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:

/>
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. 

[Uspring]

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.

[alan sailer]

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.

[davekni]

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.

[alan sailer]

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.

[davekni]

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.

[alan sailer]

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.

[davekni]

Absolutely fascinating!  Thank you so much for sharing.  Happy that you are feeling better too.

[alan sailer]

I decided to break out part of my old plasma filling system (valves, pumps, gauges etc) and try out different gasses with the
10MHz oscillator. After the usual reeducation ("leaks" are sometimes just out-gassing) I got the system set-up and pumped to 30mT.
I'm using a 1000ml round flask with a 24/40 ground glass joint. This lets me set-up and run without waiting months for real glass
from a glassblower. The gauges are two Baratron 10 and 1000torr. I used a Convectron to calibrate them years ago so they are probably OK. At least they agree in the crossover pressure.

I tried three gasses in the range from 50 to 1 torr air (cheap), argon and neon. The 10MHz oscillator was set at 19V/3Amps. I used the vacuum leak oudin coil to start things up. Also played with the current up to 5 amps. I could not find any toroid formation in any of these gases.
I have one left to try krypton but have not found the tank yet.

I am also planning to try the set-up with sodium and potassium. I'm hoping that the path between the globe and the gauges is long enough that the metals can condense out and not hurt anything.Evidently under vacuum these metals when heated moderately can vaporize enough to ionize. They both have such low ionization potentials that they may do something interesting with the oscillator.

[klugesmith]

Glad to hear stories about digging out old playthings.  Can you produce conditions today that are similar to those in your old sealed bulbs that do display toroids?

Regarding the metals, for example sodium.    We know that sodium vapor lamps are filled with rarefied Ne or Ar to get the arc started and warm things up.    Partial pressure of metal vapor can never exceed equilibrium where it condenses at coldest place on vessel wall.   
So I bet the atom fraction in low pressure sodium lamps is never more than 0.1% metal, but for some reason that generates most of the light.  Like Hg in regular fluorescent lamps and non-red neon lighting:   a few microns of Hg pressure and many torr of inert gas pressure.

OTOH, Hg vapor rectifiers with hot cathode manage to ionize at 10 or 15 volts.  As far as I know, the gas content is nothing but Hg vapor, at pressure than can't be more than a few microns.

[davekni]

I tried three gasses in the range from 50 to 1 torr air (cheap), argon and neon. The 10MHz oscillator was set at 19V/3Amps. I used the vacuum leak oudin coil to start things up. Also played with the current up to 5 amps. I could not find any toroid formation in any of these gases.
I have one left to try krypton but have not found the tank yet.

Thank you for for the experiments and results!  I would have expected argon to form a toroid as easily, just not be as bright as xenon.  I think dielectric strength of argon is slightly lower than xenon at any given pressure.  Must be some other characteristic hindering toroid formation.

We know that sodium vapor lamps are filled with rarefied Ne or Ar to get the arc started and warm things up.    Partial pressure of metal vapor can never exceed equilibrium where it condenses at coldest place on vessel wall.

My small (perhaps 800mm, bent into a U, so 400mm long) low-pressure sodium vapor lamp is in a double glass housing.  Inner U-bent tube surrounded by a larger cylinder glass cover for thermal insulation.  I'm not sure if the space between glass walls is vacuum or filled with some low-thermal-conductivity gas or vented to atmosphere.  It takes a long time (20 minutes) to warm up to full brightness.  Initial glow is just the noble gas color.  There is a tiny bit of the noble gas colors remaining at full brightness, but sodium lines are orders-of-magnitude stronger (by visual judgement through a diffraction grating).  I'd guess that the sodium pressure is significant at full brightness, probably higher than that of the starting gasses.

[klugesmith]

It looks like Dave and I are in the small club of people with big LPS lamps at home.  In my case, parking lot luminaires that were replaced with LED's.  The only voltage taps on these ballasts are for 208 V and 277 V.  I haven't observed the spectrum, but this outdoor picture on a sunny day in 2018 gives an idea of the luminance.

Dave, do your lamps (SOX series ?) have little dimples at intervals along arc tube, as intentional cold spots for sodium to go when lamps are off?

Following up on my guesses about vapor pressure, Internet search found this familiar classic chart.
https://www.powerstream.com/vapor-pressure.htm
Suggests that tube must be pretty darn hot to get sodium pressure higher than 1 micron.

