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.
Highlights
The Free-Electron Laser in Hamburg (FLASH) [1,2] and European X-Ray Free-Electron Laser (European XFEL) [3,4,5] are single pass free-electron lasers (FELs), generating high-brilliance radiation by self-amplified spontaneous emission [6]
Acceleration of the driving electron bunches is achieved by using superconducting TeV-energy superconducting linear accelerator (TESLA) [7] -type cavities
The principle of the above-described procedure for the measurement and data analysis was applied to the four accelerating modules (ACC2–5) at FLASH, which are equipped with higher-order modes (HOMs) readout electronics
Summary
The Free-Electron Laser in Hamburg (FLASH) [1,2] and European X-Ray Free-Electron Laser (European XFEL) [3,4,5] are single pass free-electron lasers (FELs), generating high-brilliance radiation by self-amplified spontaneous emission [6]. In order to determine the transverse cavity position, we chose a method based on beam-excited dipole modes in the cavities. These modes have a linear dependence on the transverse beam offset and angle with respect to the cavity axis. Because of the low beam energy, and beam sensitivity to off-axis fields, the injector module (ACC1) has the largest impact on the intrabunch-train trajectory variation, and knowledge on its cavity alignment is the first priority. Dedicated studies to derive accelerating structure misalignments from multibunch rf and beam position measurements [11] at the injector module have not been conclusive. The results of cavity misalignment measurements at five accelerating modules at FLASH are. The measurement at the injector module was the most challenging part of the experiment
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