A. Cravatta*1, S. Aderhold2, C. Adolphsen2, T. Arkan1, A. Benwell2, B. Chase1, C. Contreras-Martinez1, L. Dootlittle3, J. Fuerst2, D. Gonnella2, B. Hansen1, E. Harms1, B. Hartsell1, G. Hays2, J. Holzbauer1, S. Hoobler2, J. Kaluzny1, T. Khabiboulline1, M. Kucera1, D. Lambert1, R. Legg2, F. Lewis1, J. Makara1, H. Maniar1, J. Maniscalco2, M. Martinello1, J.

Nelson2, S. Paiagua3, C. Pareja3, Y. Pischalnikov1, S. Posen1, P. Prieto1, J. Reid1, M. Ross2, N. Solyak1, A. Syed1, G. Tatkowski1, R. Wang1, M. White1, L. Zacarias21Fermi National Accelerator Laboratory, Batavia, IL, USA – 2SLAC National Accelerator Laboratory, Menlo Park, CA, USA – 3Lawrence Berkeley National Laboratory, Berkeley, CA, USA

LCLS-II-HE Verification Cryomodule Testing at Fermilab

Abstract: After successfully testing 25 cryomodules for the LCLS-II program, Cryomodule Test Stand 1 (CMTS1) within the Cryomodule Test Facility (CMTF) at Fermilab has shifted focus to the LCLS-II High Energy upgrade (LCLS-II-HE). Building upon a well-established cryomodule testing program, cryomodules with state of the art 1.3 GHz SRF cavities are now being tested, beginning with a prototype – the verification cryomodule, or vCM. LCLS-II-HE cavities present new challenges to the facility, with higher achievable gradients and the prospect of nominal gradients lying within the multipacting band. We present acceptance criteria of the vCM and LCLS-II-HE program as well as preliminary results.

New Challenges:

At higher gradients come new challenges:

• Cryogenic loado 137𝑊 spec for heat load at full vCM total voltage – a challenge for the cryogenic plant, cryo distribution system, and

avoiding flow instabilities. • Fundamental Power Couplers

o Coupler vacuum outgassing events observed in SELAP, causing vacuum trips.

• Multipacting (MP)o Nominal gradient lies in MP band.o Budget time for MP processing is required.

• Magnetic field sensitivityo Magnetic hygiene and flux expulsion crucial.

Acceptance Criteria:

Introduction: At the conclusion of Fermilab’s cryomodule testing program in February 2021 for LCLS-II, the next generation CW SRF linac for hard x-ray production at SLAC, a prototype cryomodule, the vCM, for the high energy upgrade, LCLS-II-HE, has been installed and testing is currently underway.

A 2N0 doping recipe for the eight state of the art, nine-cell, 1.3 GHz SRF cavities in the vCM has been approved to the vendor after meeting or exceeding acceptance criteria for nominal 𝐸!"" 20.8 𝑀𝑉/𝑚 and 𝑄# 2.7 , 10$# .

Preliminary Results: Record setting total CW voltage:• SELA – 200.6 𝑀𝑉 193.2 𝑀𝑉/𝑚

o 8 cavities in amplitude locked RF mode.

• SELAP (GDR) – 200.0 𝑀𝑉 192.7 𝑀𝑉/𝑚o 8 cavities in amplitude locked, fixed

frequency RF mode.

Maximum allowable CW cavity gradient:• Cavity 4 – 26 𝑀𝑉/𝑚 (SELAP/GDR)

o Amplitude locked, fixed frequency RF mode.

o Limited by admin limit

Cavity performance exceeds specifications:• Max Gradient comparable to VTS• Usable Gradient far exceeds spec

16 𝑀𝑉/𝑚• 𝑄! meets or exceeds spec

2.7 / 10"!

Clean cavities:• One cavity with low-level field emission,

processed fully during Unit Test.• Radiation at background during normal

operation.Vertical Cavity Test Stand vsCryomodule Test Stand

Conclusion: With a mature program established thanks to expertise with LCLS-II cryomodule testing, the LCLS-II-HE program is successfully underway.

• Acceptance criteria nearly fully met or exceeded:o Test program ongoing, runs through end of July.

• Remaining tests:o Extended range tuner test / off frequency operation (OFO).o 𝑄! degradation studies.o Plasma processing.

The vCM has met or exceeded specifications. These preliminary results could not have been accomplished without numerous experts from Fermilab and partner labs.

This work was supported by the United States Department of Energy, Offices of Basic Energy Sciences and High Energy Physics under Contracts No. DE-AC05-06OR23177 (Fermilab) and No. DE-AC02-76F00515 (SLAC).

Related Posters & Papers AT SRF’21:

• THOTEV08• TUPFDV003• MOPTEV002• THPFAV005• THPCAV009• THPTEV003• THPTEV017