Below are some forums and communities involving cyclotrons directly, or related technologies – DIY vacuum systems, high voltage electronics, electromagnetics, RF, radiation and particle detection, etc.
Facebook Guild of Cyclotron Builders – active group of cyclotron enthusiasts
Fusor.net – centered around fusion and DIY fusors, excellent resource on DIY vacuum, HV, radiation detection, and other topics in the forum archives
Facebook Guild of Accelerator Builders – not much activity, dedicated to general accelerator construction
Facebook Beam on Target Technology Group – Forum dedicated to the technology, research, and construction of amateur built beam-on-target systems.
High Voltage Forum – active forum for high voltage DIY projects, “successor” to 4hv.org
4HV.org – older forum for high voltage DIY projects
eevblog.com – large active forum on all aspects of electronics, electrical engineering
Nuclear Fusion and High Energy Research Forums – amateur fusion forums run by Mark Rowley
Nuclear Physics Lab – projects by group led by Tim Koeth, the “father” of modern DIY cyclotrons
Physicsforums.com Particle Physics Forum – popular general physics forums
Doug Coulter’s Science/Engineering/Technical forums – centered mostly on DIY fusor, vacuum, HV, and machining
DIY vacuum tube forum – good vacuum info, including making your own diffusion pump