  A high-quality factor microwave resonator in the presence of a strong
magnetic field could have a wide range of applications, such as axion dark
matter searches where the two aspects must coexist to enhance the experimental
sensitivity. We introduce a polygon-shaped cavity design with bi-axially
textured YBa$_{2}$Cu$_{3}$O$_{7-x}$ superconducting tapes covering the entire
inner wall. Using a 12-sided polygon cavity, we obtain substantially improved
quality factors of the TM$_{010}$ mode at 6.9 GHz at 4 K with respect to a
copper cavity and observe no considerable degradation in the presence of
magnetic fields up to 8 T. This corresponds to the first demonstration of
practical applications of superconducting radio frequency technology for axion
and other research areas requiring low loss in a strong magnetic field. We
address the importance of the successful demonstration and discuss further
improvements.
