Rotating a Plasma in a magnetic field

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todtheexplorer
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Rotating a Plasma in a magnetic field

Post by todtheexplorer »

16 years ago I became fascinated with making sparks in a vacuum (plasma). I conjured up all sorts of experiments, some were downright dangerous some were comical, but one has stuck with me ever since. It was rather simple, I had a 18 in sphere with an electrode around the equator and one at a pole. I then wrapped wire around the sphere parallel to the equator (See fig1). When I applied 4kv to the electrodes and 14kw to the coil the plasma rotated as I had expected.

http://www.youtube.com/watch?v=rgl1VOYAcNA
In the video I’m adjusting the HV along with the electormagnet.

What I didn’t expect was for the plasma to form a ball in the center of the sphere. It looked like a very easy way to confine plasma. A fusion reactor started to come to mind. In all my searching I have not found any experiment rotating plasma in a 3 dimensional sphere. I have seen it done in 2d. Is this worth pursuing? I have a concept that would turn this into something that looks like a flying saucer (don’t laugh I’m not an alien freak) in order to get a stronger magnetic field at the equator and towards the middle of the sphere (See fig 2).
Tod Patterson
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Chris Bradley
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Re: Rotating a Plasma in a magnetic field

Post by Chris Bradley »

Hello Tod.

I had similar ideas of rotating plasmas 25 years ago whilst at school and being introduced to nuclear energy and the problems of fusion. It immediately struck me that if ions are rotated by RF sources at just a few MHz in a device ~10 cm, then they'd have fusible energies.

But I am a bit more of an armchair theorist than experimentalist and it was not until a few years ago that I got around to seeing if I had the experimental skills to turn words into reality.

I did not think, then, that I did, and I'm still not sure whether I do (!) but I came onto the board to describe this work. So far, I have pinned up some of my first results; viewtopic.php?f=18&t=7852#p55811 , and more will follow soon once I collate some data from last year.

On to your demonstration - very nice! Thanks for sharing. I presume you know that you have a type of 'Penning' arrangement. I'll presume the central ball is the cathode. So, my interpretation is that ions accelerating inwards impart rotational inertial to the plasma they are going through due to their rotation in the magnetic field [note; this is a simplistic broad-brush description, there are various ambipolar behaviours of both ions and electrons that might do this]. There is some condition for magnetic and electric fields where the outward electric field force stabilises against the inward magnetic field force. If I understand what you are doing, then this is where, at the boundary of that plasmoid, its rotation, w, satisfies the condition w^2>2Eq/mr (for q and m of the dominant ion species [or most massive ion, if none dominate] and r for the edge radius of the plasmoid).

The essential problem with this is that the stability condition changes as the plasmoid grows, because the electric field at that point r will change according to the plasmoid's size. It is fantastic to see your experiment and that you can actually achieve at least a momentary stability. I would not have guessed this would be particularly controllable under these conditions. How long can you keep it stable for? Whatever you have done, I think you must have been very accurate with your geometry!

To overcome instabilities, my concept is to actively drive a series of 'equatorial' electrodes in a rotating manner, in place of your single electrode, thereby actively determining the rotational frequencies of ions in the plasmoid edge. So the plasmoid size can be controlled by the rotational frequency, rather than these two values being interdependent (and, ultimately, unstable).

In the configurations I am aiming at, I also stick the electrode through the full axis of the device, so that ions don't hang around the centre and the rotation is at larger radii (and, thus, the ions have more energy for a given rotational velocity). If I try without a central electrode, as per your experiment I get a more diffuse oblate plasmoid that is a bit 'skittish'.

I think your demonstration is very well presented.
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Re: Rotating a Plasma in a magnetic field

Post by Andrew Seltzman »

There has been significant research on this here at UW Madison for the study of plasma dynamo effects and magnetorotational instabilities using exactly the same concept. The plasma is trapped in a multicusp field, and VxB force is used to make it rotate. The machine setup is very similar to yours except with many magnet rings and electrodes.

