# Is it possible to make cathode ray or electron gun without a vacuum?

I stumbled upon this picture on the internet here is the link https://www.reddit.com/r/ElectroBOOM/comments/16azeq5/cathode_ray_succesfully_shooting_a_plasma_beam_in/

I wasn't able to get info from the poster and I won't let it pass since I tried making one like this in the past but never got a line like this, it just goes directly to the plate even with a vacuum. This picture maybe fake but in anycase is it possible in to make electron gun in the open air?

It is easy to generate plasma channels in air (as shown in the photo, as used millions of times a day with TIG welders, and as present every time there is a spark), but that is not the same as an electron "beam".

Plasma channels are formed by electron/ion flow in an ionized gas, typically as a result of an external electric (or electro-magnetic) field.

The ions and electrons repeatedly hit normal gas atoms/molecules, causing secondary ionization, losing energy and eventually recombining. At atmospheric pressure, the resulting "mean free path" between collisions is very low (<<1um), and whilst there is net transport of electrons and ions along the channel, individual particles will tend to follow a biased random walk, rather than a directed high-speed beam.

If you want a true "beam" you either need to stick to very low pressures (so the mean free path increases to visible lengths) , or you can "prepare" a very high energy beam in a vacuum, and accept that when it hits the air it will quickly dissipate due to collisions.

An example in the former regime, is this spherical accelerator, which I built myself, running at about $$\10^{-5}\$$ Bar and 10KV. You can clearly see two electron beams. (The photo is also used for my profile picture.)

An example of a beam created in a vacuum entering the air, can be found near the top of the Wikipedia page for the Cyclotron

"No, it is no possible to make electron beams without a vacuum, because the mean free path between collisions is too short"

• Just one more question..... so according to what you said , if I put an ion detector (charged particles detector) infront of the beam in my post I won't get readings ?
– Majd
Sep 12, 2023 at 8:57
• It depends how far in front you put the detector. The plasma channel shown is being excited by a high voltage, high frequency field. Your ion detector is probably grounded, and there may be enough of a field to create a secondary discharge. There may also be "free" ions floating around which didn't get a change to recombine with their partner electrons. However, you definitely don't have a "beam" that is made up of high velocity electrons, because they will quickly lose their energy through collisions. Sep 12, 2023 at 9:03
• @colintd By "mean free path", do you mean an average length of path (in air, in this case) electrons can travel before colliding? Sep 14, 2023 at 19:58
• Yes, that is exactly what I mean. As you increase vacuum this goes up, and by the time you are into medium vacuum it is up to cms as opposed to ums Sep 14, 2023 at 20:03

I would say it is. The reason why vacuum is used is because electrons interact with air and lose energy before reaching their destination which is typically an anode in a vacuum tube.
As the guy who posted the pictures stated, this uses 10A at 36V at the primary of the HV transformer, which tells you how much energy is needed to create this beam, make it visible and make it travel those few centimeters.

You never mention the details of your setup, so it's hard to tell why yours didn't work like this.

• @Majd If it wasn't a straight line, then it wasn't a "beam", but an arc. The point is to have a high voltage (like 20kV in this case) accelerate the electrons to the point of travelling in a straight line. Sep 12, 2023 at 8:00
• @Majd Mahdi (ElectroBoom) mentioned that he used a sharp point on the negative side, and 20kV DC (flat DC) on the positive side. I think he's being honest. He's not the type that makes fake videos. Sep 12, 2023 at 8:02
• As I've noted in my answer below, the pictures don't show a particle "beam" rather a plasma channel. The mean free path of individual particles is <<1um, and the net velocity along the beam is nowhere near the relativistic velocities you often have with a true electron beam. What you have is much more akin to an electric spark than a particle "beam". Sep 12, 2023 at 8:36
• @colintd Unfortunately, I would have to agree. I saw his video and he didn't achieve such a beam. Personally, I would experiment with it if I had the time. I do have a couple of old CRTs I could use. Sep 12, 2023 at 12:14
• @EdinFifić The picture is from some Polish person, not from ElectroBoom. Hard to say if it’s real or not. That washer is awfully rough. I’d expect this to work after sand- or bead-blasting the washer to make it smooth, then polishing it. As it’s shown it’s probably rather sensitive to layout and a slightest movement may destroy the effect and arc directly to a bump on the washer. This may also be AC not DC. I don’t hold much faith in pictures posted with no further elaboration. It’s not a cathode ray for sure - those don’t shine purple in air. Sep 12, 2023 at 13:51

Without vacuum at all, or just at some point(s) along the path?

Electron-beam welding is an industrial process, and can be done in the air (in part). Example (no affiliation, just top Google hit):

https://www.sst-ebeam.com/en/application-areas/welding-at-atmosphere.html

My understanding is, by a combination of having an extremely small aperture (tightly fitting the beam), a "force field" (really more of an ion pump action), and multiple pumping stages, the high-vacuum gun assembly can be kept in relative cleanliness while actually shooting useful beam out into plain air.

The beam diverges rapidly in air of course, due to scattering, but at the close ranges used here, that's acceptable. It scatters even more once it hits the metal, after all (but that's where it's doing useful work, heating a very small spot).