Plasma Toroid

[alan sailer]

I'm sorry if this a wrong place to put this post. But it involves plasma, flame tesla circuits and oudin coils.


A few days ago I found out about experiments that involve the creation of a stable plasma toroid in low pressure xenon. The exciting field is created by using the primary of a 10MHz flame tesla coil. Since I have built both plasma globes and flame tesla coils it seemed like a nice idea to repeat the experiments.

https://www.flickr.com/photos/8763834@N02/52470073982/in/photostream/

https://www.flickr.com/photos/8763834@N02/52471107458/in/photostream/

The descriptions on the Flickr pages do a first pass at explaining the set-up. I can clarify if anyone cares.

I also briefly tried a neon based globe and a krypton/iodine globe and could not  get anything like this.

[alan sailer]

I had noticed earlier that unlike an ordinary plasma globe discharge, the xenon toroid is relatively unaffected by touching the globe.
So I got a small but powerful NIB magnet and was happy to see that the Maxwell laws hold quite nicely.

I wish I could explain why the toroid exists at all and why it is not effected by body capacitance like a regular plasma globe discharge.

https://www.flickr.com/photos/8763834@N02/52471564418/in/dateposted/

[klugesmith]

Excellent visual production values as we'd expect from Alan! Did you do the xenon fill yourself?

Does the toroid produce enough heat that glass globe might be weakened or softened?

[alan sailer]

klugesmith,

What a nice thing to say. The only reply I can give is that I dislike the videos where the camera waves from one vantage to another. They make me dizzy and make it hard to picture what is going on.

The globe gets warm but not alarmingly so. Power input to the circuit is about 60 watts and I'd imagine much less than that is getting into the globe. Weakened or softened is not an issue.

Finally the xenon is an old fill back when I was making dozens of tubes/globes. The green is due to adsorbed oxygen. I made a xenon globe for a friend many months later after I figured out this issue and he reports the globe is white, no green. It required much baking under vacuum to get rid of the green.

[davekni]

Excellent visual production values as we'd expect from Alan!

I quite agree!  Watched quite a number of other plasma toroid videos.  Yours are much better!
One question:  Do you know roughly the voltage across your 3-turn coil?  Volts/turn is the important parameter.

I also briefly tried a neon based globe and a krypton/iodine globe and could not  get anything like this.

If these are the commercial plasma globes, I believe they are much higher pressure, around 500 Torr rather than the 15 Torr I've seen in other toroid videos.  BTW, what pressure is in your Xenon tube?  I've thought about duplicating this with argon and/or helium, the only noble gasses I have around.  Won't be as bright as Xenon, but still fun to play with.

I had noticed earlier that unlike an ordinary plasma globe discharge, the xenon toroid is relatively unaffected by touching the globe.
So I got a small but powerful NIB magnet and was happy to see that the Maxwell laws hold quite nicely.

I wish I could explain why the toroid exists at all and why it is not effected by body capacitance like a regular plasma globe discharge.

I don't have any definitive answer, but behavior generally makes sense.  Electric field inside and around the coil is a combination of capacitive coupling from coil voltage and circular field induced by coil's magnetic field.  I expect the capacitive coupling dominates for the more random thin streamers before a torios forms.  Capacitive-coupled current will be relatively low as with a standard plasma globe.  I'd guess that these random streamers are sensitive to touch.

I think the toroid plasma is much higher current and lower voltage, dominated by magnetically-induced current.  This makes it less sensitive to hand capacitance and more sensitive to magnetic field.  I expect the primary magnetic effect is due to gradient of permanent magnet's field component perpendicular to toroid (perpendicular to current flow).  Since current is AC, there will be no net force due to any uniform magnetic field.  Instead, the field makes electrons travel in spiral paths.  Those longer paths have more collisions so increase resistance, causing the current flow to move towards areas where magnetic field is lower.

