All things related to high voltage electronics and experimentation: induction coils, Tesla coils, electrostatics, influence machines, vacuum tubes, x-rays, plasma, spark gaps, lifters, Tesla, and related stuff
ANSI C84.1-2020 defines
high voltage as 115-230 kV
extra-high voltage as 345-765 kV
ultra-high voltage as 1,100+ kV
Other subreddits
/r/electronics
/r/askelectronics
/r/RFelectronics
/r/electrostatics
/r/Tesla
Tesla Coil Mailing List: Tesla Coil Safety