitting was applied. For the first time, the tilts of all cavities inside a cryo-module are measured. The ratio between the offset and angle dependence of the HOMs agrees well among the different cavities.