FRXAA03
Fermilab, Batavia, Illinois, USA
Continuous wave (CW) operation of LCLS-II cryomodules require substantial design changes to previously designed 1.3 GHz ILC type cryomodule. The new design uses innovative nitrogen doped cavities together with a helium vessel that provides high thermal gradient on the cavities during a fast cool down. Magnetic shielding uses two layers outside of helium vessel and a single layer that completely encapsulates the end groups except the beam apertures. Sophisticated magnetic hygiene includes vacuum vessel demagnetization, magnetic screening of the components and final demagnetization to the assembled cryomodule. Fermilab successfully assembled a preproduction LCLS-II cryomodule that achieved unprecedented cavity quality factors of 3.0·1010 at a nominal cryomodule voltage. The ambient fields in the cavities were near zero. This article describes historic cavity performance under various cool down parameters. Cavity performance retained very well compared to that of vertical tests.
