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

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.

Transverse diagnostics based on dipole mode signal fitting method in TESLA-type accelerating cavities at the free-electron laser FLASH

“…Thus the amplitude excited by bunch tilt can be ignored. Also, compared to the beam offset, the trajectory tilt contributes much less to the dipole mode amplitude [16] and can be neglected. Therefore, the dipole mode amplitude can be seen in our case as only linearly related to the beam offset.…”

Submacropulse electron-beam dynamics correlated with higher-order modes in Tesla-type superconducting rf cavities

“…Since there were different filter boxes being tested, a patch panel was used to quickly switch between cavities and boxes. Unlike the downconverter electronics design with a narrow band filter on the TE111 mode used by others [12,13], the HOM detectors' bandpass filters were optimized for two dipole passbands from 1.6 to 1.9 GHz, and the 1.3-GHz fundamental was reduced with a notch filter.…”

“…The cryomodule configuration with eight 9-cell cavities is currently the drive accelerator for the FLASH free-electron laser (FEL) [4], the European XFEL [5], the under-construction LCLS-II XFEL [6], the proposed MaRIE XFEL at Los Alamos [7], and the International Linear Collider (ILC) under consideration in Japan [8]. A recent study at FLASH using one specific TE111 HOM showed that the root mean squared (rms) relative alignments were about 342 µm for the 40 cavities in the 5 cryomodules with some close to 600 µm off axis [9]. The assessment of the effects on beam quality of such implementations warrants further study as the thrust for brighter electron beams at higher powers continues.…”

“…The upstream corrector magnets allowed controlled changes of the trajectories into the first capture cavity (CC1) which were monitored by the bunch-by-bunch capable rf beam position monitors (BPMs). In each cavity, there are two HOM couplers [e.g., see 9] which are designed to outcouple and dampen many of the modes. The signals from these couplers are processed by four matched detector circuits to track the transverse modes in the first two dipole passbands of the cavities.…”

“…In this article, we report on a unique opportunity to study HOM effects on beam parameters from two 9-cell TESLA-type cavities arranged in series after a photocathode (PC) rf gun in the electron linac at the Fermilab Accelerator Science and Technology (FAST) facility [10]. These studies focused on the correlated HOM effects on the beam itself, unlike several studies that used the HOMs to determine cavity field centers or cavity misalignments [9,[11][12][13][14]. Moreover, there was the possibility for an additional ~10 m drift with all quadrupoles powered off prior to the dipole for the low-energy spectrometer.…”