https://www.sciencenews.org/article/moon-particle-collider-accelerator-physics
Doesn't have to encircle the moon, much smaller accelerators count too. They just need to be permanently attached to the moon. (i.e. an accelerator in a spaceship that temporarily lands on the moon does not count.)
Must have been built for research purposes.
1,000I think you should specify a minimum energy, otherwise an old-fashioned cathode ray tube would already count and I don't think that's what you have in mind.
@ErwinRossen Good point. Minimum energy or minimum size? Or maybe specify that it needs to be for research purposes?
@IsaacKing I think any of these three would help. It's hard to say upfront what to specify when looking almost 30 years into the future.
@ErwinRossen A low threshold could be if it can be carried at once or if it needs to be built in situ