BTW, if I get around to trying this, plan to use a single-turn coil to minimize voltage.  Same volts/turn will induce same toroid current but reduce capacitive voltage so hopefully make toroid more stable compared to random streamers.

Uspring may have a more precise and detailed explanation.

[Uspring]

These are very pretty videos. Davids explanations are excellent, regarding both the effects of the electric and magnetic fields of the coil and also of the effect of the permanent magnet. There is little I can add to this.
The magnetically induced circular electric field, which is the source of the torus discharge and the electric field, which comes from the voltages of the coil wire both fill all af the xenon tube. So it needs some explanation, why the discharges are quite localised.
I believe several opposing effects to be responsible for this:
1. Warming up of the gas will cause a lowering of gas density, which increases conductivity, since electron movement is less obstructed. That will cause more current to go into the already warmer regions causing a constriction of the flow path.
2. Heat is conducted away from the warm region, causing the the flow path to widen.
Possible also:
3. Parallel flowing currents attract each other, causing a constriction of the flow path.
4. Diffusion caused by electron scattering on the gas molecules will widen the path.

To establish the magnitude of these effects one would need to know the gas density and the currents flowing.

[alan sailer]

David, Uspring,

Thanks for the excellent explanations. The random pre-toroid streamers are typical plasma globe effects and are very touch sensitive.

I have an as yet unlocated notebook that has all the plasma tube fill data. All the globes are tried are homebuilt. It might be interesting to make a low pressure neon and krypton globe but restarting that old project would be pretty involved.

I'll se about measuring the loop voltage. I have a Tek HV probe but am unsure as to how to measure a floating voltage like that. I am not willing to kill my scope. If you have any tips please advise. The probe is one of the P6015 types.

[klugesmith]

Regarding number of turns in the coil: Why would it matter, as long as ampere-turns are the same?  More turns requires more voltage between terminals, but how does that affect the electric field around the coil, especially since the voltage per turn is the same?   

I invite Alan to try an electrostatic shield around the coil. As used with H-field probes (for RF emission compliance testing) or loop antennas for radio reception.
Wrap the coil with conductive metal foil, or braid, leaving a narrow air gap so there's no shorted turn.  If the drive line is single-ended, then shield can be connected to coaxial ground.

One reference just found on Internet is https://www.n5ese.com/loop_ant.html

[Weston]

Cool to see this effect! One day I want to own a plasma globe of the right fill and pressure to do this.

We were discussing a video of the same effect on IRC last week:

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The plasma ring is an inductively coupled plasma, as opposed to the capacitively coupled plasma in a plasma globe. There is a decent amount of literature on the effect, as both capacitively coupled and inductively coupled plasmas are used for industrial applications. Inductively coupled plasmas are typically more dense, and are easier to control the location of than capacitively coupled plasmas.

https://en.wikipedia.org/wiki/Inductively_coupled_plasma

[klugesmith]

Plasma rings are the reaction mass in Pulsed Inductive Thrusters.

Set up a flat spiral work coil with high voltage capacitor and switch, as in disk launchers discussed under Pulsed Power in this forum.   
In vacuum, with a puff of inert gas in place of annular metal projectile.
When fired, the induced voltage ionizes the gas and develops a strong circulating current.
Plasma is accelerated to very high velocity. Like the toroids in this thread, and unlike in ion beam thrusters, there's no contact between plasma and electrodes.

https://en.wikipedia.org/wiki/Pulsed_inductive_thruster
Wiki editor mentions a variant. "FARAD uses a separate inductive RF discharge to preionize the propellant before it is accelerated by the current pulse. This preionization allows FARAD to operate at much lower discharge energies than the PIT (100 joules per pulse vs 4 kilojoules per pulse) and allows for a reduction in the thruster's size."

[davekni]

I believe several opposing effects to be responsible for this:

My personal guesses to most important effects:

1. Warming up of the gas will cause a lowering of gas density, which increases conductivity, since electron movement is less obstructed. That will cause more current to go into the already warmer regions causing a constriction of the flow path.

