Adding Magnetism to a Fusor

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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VernonNemitz
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Adding Magnetism to a Fusor

Post by VernonNemitz »

Years ago, when I first heard about Bussard wanting to modify the Farnsworth-Hirsch design by using magnetic fields, there wasn't enough data for me to know what he had in mind, so I began to imagine....

This notion therefore may be totally off-the-wall and useless. It may have some entertainment value, though, so...

Have you ever seen an incandescent light-bulb filament under a magnifier? Tungsten is actually a pretty good electrical conductor, so a long long filament is needed to get enough electrical resistance. To fit such a long filament into the bulb, the filament is coiled, and then the coiled filament is coiled again. Some high-output bulbs have a third level of coiling of the filament.

What if the electrostatic grid in a Fusor was coiled, and a DC current fed through it? Then it would have a magnetic field that should mostly deflect collision-course ions. This idea would require that the entire electrical circuit be isolated from other circuits (including parts of it outside the Fusor), so that its wiring, as a whole, can be given an electrostatic charge (and DC induced to flow through its circuit), but I don't see that as a particularly difficult thing to accomplish.

I do think there still might be a few spots where ions could impact the grid. Will enough of them be deflected to make the Fusor practical? I have no idea...have fun!

D Tibbets
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Post by D Tibbets »

One flaw in this concept is that the central cathode in a Farnsworth type fusor is not a conducter to ground, ie current does not pass through the wire from a negative end to the positive end. So there is no current through alot of amp turns to creat a significant magnetic field. The current flow is from the wire at random (sort of) points on its surface through a conducting plasma to the anode which is a more periferally located grid, or the grounded metal walls of the vacuum chamber itself. The current carriers are electrons emitted from the cathode through thermoionic emission and possible other means (?) These free electrons then ionize neutral gas forming pos. ions and more free electrons which greatly increase the conductivity, with the ions hitting the cathode heating it further, etc, etc. Some method of restricting most of the electrons from leaving the wire (perhaps a porous ceramic coating?) with an appropiate resistance in the cathode circuit, combined with alot of turns, you might reach a compromise with adiquite electron emission (also possibly utilizing ion and electron guns) combined with enough wire conducted electrons to get interesting results. Another method would be to seperate the magnetic field generating coils from the cathode wires. Placing the electromagnet in a insulated can with the cathode lieing next to it. There was a link here several months ago where someone was advertising this approach. In some ways this is similar to the Polywell approach in part. The cathode would be shielded from ion collisions. The next problem would be that the electrons would still be flying to the walls unimpeaded. But, if you repeated the process with a magnetically shielded anode grid, the electrons would fly around the shielded anode and perhaps not reach the walls (but upscattered ions reaching past the anode would then be acellerated to the walls (unless it was designed like the Polywell where there are only tiny cusps where the ions can escape-which would in effect be a Polywell with the added complecity of the central physical cathode). You are now getting close to a polywell, except you have a shielded central cathode instead of a virtual cathode. The issues of recirculation, connecting wires, radiation loads, arcing, would all have to be addressed, and I'm guessing they would be more challenging than the Polywell design.

Dan Tibbets
To error is human... and I'm very human.

chrismb
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Post by chrismb »

I see no advantage possible. A fusor runs a kind-of self-ionising system along the beams, so that a large fraction of the fast ions self-align themselves through the gaps. There are papers predicting this with numerical models, and supported by experimental measures.

The idea that a wire would produce a sufficient 'magnetic boundary' around it so that ions couldn't get to the wire is far fetched. The current it'd need to carry would be enough to vaporise it because we're looking here at the 'parallel plate-magnetron condition' such that the magnetic field has to be at a certain level to prevent charged particles in a given electric field from moving along the electric field rather than in an ExB drift. As you get closer to the wire, so the electric field goes up, so the magnetic field you'd need would go up, etc.... mathematically it all falls down into 1/r^2 singularities, so you're on your own trying to calculate that.

Suffice to say, in a vacuum, you're talking red to white-hot tungsten at 10's of mA and when you get that condition in a fusor, the ions still run stright lines past, and into, the grid. So how much hotter will the grid need to get to be able to magnetically deflect ions?? That's not even beginning to consider the likely transparency of that grid, to work out how many ions will even make it through.

Appying a global magnetic field to a fusor is likely to help, however, because it causes the electrons to execute ExB drifts which reduces conduction currents. However, for a spherical fusor you'd need a spherical magnetic field and that doesn't exist. I have proposed a toroidal fusor, with a solenoidal field, for that purpose, but as fusor ions loose all but 1 trillionth of their energy, on average, to thermalisation, what's the point!?

