Study of Beam Diagnostics with Trapped Modes in Third Harmonic Superconducting Cavities at FLASH

TL;DR

This paper explores using trapped dipole modes in third harmonic superconducting cavities at FLASH for precise beam position monitoring, demonstrating a correlation between HOM signals and beam alignment through simulations and measurements.

Contribution

It investigates the potential of the fifth dipole passband for cavity-specific beam position measurement, utilizing both simulations and experimental data.

Findings

Good correlation between HOM signals and beam position.

Trapped modes enable cavity-specific beam diagnostics.

Simulation and measurement results agree well.

Abstract

Off-axis beams passing through an accelerating cavity excite dipole modes among other higher order modes (HOMs). These modes have linear dependence on the transverse beam offset from the cavity axis. Therefore they can be used to monitor the beam position within the cavity. The fifth dipole passband of the third harmonic superconducting cavities at FLASH has modes trapped within each cavity and do not propagate through the adjacent beam pipes, while most other cavity modes do. This could enable the beam position measurement in individual cavities. This paper investigates the possibility to use the fifth dipole band for beam alignment in the third harmonic cavity module. Simulations and measurements both with and without beam-excitations are presented. Various analysis methods are used and compared. A good correlation of HOM signals to the beam position is observed.