Materials path toward improved energy efficiency for High Energy Physics accelerators
- College of William and Mary, Williamsburg, VA (United States)
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
For nearly fifty years researchers working with particle accelerators have envisioned reducing their initial and operating costs by substituting the intermetallic compound Nb3Sn for Nb as the cavity interior surface. The goal has remained elusive, stymied especially by the fall of cavity quality factor (representing increasing energy consumption) with increasing gradient (the amount of accelerator need to reach an energy goal). While one research group reported making a few cavities free of the problem, no one has reported how this comes about or has reproduced the improvement. We believe that insufficient understanding is responsible. We report here findings of a detailed, systematic study of the formation of Nb3Sn coatings on Nb, relating structure and properties of the coating to the process parameters and substrate materials on the one hand, and RF surface impedance and SRF cavity performance on the other was carried out, focusing on the two step (nucleation – deposition) tin vapor diffusion process. Major outcomes are: The choice of initial surface preparation (BCP etc) has no discernable impact on any quality of the final coating; Though tin chloride serves as the coating nucleant, no trace could be found by any means of chlorine after nucleation or coating; No evidence was found for any substantive impact of the details of the nucleation on the ultimate coating, as long as it was done within the frequently used range; The coating forms as Nb3Sn crystals by tin from the vapor diffusing down grain boundaries to the Nb interface and growing there. Higher temperature and longer time lead to larger, but otherwise substantially identical crystals; All coatings exhibited roughness on the micron scale. Testing the effect of coating surface modification technologies in use by the community, only a carefully tailored electropolish showed significant benefit; and, Preliminary results indicate that Ti contamination may be associated with falling gradient. Two particular item appear promising for future work. The effect of surface topography in the range seen here on performance is significant for niobium. The nature and impact on Nb3Sn, if any, needs to be understood and managed. At what level Ti, Ta or other impurities affects the performance of Nb3Sn needs to be understood.
- Research Organization:
- College of William and Mary, Williamsburg, VA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- DOE Contract Number:
- SC0014475
- OSTI ID:
- 1467442
- Report Number(s):
- DOE-WM-0014475; TRN: US1901693
- Country of Publication:
- United States
- Language:
- English
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