Nb₃Sn Superconducting RF Cavities: R&D Progress at Fermilab and Opportunities [Poster]

Abstract

If Nb 3 Sn cavities can reach H sh , this would correspond to a gradient of nearly 100 MV/m, which would be extremely beneficial for high energy accelerators. This field has not yet been reached, and experiments suggest that defects in the film are the limitation. Improvements in film quality has led to steady progress in maximum gradient. In 2019, a Fermilab cavity reached a new record gradient of 24 MV/m. This is already useful in many applications. For example, this exceeds the operating gradient requirements of LCLS-II and PIP-II cavities. Left: Improvement in maximum accelerating gradient of Nb3Sn cavities over time. Fermilab R&D recently led to reaching 24 MV/m in a single cell 1.3 GHz cavities. Right: Microscopic images of a Nb 3 Sn film from above (a) and cross section (b).

Low-Temperature Baking Effect of the Radio-Frequency Nb3Sn Thin Film Superconducting Cavity

“…To remove the performance-limiting defect, Posen carried out a light removal of 30-50 nm for the Nb 3 Sn thin film by both HF rinse and centrifugal barrel polishing (CBP). [8] However, some residues appeared on the surface of Nb 3 Sn grains after the HF rinse, leading to a stronger increase in the 𝑄-slope. The performance degradation after CBP is even more serious than that after the HF rinse.…”

“…Compared to the high-purity Nb used for accelerator applications, Nb 3 Sn has a theoretical superconducting transition temperature (𝑇 C ) of about 18.3 K (∼9.25 K for Nb), an energy gap 𝛥 of about 340 mV (∼140 mV for Nb), and a superheating magnetic field (𝐻 sh ) of about 425 mT (∼240 mT for Nb). [8] Therefore, the Nb 3 Sn thin film SRF cavity can be operated at 4.2 K or even higher temperature, having a theoretical accelerating field two times higher than the Nb SRF cavity. This will not only have a huge impact on the large-scale facilities such as International Linear Collider (ILC), but also make the SRF technology have bright application prospects in small scientific research platforms such as compact light sources [9] and photo-neutron sources, [10] as well as in industrial applications such as wastewater treatment [11] and medical isotope production.…”

“…This will not only have a huge impact on the large-scale facilities such as International Linear Collider (ILC), but also make the SRF technology have bright application prospects in small scientific research platforms such as compact light sources [9] and photo-neutron sources, [10] as well as in industrial applications such as wastewater treatment [11] and medical isotope production. [12] In addition, the maximum secondary electron yield (SEY) of Nb 3 Sn is very similar to Nb, [8,13] which means that the multipacting and field emission behaviors in Nb 3 Sn thin film cavities should be comparable to that normally observed in Nb cavities. Thus, the optimized structures of the Nb cavities can be directly applied to the Nb 3 Sn cavities.…”

“…Researchers [17] at University of Wuppertal reported a 50% degradation of the low field 𝑄 0 and the onset field of 𝑄-slope after light oxipolishing. Posen [8] repeated the oxypolishing treatment of the Nb 3 Sn thin film cavity at Cornell University to remove the possible excess Sn on the surface. However, the 𝑄 0 at low fields was reduced and the 𝑄 0 degradation with the increasing accelerating field (𝑄-slope) was even stronger at lower fields.…”

“…After the vertical tests before and after low temperature baking treatment, the Nb 3 Sn thin film of X2X4-1 cavity was removed by BCP with about 40 µm material removal. Then the cavity labeled as X2X4-2 was recoated using a recipe based on the temperatures and times specified by University of Wuppertal [20] and Cornell University, [8] the temperature of the Nb substrate cavity and the tin source recorded by thermocouples were 1100 ∘ C and 1250 ∘ C, respectively. The detailed data of temperature and pressure during the two coating processes are shown in Fig.…”