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First Plasma

Posted: Fri May 18, 2018 11:14 pm
by Ryan Copeland
I was running some experiments with a steel vacuum chamber (really a steel cooking pot with acrylic on top) and I decided to make a quick grid from copper wire and use the vacuum line as a HV feed-through. My supply is the United Nuclear 25KV DC power supply and runs off a motorcycle battery. My pump has an ultimate vacuum pressure of 0.3Pa. Also my vacuum gauge that came with the pump broke and is leaking oil into the chamber so if anyone could tell me the ballpark pressures this plasma is at, it would be greatly appreciated (colors not altered).

Inner grid (sorry about the reflection)
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Inner grid at night
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Both grids
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Image

Re: First Plasma

Posted: Sat May 19, 2018 3:55 am
by Richard Hull
With a glowing stalk, your pressure is above 300 microns.

Richard Hull

Re: First Plasma

Posted: Sat May 19, 2018 3:59 am
by Ryan Copeland
Thank you Richard! I was also wondering if this would be enough to get into plasma club or would I have to drop the pressure even lower?

Re: First Plasma

Posted: Sat May 19, 2018 4:17 am
by prestonbarrows
The fact that there is a visable glow discharge around your electrode shows you are making a plasma. The visable light comes from free ions and free electrons (a plasma) relaxing back into a neutral gas and spitting out the excess energy as a photon.

~300 microns or 0.3 Torr, is a very high pressure in the fusion world. This puts the mean free path in the range of around a fraction of a millimeter.
http://library.psfc.mit.edu/catalog/onl ... ARY_13.pdf

This is in agreement with your images. Expect the visible glow discharge to be a few MFP's, or a few mm in your case.

You are applying a potential of ~25 kV with your power supply. This results in a field (in volts per distance) of, say 2.5kV/cm depending on your chamber geometry. Given the MFP discussed above, each accelerated ion gets very roughly .025kV (or 25 volts) of acceleration before colliding with a background particle. The DD fusion cross section at this energy is negligible; so the ions you are creating and accelerating are basically just going into heat and never producing any fusion.

To get fusion going, you need to reduce your background pressure by a factor of 100 or so. Then, any individual ion you accelerate will gain enough energy by falling though the potential well that it can produce fusion events when it collides at the end of its MFP.

Re: First Plasma

Posted: Mon May 21, 2018 12:16 am
by Ryan Copeland
Thanks for the information Preston! I will continue to try to get my chamber down to the specifications on my pump (0.3Pa or ~2.75 microns) to hopefully achieve a better effect.

Re: First Plasma

Posted: Tue May 22, 2018 3:07 am
by prestonbarrows
What kind of vacuum gauge were you using before it broke? What makes you think the gauge is broken? What makes you think the gauge is leaking oil into the vacuum?

What type of pump are you using?

Re: First Plasma

Posted: Wed May 23, 2018 2:37 am
by Ryan Copeland
I am certain the gauge is broken and is leaking oil into the chamber. When I pull a vacuum for about 5 minutes, the gauge still reads zero while plasma is created. Also at these pressures the plastic that is used to view the numbers is being pulled into a concave shape and tiny bubbles can be seen rising from the gauge oil to the air above the oil. When re-pressurized to 1 atm, the gauge can be heard "gulping" in air. The pictures below show the gauge on the setup, the oil from the gauge in the bottom of chamber, and the pump I am using.

Gauge on chamber
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Oil in chamber (it isn't water or anything believe me)
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Vacuum Pump
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Re: First Plasma

Posted: Wed May 23, 2018 4:37 am
by Richard Hull
The gauge, even if in perfect working order and purchased brand new, is worthless for reading a vacuum. It tells you nothing at all about the real pressure in the system. Read the FAQs and look at the photos.

Richard Hull

Re: First Plasma

Posted: Wed May 23, 2018 1:29 pm
by Ryan Copeland
Yes that gauge came with the vacuum pump a long time ago and I am looking for one that will work for fusion applications. The gauge would just tell me if I got to a low enough pressure for plasma to form.

Re: First Plasma

Posted: Thu May 24, 2018 12:25 pm
by Dennis P Brown
Any gauge that contaminates your system tells you nothing except how to waste your time and efforts. Throw that junk away. Also, those connections on the chamber (besides the acrylic window) are not suitable for any real vacuum system.

Creating a vacuum has enough issues for a novice without using a leaking gauge.

More to the point, you need to decide what you plan on doing; if just creating a plasma, you don't need any gauge because the plasma tells you more than enough. If you are serious about a demo fusor, then you will need a pump that gets into the mircon range and a gauge that can read in that range.

That pump may get you in the lower end of the mircon range (once you have decent connectors and gauge) so getting an inexpensive gauge that reads microns (from 1 to 300/1000 or so; scales vary) to confirm your pump's performance is essential; you primarily need one that reads microns and has some decent resolution in the 1 - 20 microns scale and, of course, some values much higher. See ebay for inexpensive units but do your homework before buying - inexpensive gauges are often old and can have issues. More expensive units can read higher pressures and lower as well as microns (or pascal or torr.)

The unit you decide upon, again, depends on your long range goals (a real fusor requires high vacuum among many things.)