Higher Order Modes in Third Harmonic Cavities for XFEL/FLASH

TL;DR

This paper investigates higher order electromagnetic modes in third harmonic cavities at DESY's FLASH facility, comparing experimental measurements with computational predictions to understand mode behavior and coupling effects.

Contribution

It provides the first detailed experimental and computational analysis of higher order modes in 3.9 GHz cavities used for beam shaping at FLASH.

Findings

Modes below cut-off are single-cavity modes.

Modes above cut-off are coupled across multiple cavities.

Experimental results align with finite element and finite difference simulations.

Abstract

We analyse higher order modes in the 3.9 GHz bunch shaping cavities recently installed in the FLASH facility at DESY. We report on recent experimental results on the frequency spectrum from probe based measurements made at CMTB at DESY. These are compared to those predicted by finite difference and finite element computer codes. This study is focused mainly on the dipole component of the multi-pole expansion of the wakefield.. The modes are readily identifiable as single-cavity modes provided the frequencies of these modes are below the cut-off of the inter-connecting beam pipes. The modes above cut-off are coupled to the 4 cavities and are distinct from single cavity modes