There's an old stubby version, the NA-1 lamp, once used as a laboratory light source.  Normally operated inside a double-wall vacuum Dewar to get hot enough at 5W or 10W power level. http://lampes-et-tubes.info/dlna/dl074d.jpg

We could compare elements' vapor pressures at their normal melting points.   Zn and Cd are familiar as "volatile" in that sense, and noted in high-vacuum literature.   Hg, Pb, and I think Na are in bottom quartile.  In and Ga are in a class of their own at the bottom of vapor pressure charts (at their respective melting points) IIRC.

[edit] found one reference giving tube temperature.
http://www.soxlamps.org/soxdiagnostics_contents.htm
4. Failing to warm up fully to final colour — i.e. more orange or red than normal
If the lamp does not reach its optimum operating temperature of 260°C, the sodium inside may not vaporise and the red colour of the neon in the lamp tube would be the result. This is normally attributable to a leak in the outer jacket: when the outer vacuum fails, the thermal insulation of the arc tube deteriorates, and the lamp can only partially warm up. ... (ref: Colin Grimes, James Hooker, Mike Docherty)
260 °C is 533 K, where Honig chart shows sodium at 5 microns vapor pressure.
I think that what counts is the temperature of coldest spot on tube wall.   But maybe non-uniform partial pressure along the length of an arc tube can persist for a while.  I guess neon lamp tubes and curly CFL's charged with Hg can take minutes to get uniformly bright.  How can diffusion timescale be more than a few seconds, in 1 meter length of hot and rarefied gas?

[Uspring]

I believe, that the gases you used are too thermally conductive, requiring more power than available to sustain a hot torus. Possibly you could see a self sustaining glow discharge at the lower pressures. But these were cooled too well to reach the lower limit temperatures of hot plasma conductivity (4000 - 5000K). Or the gas was too dilute to be able to carry much current.
I think you have a better chance of success with Krypton, which is less thermally conductive than Argon or air. Or you increase amp turns of your coil.

I really liked the slow motion video. Pretty and interesting. It looks like the torus flashes brightly initially. Difficult to explain.

[kurtrox]

Good evening Alan! I too have joined in this quest to make shiny bright xenon donuts, I finally received my 2L 15Torr xenon envelope (sphere). I am currently only using the basic 10Mhz oscillator (SSTC) from teslundmehr on youtube flame candle design. I am having extreme difficulty keeping my variable bench supply calm from some sort of back EMF. or RF noise. I also keep killing the IRFP460 and IRFP260 Mosfets I'm trying to use. I would love some help shedding some light on your circuit design.

https://www.circuit-diagram.org/circuits/24ddc75d2c05422cb0dfbe5f598448c6

I have chosen to use 2x 1KOhm Resistors and only a 4.7nf and also tried a 5.6nf capacitor once that is in parallel with the TVS. I will admit, I was super excited to make the flame candle work, and then the globe, but I would really like to get it to run a bit longer ( i do have the mosfet heatsinked and fans). Any help would be appreciated.

I am super new to the idea of winding my own coils. I have tried to get as close as possible winding my own magnet wire around small pieces of plastic tubing. I know the plastic effects things but unclear as to how much, I see your coil on the glass is directly on the globe, and also the windings are bundled. Does this help performance? Thanks!

I forgot to mention some other information. I have tried to get the toroid to form at 19v as suggested by other people doing similar things, but Ive only been able to get tendrils to form. I can achieve a toroid at 30V and roughtly 3 amps.

[alan sailer]

kurtrox,

I'm sorry that you are having such problems. I used the exact same diagram that you linked to and it worked on turn-on.

I happen to own an LCR meter so measuring the coils was easy and accurate. Plastic is an ideal coil form and should have no effect on the inductor unless you are using some weird plastic. PVC tubing works great.

If the MOSFET is heating too much then the circuit is probably not operating class E and needs to be tuned.

I bundled the output coil because I wanted to keep it somewhat neat. Keeping the coil bundled does affect the inductance somewhat.

Is your coil running at 10MHz? How are you establishing the inductance? You can measure an unknown inductance be resonating it with a known capacitance...

[davekni]

Looks like Kludgesmith is correct about relative pressures.  On the order of 1000Pa for noble fill gasses and 0.4Pa for sodium vapor.  Found another reference:
http://www.lamptech.co.uk/Documents/SO%20Vapour.htm
http://www.lamptech.co.uk/Documents/SO%20Gas.htm

I believe, that the gases you used are too thermally conductive, requiring more power than available to sustain a hot torus.

