Results of High Order Modes Spectra Measurements in 1.3 GHz Cavities for LCLS-II

TL;DR

This paper details the measurement procedures and results of high order mode spectra in 1.3 GHz cavities used in LCLS-II, highlighting their frequencies, quality factors, and implications for heat loads and beam dynamics.

Contribution

It introduces a comprehensive procedure for measuring HOM spectra in superconducting cavities and provides statistical analysis across different vendors.

Findings

HOM frequencies and quality factors vary among vendors.

HOMs contribute to heat loads and affect beam dynamics.

Measurement procedures enable quality control and qualification.

Abstract

Fermilab recently completed production and testing of 1.3 GHz cryomodules for the LCLS-II project. Each cryomodule consists of eight TESLA-shaped superconducting elliptical cavities equipped with two High Order Mode (HOM) coupler ports. Measurement of the HOM spectrum is part of the incoming quality control of cavities at room temperature and the final qualification cold test of cryomodules at the Cryomodule Test Facility (CMTF). In this paper we describe the procedure for measuring the HOM spectrum along with further data processing. Finally, we present accumulated statistics of individual HOM frequencies and quality factors related to various cavity vendors and discuss the possible contribution of HOMs to heat loads and beam dynamics.