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Vacuum for high voltage plasma

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Toasty

Wed Feb 17 2016, 01:41PM

Registered Member #56785 Joined: Fri Aug 28 2015, 02:54PM
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Hazmatt_(The Underdog) wrote ...

Yea you can do that, but make sure to tape the mason jar with electrical tape in a spider-web pattern. If you don't, and there is a weak spot on the jar it will implode then explode sending glass at like 100 feet per second in every direction.

That's why you buy vacuum rated heavy wall bell jars for this kind of thing, and even the big ones use steel cages for this reason.

Thanks, its pretty thick glass but i got a plastic sheet in front of it so i should be fine if it implodes.... i hope

Still... what kind of vacuum pressure should i have to get some decent plasma going?

jpsmith123

Wed Feb 17 2016, 02:48PM

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Generally speaking, the results you get will depend on geometry, the electrodes, the applied voltage (AC, DC, pulsed), the type of gas, etc.
You can even make a plasma at atmospheric pressure; i.e., under certain conditions you wouldn't even need a vacuum pump. (See, e.g., US Patent 8471171).

If you just want to make a glow discharge plasma inside a glass tube, for example, then you probably want a pressure in the range of say, 5 to 20 torr, or something like that, so even a cheap single stage AC type vacuum pump would probably work.

I imagine you would evacuate the container down to the limit of the pump and then open a leak valve (could be just some rubber tubing with a clamp or something like that) and then see at what point a glow discharge starts for your particular set up.

woodchuck

Wed Feb 17 2016, 04:35PM

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There is a book called X-Rays, by a fellow named Kaye, that is available at archive.org (written sometime around 1917). It is a great read, and by the time you've finished the first few chapters, you'll be something of an authority on the subject of electrical discharge phenomenon at reduced pressure.

2Spoons

Thu Feb 18 2016, 02:02AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Toasty wrote ...

However what kind of vacuum pressure should i look for when i want some decent looking plasma?

Based on the vapor pressure of water at 15C, my water aspirator would not have gone below 13 Torr. I found the air plasma (roughly a 10cm discharge) I had going in a small jar starting to fade if I left the aspirator running. So that shows the vacuum requirements are rather modest.

Ash Small

Thu Feb 18 2016, 09:48AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
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Most commercial vacuum plasma systems ignite the plasma at a higher pressure than the working pressure. Once the plasma is established the pressure is then reduced to the working pressure.

Paschen curves for various gases should give you some idea.

klugesmith

Fri Feb 19 2016, 04:02PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
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>> Still... what kind of vacuum pressure should i have to get some decent plasma going?

You can quickly see lots of plasmas at different pressures by poking around on the fusor forum.

Neon signs and fluorescent lights work in the 10 to 100 torr region.

Rotary pumps often quote ultimate pressures on the order of one micron (millitorr,microbar,decipascal) but don't actually get there, even with gauge & nothing else on the pump. Even industrial pumps from reputable manufacturers. One excuse would be testing with a McLeod gauge, that's used to characterize & calibrate electronic gauges. McLeod gauge measures the compression ratio to bring a "vacuum" sample up to atmospheric pressure, so they miss the partial pressure of condensible vapors in the sample.

Rectified HV adds interest. For example, at pressures where electrodes are coated with a layer of plasma, it's the negative electrode.

Ash Small

Fri Feb 19 2016, 11:08PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
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It may seem like you're not getting direct answers, but the question isn't that direct. You've not specified a gas, not specified the type of coupling (capacitive or inductive, I think we assume capacitive), you've not specified voltage or power.

You've not said 'I want a pump that will reach this pressure with this chamber'.

We can only reply to the questions you ask. Maybe you should read up a bit on Paschen's law and the various types of coupling, etc? With AC, even the frequency can make a difference.

Toasty

Sat Feb 20 2016, 01:04PM

Registered Member #56785 Joined: Fri Aug 28 2015, 02:54PM
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I'm learning a lot from all this, i think i will poke around first, see what happens.

Ash Small wrote ...

It may seem like you're not getting direct answers, but the question isn't that direct. You've not specified a gas, not specified the type of coupling (capacitive or inductive, I think we assume capacitive), you've not specified voltage or power.

You've not said 'I want a pump that will reach this pressure with this chamber'.

We can only reply to the questions you ask. Maybe you should read up a bit on Paschen's law and the various types of coupling, etc? With AC, even the frequency can make a difference.

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