See:
http://plasma.physics.wisc.edu/pcx
http://plasma.physics.wisc.edu/viewpage.php?id=mpdx


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Re: Rotating a Plasma in a magnetic field

Post by jelopez777 »

It looks like a vertical axis magnetic mirror which means the plasma will create a helix that drifts north and south along the Y-axis. The equator anode stabilizes the plasma along the Y-axis, while the north pole cathode will stabilize the plasma along the X and Z-axis preventing precession drift. The challenge is when the plasma is heated to fusion energies which means ions collide in fusion velocities and break containment. This would require stronger electrostatic and magnetic containment forces to prevent sputtering the chamber. Discussion of alternative magnetic and electrostatic configurations are interesting. I am sure there is a solution somewhere.
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Carl Willis
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Re: Rotating a Plasma in a magnetic field

Post by Carl Willis »

Hi Tod,

Interesting video.

At first glance I thought this was essentially an egg-of-Columbus analog, but on reading the description I am not so sure. Can you clarify the "14kW" figure regarding the magnet windings? Is this AC or DC power (and how are you getting rid of all the heat)? Basically, in the configuration described, I can't figure out why the plasma would feel a rotational force about the vertical axis due to a single solenoidal coil around the vertical axis.

Thanks for sharing.

-Carl
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Chris Bradley
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Re: Rotating a Plasma in a magnetic field

Post by Chris Bradley »

There is a magnetic field parallel to the axis, so ions heading towards the axis will experience a crossed-product force perpendicular to their motion. The ions' radial momentum will be translated into a circumferential momentum, by reaction against the magnetic field, and that angular momentum gained by those ions will then pass to plasma with which those ions interact.

In any radial current with a perpendicular magnetic field, there will be a torque reaction.
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Carl Willis
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Re: Rotating a Plasma in a magnetic field

Post by Carl Willis »

Maybe that is the whole story.
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todtheexplorer
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Re: Rotating a Plasma in a magnetic field

Post by todtheexplorer »

Chris Bradley wrote:
How long can you keep it stable for? Whatever you have done, I think you must have been very accurate with your geometry!
>
Chris,
The plasma was very stable as long as the vacuum pressure, magnetic field and the high voltage remained constant. I did have excellent control keeping the plasma centered. What you see in the video is me playing with all of the above just to see their effects. I could only run the high magnetic field for a very short time do to coil heating. I had even constructed a water jacket around the coil but that just allowed me to cool it down faster to run more tests. Even than I could only run for 15 seconds before I had to cool it down.
As for the accuracy of the system the vacuum chamber was made from 2 squirrel baffles with a machined ring in between. I found these to be very good and cheap vacuum chambers.
I did not know off the Penning type of confinement so that gave me yet something else to research. I did see the similarities with the magnetic mirror.
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Re: Rotating a Plasma in a magnetic field

Post by Richard Hull »

How many turns of wire of what gauge and how many layers of wire were in you magnetic coil? I assume you had two coils above and below the equator to be free of the anode ring. A bit more detail would be nice. 14kw is a lot of sauce poured into that magnet. What was your applied voltage and current to the coil.

Richard Hull
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todtheexplorer
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Re: Rotating a Plasma in a magnetic field

Post by todtheexplorer »

I made the coils 18 years ago and don’t have them any more along with most of my notes. The wire I used was 18 gauge solid copper with insulation over it (.055 dia). I got used rolls for free from an alarm installation company. I do not recall how I wound the coils but they were about 15” in diameter, 3” tall and maybe 3/8” thick. The resistance was around 7 or 8 ohms for each coil. I think I ran 2 wires parallel for 3.255 ohms/1000’ = 2300 feet/(15*3.14) = 48 turns/coil in 2 layers. The power supply was a 240v variac and I think I drew 60amps max thru 6 rectifiers.
The HV power supply was from a UV curing oven and the spec for that was 4kv@200ma.
Hope this helps, I would love to get involved with this again now that I have a little more money and time.
Tod Patterson
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Re: Rotating a Plasma in a magnetic field

Post by Richard Hester »

It's instructive to check the Samuel Glasstone book "Controlled Thermonuclear Reactions", as the book descrbes a lot of the experiments done in the earlier stages of fusion research, before Tokomaks ate all the funding. One area of research was rotating plasmas.
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Steven Sesselmann
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Re: Rotating a Plasma in a magnetic field

Post by Steven Sesselmann »

Todd,

Thanks for sharing, .... nice work, with some interesting engineering challenges.

Not to mention a few mental challenges

Unfortunately this kind of plasma confinement has limitations for fusion, but interesting anyway.

Steven
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