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TY - UNPB AU - Posen, S. AU - Gonnella, D. AU - Grassellino, A. AU - Harms, E.R. AU - Melnychuk, O.S. AU - Palczewski, A.D. AU - Peterson, T.J. AU - Sergatskov, D.A. AU - Solyak, N. AU - Wu, G. ED - Boland, Mark ED - Tanaka, Hitoshi ED - Button, David ED - Dowd, Rohan ED - Schaa, Volker RW ED - Tan, Eugene TI - Flux Expulsion in SRF Cavities: Discovery of Influencing Parameters and Implementation in LCLS-II Cryomodule Production J2 - Proc. of IPAC2019, Melbourne, Australia, 19-24 May 2019 CY - Melbourne, Australia T2 - International Particle Accelerator Conference T3 - 10 LA - english AB - For decades, magnetic flux trapping in superconducting RF cavities has been poorly understood, with various reports of all ambient flux being trapped in the superconductor during cooldown or unreproducible behavior. This recently changed, when an R&D study at Fermilab showed that thermal gradients over the surface of the cavity can expel flux from the superconducting material. Further R&D studies showed that the expulsion behavior also depended strongly on the high temperature heat treatment of the cavity. These studies were timely for LCLS-II cryomodule production, in which stringent requirements on the quality factor make trapped flux a significant problem. To minimize degradation due to trapped flux, the recommendations from the R&D program were applied to cryomodule production. This is the first implementation of several new paradigms to minimize trapped flux: fast cooldown, high temperature heat treatment to minimize flux pinning centers, and magnetic hygiene controls. In this contribution, we review the R&D studies and the implementation of the lessons learned in cryomodules for LCLS-II. We present performance statistics from cryomodules as a function of heat treatment temperature and helium mass flow during cooldown. We show that with these modifications, LCLS-II production cryomodules are now achieving an unprecedented Q₀ of ~3x10¹⁰ or higher, approximately 3x higher than the state-of-the-art 5 years ago. PB - JACoW Publishing CP - Geneva, Switzerland ER -