LCLS-II-HE verification cryomodule high gradient performance and quench behavior

TL;DR

This paper reports on the successful testing of an 8-cavity cryomodule with nitrogen doping, achieving high gradients and Q0, and discusses operational issues and mitigation strategies.

Contribution

It presents the first verification of a high-gradient LCLS-II-HE cryomodule with nitrogen doping, exceeding performance specifications.

Findings

Achieved 200 MV in CW mode, surpassing 173 MV specification.

Average Q0 of 3.0x10^10 exceeded the 2.7x10^10 requirement.

No field emission observed up to maximum gradient.

Abstract

An 8-cavity, 1.3 GHz, LCLS-II-HE cryomodule was assembled and tested at Fermilab to verify performance before the start of production. Its cavities were processed with a novel nitrogen doping treatment to improve gradient performance. The cryomodule was tested with a modified protocol to process sporadic quenches, which were observed in LCLS-II production cryomodules and are attributed to multipacting. Dedicated vertical test experiments support the attribution to multipacting. The verification cryomodule achieved an acceleration voltage of 200 MV in continuous wave mode, corresponding to an average accelerating gradient of 24.1 MV/m, significantly exceeding the specification of 173 MV. The average Q0 (3.0x10^10) also exceeded its specification (2.7x10^10). After processing, no field emission was observed up to the maximum gradient of each cavity. This paper reviews the cryomodule performance and discusses operational issues and mitigations implemented during the several month program.