High-gradient srf R&D for ILC at jefferson lab

Jefferson Lab plays an active role in high-gradient SRF R&D in the frame work of the internationally coordinated International Linea Collider (ILC) S0 program. The S0 aim is to push the yield at 35 MV/m in 9-cell cavities. So far, twelve cavities have been electropolishing (EP) processed and RF tested by using the state-of-the-art recipes at JLab, in close collaboration with FNAL and KEK. Seven of them reached a best gradient of over 31.5 MV/m. Understanding gradient limiting mechanisms in real 9-cell cavities is an important component of our studies. Thermometry and high-resolution optical inspection are used to locate and understand the source of gradient limits. Experimenting with selective cavities is still a necessary method for process optimization. One example is the first demonstration of 35 MV/m without detectable Bremsstrahlung X-ray after a light EP is applied to a previously heavy chemical etched 7-cell cavity. Some new understanding has been gained with regard to quench behaviors, field emission behaviors as well as optimized processing. Progress has been made as a result, exemplified by the recent achievement of ≥ 35 MV/m in two cavities, each after the first light EP. Several exploratory studies are under way at JLab, aiming to covert the new understandings into further improved cavity gradient results.