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

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

Electronics and algorithms for HOM based beam diagnostics

The signals from the Higher Order Mode (HOM) ports on superconducting cavities can be used as beam position monitors and to do survey structure alignment. A HOM-based diagnostic system has been installed to instrument both couplers on each of the 40 cryogenic accelerating structures in the DESY TTF2 Linac. The electronics uses a single stage down conversion form the 1.7 GHz HOM spectral line to a 20MHz IF which has been digitized. The electronics is based on low cost surface mount components suitable for large scale production. The analysis of the HOM data is based on Singular Value Decomposition. The response of the OM modes is calibrated using conventional BPMs

CAS course on "RF for Accelerators", 18 June - 01 July 2023, Berlin Germany

Higher Order Modes (HOM) in accelerating structures are resonant electromagnetic fields excited and left behind by charged particle bunches. They can dramatically degrade the beam quality in accelerators, such as the multi-bunch energy spread and transverse emittance and therefore, one has to study and make them harmless carefully. In this lecture, various methods of HOM mitigation are reviewed. On one hand, one tries to avoid their existence, for example, by careful design of the accelerating structures and damping systems. On the other hand, one can reduce the effect of the excited HOM fields, for example, by means of fast feedback or beam alignment. Also, a few ways to make HOM signals useful are referred to, such as beam diagnostics or accelerating structures

HOM Characterization for Beam Diagnostics at the European XFEL

HOM-based diagnostics in SC accelerating cavitiesPrincipleExperience at FLASHThe European XFELSpectra of 1.3 GHz cavities in the E-XFELBeam measurements in 1.3 GHz cavities in the E-XFEL injectorSummar

Overview of HOM-based Diagnostics at FLASH and the European XFEL

Principle, benefitsHOMBPMs for 1.3 GHz cavitiesFLASHE-XFELHOMBPMs for 3.9 GHz cavitiesFLASHE-XFELHOM-based beam phaseMeasurement of cavity alignment at FLASHSummar