Good point.  I hadn't thought about thermal conductivity.  Some day, after many other projects on my list are past, I hope to try argon with higher field strength.

[DashApple]

Dave, do your lamps (SOX series ?) have little dimples at intervals along arc tube, as intentional cold spots for sodium to go when lamps are off?

I have a few newer lamps in the 55W to 180W range with the dimples, I think one reason for them is they are there to provide more even distribution of sodium during the warmup phase on the longer tubes and perhaps a longer life,  after enough time the sodium in the dimples will go and never come back .

Some of my older 135W lamps lack the dimples and the sodium sits in the U at the lamp top

[klugesmith]

Thanks for dimple observations.  And thanks to Dave for showing us that website, with a wealth of details about a lamp technology that filled an important niche for more than a half-century.  Check out the page about sodium migration.   It confirmed that sodium partial pressure can differ substantially from place to place along the tube.  I guess the conditions make diffusion very very slow (for a gas).  Site includes this picture of a demonstration lamp whose heat-retaining coating was removed from right half of arc tube. Outer envelope is warm in left half and hot in right half.


I respect the site's choice to report pressure values in pascals. 0.4 Pa is very close to 3 microns.
In olden days, the exact conversion factor would depend on density of Hg at some temperature, and acceleration of gravity at some place. Now it's easier to remember:
Torr is defined to be 1/760 standard atmosphere. Standard atmosphere is defined to be 101325 pascals.
Hmm, I bet the 101325 was adopted to reasonably match 760 mm of mercury in centuries of barometric instrumentation.

Back to nominal subject of this thread: I guess Alan has his work cut out for him. Get back to xenon wrangling and demonstrate a sodium-doped plasma toroid.

[kurtrox]

Thanks for the info, Ill get the other resistors and caps and get the exact numbers dialed in. I have also never really wound my own coils before time for some tuning!

[alan sailer]

So today was the sodium trial. I back-flushed the 100mL flask with argon after pumping it down to ~70mtorr.
Add a small chunk of freshly cut sodium metal (~5mm^3). I useda small alcohol flame to heat the vessel.
Since the vapor pressure of sodium is low at these sort of temperatures I pulled a vacuum on the argon gas.

At some point a yellow glow started up. The pressure was under a torr (since the 10MHz oscillator screws with my gauges
accuracy is not easy). The glow got quit bright at the 19V/3amp. I ran it up to 24V/5amps and saw no toroid.

As the temperature got higher I saw some beaded filaments whipping around inside the globe. The view was clouded with
sodium metal condensed on the inside of the glass (this was verified when I removed the globe for cleaning and the film
vanished instantly when it hit water).

My guess is that even though the sodium vapor ionizes very easily, the low density of these ions does not allow the higher
current toroid to form.

I did film the process but due to the obscuring sodium film seeing anything nice on the video is difficult.

[alan sailer]

Today was the day for potassium. I once again filled the 100mL flask with argon after
pumping it down to ~70mtorr. Put in the metal and pumped down to a few torr. Then lit
 the small alcohol burner and was able to get the argon glowing right away. As the potassium
started to evaporate the color of the flask turned into the lovely lavender color.

After a few minutes of messing about with higher and lower current to the 10MHz oscillator I saw
what looked like a ball circling inside the container. The condensed potassium make the view quite blurry.
I did get a "plasma ring" that is shown in the linked video.

https://www.flickr.com/photos/8763834@N02/52538331999/in/dateposted/

I don't classify it as a ring like the one you see in xenon since the brightening is not there and the rind is
beaded. But it's still interesting. the horizontal bands that appear are not real and may be from the 10MHz
screwing with my camera.

I have to pause and see how much further I want to go with this. I'd have to build a cold trap between
the flask and my nice vacuum system with it's expensive gauges. the potassium is not a problem until it
reacts with air to form potassium oxide, which then hit water vapor and goes corrosive. When I was
cleaning out the flask after this run the oxide hissed alarmingly when it hit the water.

I also killed my driver since at points I was inputting over eight amps. It probably indicates that my circuit
 is not tuned to best class E operating point.

[klugesmith]

Very nice. I would call that a toroid.