VernonNemitz
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two replies

Post by VernonNemitz »

To D Tibbets, yes, the standard/existing wiring of a Fusor grid is not right for planning DC flow through it. A redesign of the grid would be needed. Keep in mind that I did specify that all the wiring involved (in that redesign) would need to be isolated from other electric circuitry. Even though we need one or more electric CIRCUITS for DC to flow, each circuit as a whole can be thought of as one piece of shaped metal, and that piece of metal can be given a static-electric charge. Therefore the parts of that metal (forming part of the overall grid), inside the Fusor, would exhibit this charge. Outside the fusor, some other part of that metal can be accessible by appropriate moving magnetic fields, such that the fields induce DC to flow in the piece of metal (any/all of the one or more electric circuits).

To both D Tibbets and chrismb, I'd like to point out one advantage of a large volume of vacuum (Fusor's don't have to be table-top sized!). The electrostatic field of the grid can be lessened. The ions are accelerated to some total number of electron-volts of Kinetic Energy by the electrostatic field. Two factors, distance-of-travel and strength-of-field, contribute to that KE. So, a larger volume means more distance-of-travel and thus the strength-of-field can be lessened, and the ions will still have enough KE for fusion to occur. I would want the electric field to be low enough so as not to encourage electrons to escape the grid wire.

To chrismb, please remember that both electrostatic and electromagnetic fields strengthen and lessen with the inverse-square law. I'm not saying that it would be easy for some DC current in a coiled Fusor grid-wire to generate enough magnetism to cause ions to be deflected, but what you wrote seems to imply that an ion encountering the two fields would see them strengthen at different rates, and that's not right, in this sense. As an example of what I'm talking about, consider a "Solar Sail" in outer space. It intercepts light from the Sun, and can use the force of the impact of that light to "hover" in the Sun's gravitational field. It can do this anywhere, both close to the Sun and far from the Sun, because both its gravitational field and its light-intensity are equally affected by the inverse-square law. Back inside a Fusor, I can agree that if the electric and magnetic fields had different total strengths, equivalent to the sail not having enough area to intercept enough light, then no such balance would be possible --in THAT sense, an ion encountering the two fields would see them strengthen at different rates (equivalent to the too-small sail falling at an accelerating rate). Anyway, it seems to me possible that with a large-enough Fusor, and a mild-enough electrostatic field on the grid, then if that grid also was the source of a magnetic field, most of the ions might be deflected from it. Let's see, NASA has a really big vacuum chamber somewhere, right? All it needs is a grid to become a Fusor...heh! [/b]

chrismb
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Re: two replies

Post by chrismb »

VernonNemitz wrote:To D Tibbets, yes, the standard/existing wiring of a Fusor grid is not right for planning DC flow through it. A redesign of the grid would be needed. Keep in mind that I did specify that all the wiring involved (in that redesign) would need to be isolated from other electric circuitry. Even though we need one or more electric CIRCUITS for DC to flow, each circuit as a whole can be thought of as one piece of shaped metal, and that piece of metal can be given a static-electric charge. Therefore the parts of that metal (forming part of the overall grid), inside the Fusor, would exhibit this charge. Outside the fusor, some other part of that metal can be accessible by appropriate moving magnetic fields, such that the fields induce DC to flow in the piece of metal (any/all of the one or more electric circuits).

To both D Tibbets and chrismb, I'd like to point out one advantage of a large volume of vacuum (Fusor's don't have to be table-top sized!). The electrostatic field of the grid can be lessened. The ions are accelerated to some total number of electron-volts of Kinetic Energy by the electrostatic field. Two factors, distance-of-travel and strength-of-field, contribute to that KE. So, a larger volume means more distance-of-travel and thus the strength-of-field can be lessened, and the ions will still have enough KE for fusion to occur. I would want the electric field to be low enough so as not to encourage electrons to escape the grid wire.

To chrismb, please remember that both electrostatic and electromagnetic fields strengthen and lessen with the inverse-square law. I'm not saying that it would be easy for some DC current in a coiled Fusor grid-wire to generate enough magnetism to cause ions to be deflected, but what you wrote seems to imply that an ion encountering the two fields would see them strengthen at different rates, and that's not right, in this sense. As an example of what I'm talking about, consider a "Solar Sail" in outer space. It intercepts light from the Sun, and can use the force of the impact of that light to "hover" in the Sun's gravitational field. It can do this anywhere, both close to the Sun and far from the Sun, because both its gravitational field and its light-intensity are equally affected by the inverse-square law. Back inside a Fusor, I can agree that if the electric and magnetic fields had different total strengths, equivalent to the sail not having enough area to intercept enough light, then no such balance would be possible --in THAT sense, an ion encountering the two fields would see them strengthen at different rates (equivalent to the too-small sail falling at an accelerating rate). Anyway, it seems to me possible that with a large-enough Fusor, and a mild-enough electrostatic field on the grid, then if that grid also was the source of a magnetic field, most of the ions might be deflected from it. Let's see, NASA has a really big vacuum chamber somewhere, right? All it needs is a grid to become a Fusor...heh! [/b]
As far as I understand, a solar sail has been tried - and it just sat there not doing anything. Nice theory, shame about the reality.

