Untrapped HOM Radiation Absorption in the LCLS-II Cryomodules

TL;DR

This paper investigates the effectiveness of ceramic absorbers in reducing higher order mode (HOM) radiation heating in LCLS-II cryomodules, aiming to optimize cryogenic efficiency in superconducting linacs.

Contribution

It provides a detailed analysis of HOM power absorption and estimates the beam power that must be managed at 2 K in LCLS-II cryomodules.

Findings

Ceramic absorbers significantly reduce HOM power deposition.

Estimated beam power removal requirements at 2 K.

Improved understanding of HOM heating effects in CW linacs.

Abstract

The superconducting cavities in the continuous wave (CW) linacs of LCLS-II are designed to operate at 2 K, where cooling costs are very expensive. One source of heat is presented by the higher order mode (HOM) power deposited by the beam. Due to the very short bunch length--especially in L3 the final linac--the LCLS-II beam spectrum extends into the terahertz range. Ceramic absorbers, at 70 K and located between cryomodules, are meant to absorb much of this power. In this report we perform two kinds of calculations to estimate the effectiveness of the absorbers and the amount of beam power that needs to be removed at 2 K.