Vacuum technologies
Microwave Electron Tubes – Technologies and gridded tubes

Electron tubes are ultra-high-vacuum devices subjected to high static and RF electromagnetic fields. Vacuum envelopes are made by high-temperature brazing of perfectly sealed metals and ceramics, and the quality of the vacuum during a tube's life reflects the quality of the vacuum at the end of the pump-and-bake stage. However, this vacuum does degrade: for example, the barium contained in the cathode evaporates, then redeposits on the inner walls of the tube, which can and do release atoms when subjected to heating. Residual particles adhering to an electrode can detach. Inadvertent outgassing will cause defocusing, and a particle, electric arcs, the parasitic phenomenon most detrimental to the "good image" of tubes.