# Electron gun in a vacuum -- How hard of a vacuum is needed?

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## Main Question or Discussion Point

how high does a vacuum need to be for electron gun to work? for example, will 1 pascal work, will 1 tenth of a pascal work? is there a minimum vacuum for a electron gun to work?

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davenn
Gold Member
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how high does a vacuum need to be for electron gun to work? for example, will 1 pascal work, will 1 tenth of a pascal work? is there a minimum vacuum for a electron gun to work?

what reading have you done to try and find an answer ?

how high does a vacuum need to be for electron gun to work? for example, will 1 pascal work, will 1 tenth of a pascal work? is there a minimum vacuum for a electron gun to work?
That is not a well defined question. In particular, whether or not the electron gun "works" is more like "does it work in this particular instance?" The typical pressure i a CRT for example, is around $10^{-6}$ Torr. "Works" could mean different things, like "does it work for this particular process you are going to use it for?", not whether or not the electrons make to the target.

as long as it generates an electron beam that is 3 centimeters long or longer, it works.

berkeman
Mentor
as long as it generates an electron beam that is 3 centimeters long or longer, it works.

as long as it generates an electron beam that is 3 centimeters long or longer, it works.
That still isn't well defined. An arc is an electron beam of sorts and an arc can happen at atmospheric pressure rather easily. You need to consider things like why you need a vacuum in the first place. You can get electrons from point A to point B in a lot of different environments. What does the vacuum buy you?
The answer to that question and the type of electron gun you have will determine your answer. Is it a pocket gun or is it just some electrodes and a filament producing a beam?

gleem
make an electron gun for a science fair. i am thinking thermionic emission.

256bits
Gold Member
make an electron gun for a science fair. i am thinking thermionic emission.
But you still did not say what you tend to investigate.
For example, maybe you want the electrons to excite a rarefied gas within a tube so you see the electron beam, or maybe you want the electrons to travel undisturbed on their way to a target.

if i want the electrons to excite a rarafied gas, is there a maximum pressure to make this work? work means, the
device produces an electron beam and you can see the beam.

DBO
My electron microscope will not turn the beam on till it hits 3 mTorr and it continues to pump till it reaches a several times XE-6 Torr. At 1E-6 Torr, it still "cracks" background hydrocarbons onto the sample making a black spot.
The questioner should read about the Paschen Curve about electrical discharges in gasses if what he wants is a discharge.

You used to get e-gun tubes with a mica window at the end andthe e-beam travelled through air, for a cm or a bit more - much like beta radiation.
The route to a general answer is to look at the attenuation vs distance at different pressures - and work out what attenuation you can stand, is it 10% or 90%? - mean free path would be at 50% attenuation
here's a start: http://www2.ece.rochester.edu/projects/bdt/files/Mean_Free_Path_MC