Anomaly in demo fusor.

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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DaveC
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Re: Anomaly in demo fusor.

Post by DaveC »

Chris O' -

My initial thought is similiar to other's here... it probably is pressure related... a tiny leak, outgassing, a bit of contaminant (grease, fingreprint, etc.) or just a place where the ionization is favored at the average chamber pressure.

Try to lower the pressure, if you can.

Dave Cooper
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Doug Coulter
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Re: Anomaly in demo fusor.

Post by Doug Coulter »

He mentioned an NST supply. What I see here with DC is that the color of the wall spot I see when the pressure is "way too high" is a different color -- brown, than what you see as the cathode glow on the grid (same as he pictured). If he tried DC, he might see that too, and that would confirm. For whatever reason, you get the two color thing with DC but can't see it with AC, something weird about what states of nitrogen make what spectral lines the most efficiently, I believe.

When I do this in my systems, and everything is nicely symmetric, I see the wall spot race around the tank walls as it pumps down. It's a nice show. If it sits on one spot, I know I don't have my grid centered.
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chrisforpower
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Re: Anomaly in demo fusor.

Post by chrisforpower »

Excuse me, but Dueg I was unable to understand your post. Is their a missing reply? Whats all this about different colored plasma? And I get the impression that you think I am using AC: In reality I have always been using DC. The neon sign transformer was rectified and the fly back transformer is internally rectified. Could you repost with clearer wording?
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Doug Coulter
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Re: Anomaly in demo fusor.

Post by Doug Coulter »

Sorry 'bout that.

I have noticed, when running a discharge during pumpdown, with air as the main thing in the tank, that the glow spot I'll often see on the outer wall is a different color (brownish) than the glow on the grid, which tends to be bluish-purple or pinky-blue (same as you show). This is with DC, grid negative. This seems to be something you get with nitrogen, as some gas-jet-type ion sources I've built also showed those colors at different points in the de-excitation of the nitrogen as the jet evolved over time (distance from the jet and electrical field origin).

There are some movies of what the fusor looks like with different gas pressures on my youtube channel, but I don't happen to have one that shows this effect, it's usually pretty fleeting with fast pumps, which is what I've got here.

http://www.youtube.com/user/DCFusor
(youtube only shows a few of the more recent ones unless you click "show all")

If you'd like, I can make a movie of it happening here -- saw it again just the other day after a teardown of my smaller fusor, it should be easy to do again. Very pretty and it impresses those who don't understand you can't get near to fusion at any pressure even close to that high. That's where you are at now -- far too high pressure even for a good demo fusor. Either you have a leak or a bad pump, as a 2 stage mech pump alone will almost get you to the point of "putting out the lights" in a tight system with some patience.

In the meanwhile, here's a couple that show pumpdowns, starting from a lower pressure than where the wall-spots exist. Not very good videography here, I was just learning how to do it.

http://www.youtube.com/user/DCFusor#p/u/5/Sb3x2yPFcB8
http://www.youtube.com/user/DCFusor#p/u/6/sY_jKJQUAlg

From what I've seen -- you're just at much too high a pressure to see what I'm seeing in the movies above -- I see the wall spots at about double-triple the pressure I started from in those movies- you're at several (at least) millibars, about factor 100 higher than where we run fusion. When I call out pressures, they're all in millibars as that's what my gages speak, and are the gage readings, uncorrected for the known errors due to gas type. This works for me as it's repeatable so I don't really care what the actual true pressure is, just that I can hit the good number over and over reliably.

After a pretty short experience with it, the appearance of discharge itself along with the power supply numbers tells you more than the gas gage's limited resolution can anyway. Once you tune your color sense, you can also tell quite a bit about any impurities in the gas as well. A pocket spectroscope is a handy accessory for that -- the poor man's mass spectrometer, a trick mentioned in John Strong's book.
Why guess when you can know? Measure!
chrisforpower
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Re: Anomaly in demo fusor.

Post by chrisforpower »

Thanks. That was much clearer.

I believe the anomaly is probably a leak as it takes a very short amount of time (about an hour) for the demo to get back to atmospheric pressure. This is in comparison to before when I was able to get it to hold a vacuum for days. Before I was also able to get a nicely defined plasma ball. unfortunately., I have no time to fix it, as the demo is going to fair today. Thanks for your help everyone.

By the way, how do I go about plugging leaks such as this.
Dan Tibbets
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Re: Anomaly in demo fusor.

Post by Dan Tibbets »

I agree with Wilfried Heil. It looks much like my glow discharge with positive current to the (anode) grid.
It doesn't matter so much which lead you hook up, what matters is the diode arrangement with that lead.
If you normally have the hot lead to the 'cathode' grid, and here you maintained the diode arrangement but hooked up the 'ground' lead while grounding the fusor case to the normal cathode lead, you have reversed the current feed- ie-a pos anode grid.

There is also probably some anomulous glow discharge due to pressure, sharp projections.

Dan Tibbets
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