As Uspring said, I expect conductive hot path is first, much like TC arcs tend to follow previous ionized path.
2) Magnetically induced electric field is strongest adjacent coil, inducing toroid to stay closer to coil.
3) However, magnetic repulsion between coil and toroid (resistively-shorted turn) tends to shrink toroid away from coil.
4) Buoyancy of hot gas induces toroid to rise.

I'll se about measuring the loop voltage. I have a Tek HV probe but am unsure as to how to measure a floating voltage like that. I am not willing to kill my scope. If you have any tips please advise. The probe is one of the P6015 types.

Wow, P6015 is really nice.  Looked up specifications.  Good for 8kV RMS at 10MHz.  Most probes derate voltage much more at high frequency.  Should be good for direct probing.  Since I don't have a probe rated for such, usually make a crude tiny capacitor to put in series with probe, such as a short section of HV cable taped to another short section of wire to which scope probe is clipped.  Requires calibrating at lower voltage where both this cap-coupled probe and another direct probe can be compared.

Regarding number of turns in the coil: Why would it matter, as long as ampere-turns are the same?  More turns requires more voltage between terminals, but how does that affect the electric field around the coil, especially since the voltage per turn is the same?

Voltage affects the initial thin streamers which (I think) are due to capacitive coupling from coil voltage.  Might be part of why toroid degrades to these thin streamers occasionally (much more often in other videos).

Fun topic!

[alan sailer]

David,

I did do some measurements with the P6015 probe of the "primary" inductor of the driver.
This is the loop that powers the plasma toroid. The circuit is identical to the low
power flame tesla that teslaundmehr popularized, a few parts have different values.

I used an isolation transformer to float the power supply. The ground lead of the P6015 was
attached to the drain of the IRF260 and the signal lead to the 100pF capacitor side of the primary.

Without the xenon globe the voltage across the inductor is 2500 volts p-p @ 9.8MHz. This is with
19V@ 3 amps from the power supply.

With the xenon globe in and a stable toroid the current drawn by the circuit goes up to ~5 amps.
The voltage across the inductor is 1000V p-p. When I adjust the current back to 3 amps this goes
down to 750V p-p.

I hope this is the data you wanted. I will keep things hooked up in case I am wrong.

[davekni]

I hope this is the data you wanted. I will keep things hooked up in case I am wrong.

Exactly what I was looking for!  Thank you!  Will help greatly if I get around to trying an argon and/or helium version.

[alan sailer]

Excellent. I am so glad to be able to give a small return favor for all the help you have given me in  the past.

[alan sailer]

I finally found my old notebook on plasma fills.

The xenon globe was filled at 50 torr.

The krypton iodine fill that I could get no toroid to form was 150 torr.

And the neon globe (with other gases) had a neon pressure of 400 torr. No toroid formed.

So my xenon globe seems to be higher pressure than the 15 torr I saw on another toroid demo.

[Uspring]

I've tried to get some information about the plasma torus from this data.
1. From some guesses at the geometry and with the help of JavaTC I obtained a coil inductance of about 1.4uH, a toroid inductance of 0.11uH and a coupling of 0.3.
2. From power input without toroid, 2500 Vpp input and the frequency of 10 MHz results a loss resistance of the coil of about 0.6 ohms.
3. I drew a diagram which describes the power dissipation of a primary coil with a series resistance coupled to a secondary coil with a resistive load (i.e. the plasma torus resistance). It is shown below. Input voltage is 1000 Vpp. On the vertical axis is the power dissipation and on the horizontal axis the torus resistance.



A power dissipation of 95 W as measured by Alan can't quite be obtained by any torus resistance, but my guesses at the geometry might well have been off a bit and the max of 80 W is close to the actual 95 W input. Interesting is the rather low torus resistance of about 7 ohms. The calculation also indicates, that about 90% of the power is dissipated in the torus and the rest in the coil.

