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

TL;DR

This paper demonstrates high-$Q_0$ surface treatments like nitrogen infusion and oxygen alloying on 1.5 GHz SRF cavities, significantly reducing cryogenic losses and enhancing performance for CEBAF's continuous-wave operation.

Contribution

It introduces effective high-$Q_0$ surface treatment protocols applicable to CEBAF cavities, including nitrogen infusion at lower temperatures and oxygen alloying for multicell structures.

Findings

Achieved $Q_0$ of approximately 2 × 10^{10} at 2.07 K and 20 MV/m.

Demonstrated nitrogen infusion effectiveness at reduced annealing temperatures.

Extended oxygen alloying techniques to multicell 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.