  Photomultiplier tubes (PMTs) are widely used underwater in large-scale
neutrino experiments. As a hollow glass spherelike structure, implosion is
unavoidable during long-term operation under large water pressure. There is a
possibility of cascade implosion to neighbor PMTs due to shockwave. Jiangmen
Underground Neutrino Observatory designed a protection structure for each
20-inch PMT, consisting of a top cover, a bottom cover, and their connection.
This paper introduces the requirement and design of the PMT protection system,
including the material selection, investigation of manufacture technology, and
prototyping. Optimization and validation by simulation and underwater
experiments are also presented.
