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When a superconducting radiofrequency cavity is cooled through its criticaltemperature, ambient magnetic flux can become "frozen in" to thesuperconductor, resulting in degradation of the quality factor. This isespecially problematic in applications where quality factor is a cost driver,such as in the CW linac for LCLS-II. Previously, it had been unknown how toprevent flux from being trapped during cooldown in bulk niobium cavities, butrecent R&D studies showed near-full flux expulsion can be achieved through hightemperature heat treatment and cooling cavities through the superconductingtransition with a spatial thermal gradient over the surface. In this paper, wedescribe the first accelerator implementation of these procedures, incryomodules that are currently being produced for LCLS-II. We compare theperformance of cavities under different conditions of heat treatment andthermal gradient during cooldown, showing a substantial improvement inperformance when both are applied, enabling cryomodules to reach and, in manycases, exceed a Q0 of ~3x10^10.

The Role of Magnetic Flux Expulsion to Reach Q0>3x10^10 in SRF Cryomodules