High-$Q_0$ Treatment of CEBAF 1.5 GHz SRF Cavities

Abstract

The Continuous Electron Beam Accelerator Facility (CEBAF) was the first large-scale accelerator to employ superconducting radiofrequency (SRF) cavities for continuous-wave operation. Ongoing research and development efforts continue to focus on increasing the intrinsic quality factor ($Q_0$) of these cavities in order to reduce cryogenic losses while maintaining operational gradients. In this work, we report on the application of high-$Q_0$ surface treatments to single-cell and multicell C100 and C75 style 1.5 GHz niobium cavities used in the CEBAF accelerator. Nitrogen infusion and oxygen alloying via medium-temperature baking were applied under heat-treatment constraints relevant to existing cavity hardware. Both processes yielded substantial improvements in $Q_0$ at moderate accelerating gradients, achieving values of approximately 2 $\times$ 10$^{10}$ at 2.07 K and 20 MV/m. The effectiveness of nitrogen infusion at reduced annealing temperatures and the successful extension of oxygen alloying to multicell cavities are demonstrated. These results establish viable pathways for implementing high-$Q_0$ treatments in CEBAF-compatible cavities and support future integration into cryomodules for reduced operational cryogenic load.

Figures (9)

• Figure 1: $Q_0(E_{acc})$ of two single cell cavities at 2.07 K after nitrogen infusion.
• Figure 2: Flux trapping sensitivity of single cell cavities after nitrogen infusion with different annealing temperature as described in text.
• Figure 3: $Q_0(E_{acc})$ of 7-cell cavities after nitrogen infusion with 600 $^\circ$C/3h furnace treatment and 165 $^\circ$C/48 h process at 2.07 K. The star symbol represents the C100 specification.
• Figure 4: $Q_0(E_{acc})$ of two single cell cavities after oxygen alloying at 350 $^\circ$C for 3 hours measured at 2.07 K.
• Figure 5: Flux trapping sensitivity of single cell cavities after oxygen alloying at 350 $^\circ$C for 3 hours measured at 2.07 K