The heat capacity of the gas in the torus volume at the given pressure is roughly 1 mJ/K. A power input of nearly 100 W will heat it up very quickly to a high temperature, which is limited by heat conduction. The discharge resembles more an arc than a glow discharge, the current inside the torus is around 3 A. All this has to be taken with a grain of salt due to my guesses, the assumption of 100% efficiency of Alans driver and that the frequency didn't change when torus appeared.

[alan sailer]

Uspring,

Very impressive. I'll read it again more carefully later.

A few comments on some things I have seen with the coil/toroid. First, when I was
buillding the circuit I noticed that the coil would heat up much more with stranded wire vs solid wire.
I had seen the same thing with an earlier flame tesla build using the same circuit.

On this build I did change to solid core wire but the insulation was only rated to 400V and it arced
over. So I went to stranded HV cable which stopped the arcing but is not optimal for the heating.

I'm sure that there is an good explanation of why the stranded wire heats up more.

Also as I think I mentioned before, the toroid does heat up the globe much more than an ordinary
flyback driven/streamer discharge. Some of this, of course, is due to the localized toroid vs the
distributed streamers.

Finally, the power from the supply jumps up to 19V/5amps but I turn it down to 19V/3amps during the operation.
I always like to go with the minimum power when doing stuff. That is why I am such a rotten tesla coil builder.

[davekni]

I'm sure that there is an good explanation of why the stranded wire heats up more.

Three reasons come to mind, all driven by skin effect:
1) Presuming copper, skin depth should be about 20um at 9.8mHz.  If strands are larger than 20um, they form a rough surface around the wire circumference.  Current will be mostly at the outer edge.  Only part of the outer 20um is copper.  Remainder is insulation.  Net conductivity drops as current is forced into the limited copper part of the surface.
2) For simple stranding such as with 7 strands, one strand is in the middle and remaining 6 around it.  For more complex stranding, strands that are on the outside at one location don't remain there along the length.  Current must move from strand-to-strand to remain within the outer 20um.  Strand-to-strand contact resistance is higher than along one strand.
3) Not directly related to stranding:  Is your stranded wire tin-plated and solid wire bare copper?  Tin's higher resistivity and current forced to outer skin layer increases loss.

Would be interesting to experiment with a single-turn coil, perhaps copper pipe or strip.  Probably requires a coupling network to match impedance with driver.  Lower voltage would presumably minimize capacitively-coupled thin streamers.  No arcing issues within the coil.

[Uspring]

A calculation of the torus temperature from heat conduction, assuming 80 W power input and some estimates about the torus size indicates a value around 3000-4000K.

I found some interesting facts about heat conduction in gases. It does not seem to depend on the pressure but rises roughly as sqrt(T) with temperature, which has to do with the average speed of the gas atoms. Also heavy gases conduct much less than lighter ones. Xe, e.g., conducts about 3 times less than Ar and 8 times less than Ne. That makes Xe particularly suitable for plasma toroids, since it takes less power to keep the temperature up.

The heat seems to be the dominant cause of free electrons, since the voltage around the ring is only about 30 V, which does not seem enough field accelerate electrons enough for impact ionisation.

There is a considerable difference in magnitude between the voltages it takes to start a discharge and to keep it running. That is similar to a spark gap. Alans toroids are probably self igniting due to the large unloaded coil voltage.

[klugesmith]

Nice work, Uspring, modeling equivalent circuits and temperature and heat conduction.

>> Also heavy gases conduct much less than lighter ones.

Yup. That's why large electric generators (say, above 100 megawatts) are generally cooled with dry hydrogen gas.  Hydrogen fill also minimizes the windage loss (mechanical drag forces).
Been that way for many decades.  Nothing does the job better than H2, and it costs much less than helium.
https://www.powerservicesgroup.com/2016/03/why-use-hydrogen-to-cool-a-generator/
In a table comparing properties of air and hydrogen, "Supports combustion" has a yes for air and no for H2.