Radio-frequency cavity field measurements through free falling bead

• Published in: Physical review. Accelerators and beams 2025-01, Vol.28 (1), p.012001, Article 012001
• Main Authors: Du, Xiaonan, Groening, Lars
• Format: Article
• Language: English
• ISSN: 2469-9888; 2469-9888
• DOI: 10.1103/PhysRevAccelBeams.28.012001

This paper introduces a novel bead-falling measurement method for the precise and efficient mapping of electromagnetic fields within radio-frequency (rf) cavities, which are crucial components in the design of accelerators. The traditional bead-pull method, while effective, involves mechanical complexities and is prone to errors from wire perturbations. The innovative method reported here leverages the simplicity and accuracy of free-falling beads to mitigate these issues. This technique eliminates the need for a wire-pulley system, thereby simplifying the experimental setup and reducing potential mechanical errors. We detail the development and operational principles of this new method, including the design of a compact, portable measurement device that integrates a bead/droplet release system and a bead detection system linked to a vector network analyzer. The device has been tested with a three-gap buncher cavity and a scaled Alvarez-type cavity, demonstrating its ability to perform rapid, reliable field measurements under various challenging conditions, including low signal-to-noise ratios and environmental vibrations. The results confirm the method’s superiority in precision and operational efficiency, potentially setting a new standard for rf cavity diagnostics and tuning.