Author Topic: Plasma Toroid  (Read 1240 times)

Offline Uspring

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Re: Plasma Toroid
« Reply #20 on: November 08, 2022, 04:29:37 PM »
Thank you, klugesmith. That calculation depends on an assumption about the diameter of the hot region of the torus and I really don't know that, My very first calculation based on some tabulated heat conduction values from the internet came out to 3600 K and I thought, wow, that looks reasonable and I didn't try to improve the estimate. I don't have literature values on plasma conductivity for Xe, but for the ones I have, e.g. for air and Ar at 1 bar, it seems temperature should be somewhat higher, like 5000+ K. So that's all a bit ballpark. In any case, we are dealing with hot plasma here, not a neon sign type glow discharge.

Offline alan sailer

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Re: Plasma Toroid
« Reply #21 on: November 08, 2022, 05:41:18 PM »
David,

I have read and thought about your three heating ideas.

The first I don't understand the insulation comment. Do you mean that that copper in the center doesn't conduct any electricity.

The second explanation makes great sense. To state it another way, the current does not stay in one strand but has to travel from one strand to another. The contact resistance between strands heats up the wire more.

The solid wire I tried (it's insulation failed) was 14AWG 1.55mm diameter. The stranded is 12AWG 19 strands each 0.4mm. Both are unplated solid copper.

Offline davekni

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Re: Plasma Toroid
« Reply #22 on: November 09, 2022, 04:31:48 AM »
Quote
The first I don't understand the insulation comment. Do you mean that that copper in the center doesn't conduct any electricity.
Center current isn't zero, but extremely close to zero.  True for both solid and stranded.  Skin depth is about 20um at 9.8MHz.  Current drops exponentially from outer edge of copper inwards.  12AWG wire is 2mm diameter, 1mm radius.  By the center current is exp(-1mm/20um) lower than the edge.  Actually, that's a formula for flat conductors.  Not sure off-hand the difference with round.  Doesn't matter much here.  Center current density is ~1E-20 of outside.  Extremely low, even if incorrect by an order of magnitude or two.

19-strand wire is likely to keep the same 12 wires on the outside, at least mostly.  1 center, 6 around that, and 12 around those.  If so, and given unplated copper, stranded loss is likely due to limited copper at outer 20um of wire circumference.  Can't think of any other likely reasons.  Perhaps someone else will.
« Last Edit: November 09, 2022, 09:05:28 PM by davekni »
David Knierim

Offline alan sailer

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Re: Plasma Toroid
« Reply #23 on: November 09, 2022, 06:47:49 PM »
David,

Thanks for the reply. It's not of any real importance. I was just curious.

I'm an OK engineer as I can usually either figure out stuff or find someone (person or book) that
gives the answer.

I'm also sort of fixated on results, so if something doesn't work well (hot stranded wire) and I find
something that does (cooler solid wire ) I generally just go forward without trying to understand
why.

Offline davekni

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Re: Plasma Toroid
« Reply #24 on: November 09, 2022, 09:19:09 PM »
There's lots of information available on "skin depth", an important consideration for many if not most AC circuits.  Even at Tesla coil frequencies, copper skin depth is ~0.2mm.  Typical copper tube primary conductors are using only outer 0.2mm of the tube.  At 60Hz line frequency copper skin depth is ~8mm, so matters only for large high-current bus bars.

BTW, edited my last post to fix the exponential formula.
David Knierim

Offline alan sailer

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Re: Plasma Toroid
« Reply #25 on: November 20, 2022, 01:27:00 AM »
Today I felt better than I have for a long, long time so I have been playing in the garage. I set-up the xenon plasma globe and blew some plasma (smoke) rings. The camera taking the video is an old Edgertronic unit that is running at about 2000FPS.

https://www.flickr.com/photos/8763834@N02/52511116424/in/photostream/

I took another video with the coil on top, which creates a stable toroid. When I stepped the video you can clearly see that the tendrils snake about until one happens to meet up with another and create a closed circuit. The ring flashes into existence and quickly tightens up into a circle. It's fascinating to watch.

Offline davekni

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Re: Plasma Toroid
« Reply #26 on: November 20, 2022, 05:08:31 AM »
Absolutely fascinating!  Thank you so much for sharing.  Happy that you are feeling better too.
David Knierim

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Re: Plasma Toroid
« Reply #26 on: November 20, 2022, 05:08:31 AM »

 


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