Sunday, September 20, 2020

Shooting an electron beam through air

 A special 100nm thick window allows 25 KeV electrons to pass from a vacuum tube to the atmosphere where they hit a fluorescent screen -- a CRT in air!

Shielded GoPro goes through a powerful electron beam: More powerful amateur electron beam in air: Deep technical resource on dielectric charging via electron bombardment: Tons of information on industrial e-beam processing: KF25 to glass tube quick adapter: 100nm silicon nitride windows: More windows: Tritium light sources (eBay removed most): E-beam crosslinking: KF25 cross $18 on Amazon: Lightbulb sockets: Hysol 1C: Applied Science on Patreon:


  1. Hey Ben, what a great project, I wanted to know what difference does the distance from filament to the anode makes. As far as emission current or voltage requirement. This is also useful for my other electron gun projects. I really wanna know the science behind the cathode to anode distance. Also would it be bad to have a higher emission current. I got mine up to 1.5ma and got a much brighter spot, but is there any downside to that’s?

    1. In theory, the distance from the filament and the anode doesn't make any difference because the electrons emitted from the filament are accelerated through the same potential different and will have the same energy. Practically, they angle of emission through the anode hole will be affected by the distance (you can use the visible incandesent light from the filament to estimate the shape of the electron beam through the anode). Also, at poor vacuums, a larger distance will cause the electron beam to become degraded and scattered. Also, a too-short distance -- depending on how the anode is mounted and vacuum quality may cause arcing. 1.5mA emission should be OK. Take a look at X-ray tube filaments to get an idea of how thick they should be be if you want to emit 10's or 100's of mA. Also, be aware of X-rays if you are accelerating above 10KV!

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  3. Hi Ben,

    Great work and explanation. Thanks a lot for the ideas and tips you shared in this project.

    Do you have any estimation or may be even some measurement of what part (percentage) of the cathode current (100 uA) is able to pass through the membrane outside in the air?

    What do you think about an idea of conducting through the membrane a trochoidal (crossed field) electron beam? What is the minimal necessary energy of an electron for passing through the membrane like you used?