A Study of Beam Alignment Based on Coupling Modes in Third Harmonic Superconducting Cavities at FLASH

TL;DR

This paper investigates beam alignment in third harmonic superconducting cavities at FLASH by analyzing beam-excited higher order modes, particularly dipole modes, to improve beam quality through optimal alignment.

Contribution

It introduces a method for beam alignment based on coupling modes in third harmonic cavities, focusing on HOM monitoring to optimize beam position.

Findings

Dipole HOMs are sensitive to beam offset and can be minimized by alignment.

Monitoring HOM power helps identify optimal beam positions.

The method improves beam quality in superconducting cavities.

Abstract

This paper presents a study of beam alignment in accelerating cavities based on beam-excited higher order modes (HOM). Among the transverse HOMs, dipole modes have the dominant contribution, and may have an adverse effect on the beam quality. Since they depend linearly on the transverse beam offset, their effect can be reduced by aligning the beam in the cavity. The study has been made on the four third harmonic superconducting cavities installed in the ACC39 module at FLASH. Modes with strong beam coupling were monitored while moving the beam and searching for the minimum excited HOM power.