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TY - UNPB AU - Gonnella, D. AU - Aderhold, S. AU - Bafia, D. AU - Checchin, M. AU - Grassellino, A. AU - Maniscalco, J.T. AU - Palczewski, A.D. AU - Posen, S. AU - Reece, C.E. AU - Ross, M.C. ED - Liu, Lin ED - Byrd, John M. ED - Neuenschwander, Regis T. ED - Picoreti, Renan ED - Schaa, Volker R. W. TI - The LCLS-II-HE R&D Program: New Insights into Improving the Performance of Nitrogen-Doped SRF Cavities J2 - Proc. of IPAC2021, Campinas, SP, Brazil, 24-28 May 2021 CY - Campinas, SP, Brazil T2 - International Particle Accelerator Conference T3 - 12 LA - english AB - Nitrogen doping has now been demonstrated to produce SRF cavities of unprecedented Q₀ values when manufactured in an industrial setting. LCLS-II has shown over 300 cavities with an average Q₀ of more than 3·10¹⁰ at 16 MV/m and represents an overwhelming success of the doping protocol. LCLS-II-HE will add an additional 23 superconducting cryomodules to the LCLS-II linac, requiring cavities to operate at similar levels of high Q₀ but at 21 MV/m instead of 16 MV/m. Nitrogen-doped cavities have been historically plagued by lower quench fields than other cavity preparation methods. Therefore, an R&D effort was launched to improve upon the quench fields of doped cavities while maintaining the high Q₀. Here we present results on single-cells and 9-cells from new doping recipe pursuits, transfer of these new recipes to cavity vendors, and results on vendor-produced 9-cell cavities. This program has led to the discovery of the importance of the cold electropolish for producing higher quench fields. Finally, we will show results from the first cryomodule produced with these new cavities operating at HE gradients. PB - JACoW Publishing CP - Geneva, Switzerland ER -