[davekni]

Alan:  Very interesting experiments even if not fully "successful"!  I'd guess that sodium or potassium would require the assembly to be in an oven or have a double-wall glass bulb with vacuum between walls and IR-reflective coating as with low pressure sodium vapor lamps.

I also killed my driver since at points I was inputting over eight amps. It probably indicates that my circuit
 is not tuned to best class E operating point.

Could be just excessive FET current or voltage.  Class-E FET current can be much higher than supply current.  FET conducts resonant current besides supply current.  Could also be excess FET gate power (internal gate series resistance) or overall power, since using a FET designed for much lower frequency.

[alan sailer]

I liked the experiment. It was not too hard to do and I got lots of pretty colors.

And yes I was pondering the ways and means of making a big globe without the condensed metal.
It was easy to see that I would need a hot globe inside a vacuum insulated visual globe. While
technically possible it would be a challenge to build with the additional problem of avoiding metal.
The oven is the best idea since it would help shield that noisy 10MHz.

And kludgesmith, although technically a torris like object I'd rather call it a lavender rosary. Hopefully other,
more explicitly religious pareidolia will not appear in the future.

[Uspring]

I don't classify it as a ring like the one you see in xenon since the brightening is not there and the rind is
beaded.

Yes, striations are typical of glow discharges. These occur in cold plasmas. The Xe toroid is one of hot plasma.
Presumably you would see much less power draw from the glow discharge, i.e. the 2500 Vpp you measured at the coil would not drop to 1000 Vpp as you saw with Xe. Also the light spectra would be continuous in the Xe case, as in thermal black body radiation, whereas you'd see line spectra in the glow discharge case.

[alan sailer]

So I have been going through my back catalog of plasma globe and got a few more interesting results.

First off is a globe filled with40 torr of xenon with a trace of iodine. Iodine turns all the noble gasses blue, different hues but all basically blue. This was the first time that I got a stable toroid above the coil.

https://www.flickr.com/photos/8763834@N02/52748239841/in/dateposted/

The second globe is a fill of 30 torr of xenon with traces of oxygen and nitrogen. this is the classic way to get a green plasma. the toroid is stable only with the coil on top.

https://www.flickr.com/photos/8763834@N02/52748650440/in/dateposted/

Finally a repeat of a globe with only xenon at 50 torr. It has the coil at the bottom and has the wonderful "smoke ring effect".

https://www.flickr.com/photos/8763834@N02/52748483559/in/photostream/

I am still trying to figure out why none of the globes I have recently filled with xenon will form a toroid. I have globes filled with the classic 15 torr that everyone is using for their toroid experiments and nothing happens. i also filled a globe with 50 torr and nothing.
i even have an old xenon globe at 110 torr that I can get a toroid.

I just ordered parts to try a Steve ward toroid driver using an FDA18N50. The only major difference I can see with this driver vs the one I am using is 2 turns vs 3 turns on the drive coil.

I'd like to get a stable 15 torr globe working for use as a permanent display. So far as I can tell people are getting the 15 torr toroid to run without a kicker coil to start ionization.

Cheers.

[davekni]

Nice videos as usual.  Enjoy watching your experiments.

So far as I can tell people are getting the 15 torr toroid to run without a kicker coil to start ionization.

Many of the videos I've seen still require at least capacitance to ground of a hand on the globe (or fill tube end) to start an arc.  It appears that starting is usually due to electric field capacitively coupled through the glass wall.  Perhaps a bit of grounded foil against the globe somewhat close to the coil would be sufficient to use coil voltage for starting.  Or perhaps a grounded spiral or other non-closed shape of wire against the globe surface would be best, providing sufficient capacitance while blocking less magnetic field.  Optimum size for added grounded electrode probably depends on glass thickness and location.  Guessing perhaps 10x glass thickness away from coil wire.  Diameter depends on whether trying to maximize field at coil or at added electrode.  Small to start arc at electrode, large to start arc at coil.
This is something I hope to explore more eventually myself, but several projects are ahead of plasma toroid in my planned list.

[Uspring]

I'd like to get a stable 15 torr globe working for use as a permanent display. So far as I can tell people are getting the 15 torr toroid to run without a kicker coil to start ionization.

You might have an impedance matching problem with lower pressure. Ignition will require less voltage, but possibly the toroids resistance will be higher compared to your previous experiments due to the lower pressure. That might require extra turns to get enough power into the toroid to heat it up to plasma temperatures.