Polywell
This came out of a discussion on how to build a sentry robot. Someone suggested a portal-style machine gun turret with air-soft guns, there was a lot of joking and it quickly escalated to the point where we needed a particle cannon. A few friends joked that such a thing couldn’t exist, so this was my first stab at a proof of concept.
NB. Please don’t weaponize particle accelerators; it’s impractical and would likely lead to lots of cool things being banned.