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Electron guns only work in near vacuum, (like in a CRT).
The cathode ray tube (CRT) is a vacuum tube containing an electron gun (a source of electrons) and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others.
How about this diagram? Does it need vacuum?
They typically make the electron gun using a tungsten filament - which would oxidise in air in a few 10's seconds or quicker if operated at the usual dull red heat.
For use in air you could use a nichrome wire (which won't oxidise)
If I had money to burn I could use a yttria coated rhenium filament which would survive being heated to bright red or even white heat in air.
I know they use them in mass spectrometers. The advantage is that they don't die if you let some air in.