Why does a magnetron antenna connect to only one cavity?

[IniquiTrance]

Why is it that an antenna/waveguide is only connected to one cavity in the magnetron, such as in a microwave? Are the resonating RF waves in the other cavities wasted then?

Thanks!

Edit: Also, are magnetrons always thermionic diodes?

 

[Bob S]

Magnetrons (like the ones in microwave ovens) are coupled cavity resonant systems so although the power might flow out from one cavity, the power comes from all. The cavities are coupled by both the bunched electrons, by coupling of the cavities themselves, and by "strapping" (I think) like in the rising sun magnetron. The electron beam comes from thermionic emission by a dc biased cathode, but the power comes from the dc voltage and current input.