Higher-order mode-based cavity misalignment measurements at the free-electron laser FLASH

TL;DR

This paper introduces a novel method using higher-order cavity modes to measure transverse misalignments and tilts of superconducting RF cavities inside cryomodules at FLASH, improving alignment diagnostics for FEL performance.

Contribution

It presents an innovative beam-based measurement technique for cavity misalignments and tilts, validated through application to FLASH modules, including the first cavity tilt measurement inside a cryomodule.

Findings

Measured rms cavity offsets match TESLA specifications.

First measurement of TESLA cavity tilt inside a cryomodule.

Preliminary results align with eigenmode simulation predictions.

Abstract

At the Free-Electron Laser in Hamburg (FLASH) and the European X-Ray Free-Electron Laser, superconducting TeV-energy superconducting linear accelerator (TESLA)-type cavities are used for the acceleration of electron bunches, generating intense free-electron laser (FEL) beams. A long rf pulse structure allows one to accelerate long bunch trains, which considerably increases the efficiency of the machine. However, intrabunch-train variations of rf parameters and misalignments of rf structures induce significant trajectory variations that may decrease the FEL performance. The accelerating cavities are housed inside cryomodules, which restricts the ability for direct alignment measurements. In order to determine the transverse cavity position, we use a method based on beam-excited dipole modes in the cavities. We have developed an efficient measurement and signal processing routine and present its application to multiple accelerating modules at FLASH. The measured rms cavity offset agrees with the specification of the TESLA modules. For the first time, the tilt of a TESLA cavity inside a cryomodule is measured. The preliminary result agrees well with the ratio between the offset and angle dependence of the dipole mode which we calculated with eigenmode simulations.