I think you mean traveling wave tubes for high power RF amplifiers https://en.m.wikipedia.org/wiki/Traveling-wave_tube https://en.m.wikipedia.org/wiki/Traveling

I think you mean traveling wave tubes for high power RF amplifiers https://en.m.wikipedia.org/wiki/Traveling-wave_tube https://en.m.wikipedia.org/wiki/Traveling-wave_tube There are also klystrons, which are oscillators, used for high power radar generation, particle accelerators, etc... https://en.m.wikipedia.org/wiki/Klystron https://en.m.wikipedia.org/wiki/Klystron