  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 (VNA). 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.
