Krytrons are fast-acting switches that use a little bit of Kr85 to assist with rapid ionization. There is also a keep-alive electrode that induces a small plasma for quick conduction.
Krytrons are special as they are believed to have been used for triggering nuclear weapons. However, they have plenty of other more mundane uses such as high speed strobe lights and low-jitter pulsed power electronics for systems like lasers (where my collection has come from). Here I’ve set up a Krytron in conjunction with a Bluemline charged cable pulser.