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TY - UNPB AU - Posen, S. ED - Schaa, Volker RW ED - He, Yuan ED - Li, Lu ED - Zhao, Ning TI - Flux Expulsion Studies on Niobium J2 - Proc. of SRF2017, Lanzhou, China, July 17-21, 2017 C1 - Lanzhou, China T2 - International Conference on RF Superconductivity T3 - 18 LA - english AB - Increased requirements for high Q0 in SRF linac applications have made it crucial to avoid Q0 degradation resulting from trapped flux. In this contribution, we show results from a new systematic study of cavities at Fermilab that show a strong correlation between the temperature gradient required to expel flux during cooldown and the material history, including 1) niobium vendor processing and 2) heat treatment after purchase from the vendor. Early results from these studies indicated that it would be important to evaluate the niobium purchased for production of high Q0 cavities for the LCLS-II project. We present measurements of both expulsion and Q0 on single-cell and bare 9-cell cavities made from this material, which led to the decision to increase the heat treatment temperature in production. We present data showing the significant improvement in flux expulsion behavior of production cavities after this modification was introduced. In addition, we present new understanding of the physics of flux expulsion, and we discuss the ramifications for procurement of niobium for future applications requiring high Q0. PB - JACoW CP - Geneva, Switzerland ER -