I mentioned the inverse square law - so you think the magnetic field goes up to an infinity field 'at' the wire and you've no problem with that? That's what I said - you end up with a singularity and a load of nonsense that you can only really test with experiments - and the experiments show your grid melts and/or ablates due to heat whilst ions are still nailing the grid straight. And that's heating just due to current passing through the discharge volume, and certainly not counting MA of current through this spindly wire of yours.

The speed of the ion is simply the electric potential applied. Yes, you can apply a bigger potential with less risk of sparking if you make it bigger, is that what you're saying? Nothing new there.

And why do you think a 'low enough' field would accelerate ions but not accelerate electrons? There's nothing to hold the electrons to the wire once it's exposed to an electric field - electrons will leave it so as to try to 'counteract' the field, as per Gauss' law. The wire's conductive - that means electrons move within (and without) it.

Rather than talking about it, you could always go build one if you think there's something in your idea. It is not immensely difficult to build a fusor.

Final nail in this idea - if the magnetic fields are so strong close up to the wire that 100keV ions will be deflected, why won't this thin wire mechanically collapse in on itself? That's exactly what happens to a bulb filament on start-up, whilst the resistance is low and current high.

Skipjack
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Post by Skipjack »

As far as I understand, a solar sail has been tried - and it just sat there not doing anything. Nice theory, shame about the reality.
where do you get that from?
From what I know, the last one was lost when the russian LV (a former ICBM) failed.

chrismb
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Post by chrismb »

Skipjack wrote:
As far as I understand, a solar sail has been tried - and it just sat there not doing anything. Nice theory, shame about the reality.
where do you get that from?
From what I know, the last one was lost when the russian LV (a former ICBM) failed.
I thought I'd read it in New Scientist - one that part opened, but still should've moved (but didn't). Don't hold me to that, though. A quick search on internet merely leads to; http://www.newscientist.com/article/dn3895

Skipjack
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Post by Skipjack »

I too read New Scientist, but can not remember reading what you said.
I might have missed it though. The Planetary Society was behind the failed attempt and I thought they wanted to try again. I have been out sick for a few weeks so I might have missed something.

Edit: You might be confusing it with the problems of air drag in LEO.
Once you have reached a higher orbit, that problem should go away.

VernonNemitz
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Joined: Thu May 28, 2009 3:18 pm

silliness

Post by VernonNemitz »

To chrismb, it is pure silliness to talk about infinitely-intense magnetic or electric fields; there are none such anywhere on Earth, including any existing Fusor. Nor are there going to be, any time soon. So stop trying to confuse the topic with "singularities", please.

Next, round wires in a vacuum are generally poor electron emitters; a sharp point is much preferred by electrons for escaping a wire. When heated, sure, they will escape more easily than when the wire is cold.

Which leads directly to the next bit of silliness; your assumption that just because I used an incandescent-bulb filament as a starting point for describing a coiled wire, I somehow intended that that model had to be copied exactly, incandescence included, in the suggestion I was making. I most certainly did not! The coiled grid wire needs to be a good conductor, and we don't want to resistance-heat it any more than can be avoided.

The next bit of silliness on your part involves the ions impacting the wire and thereby heating it; why are they doing this if the magnetic field deflects them? Having the grid wire coiled so that it forms a deflecting the field is the whole purpose of this Topic Thread. If not-enough deflection occurs, fine, then the idea isn't so good, but you seem to be assuming none at all will be deflected.

The next bit of silliness on your part is a failure to read what I wrote. I am not at all talking about applying a bigger potential in a bigger vacuum. I am talking about lessening the electrostatic field on the coiled grid wire. This is workable because, in a larger vacuum, the ions have more room to accelerate under the influence of a weaker field; the net effect is that the same kinetic energy as before can be achieved. So, it logically follows that a big-enough vacuum chamber will allow a WEAK-enough electrostatic field on the grid wire, so that if the grid wire is also coiled and is conducting some DC, it will host a magnetic field that should be able to deflect the ions. The magnetic field can be "in balance" with the electric field. Or even overbalance it.

chrismb
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Re: silliness

Post by chrismb »

VernonNemitz wrote:To chrismb, it is pure silliness to talk about infinitely-intense magnetic or electric fields; there are none such anywhere on Earth, including any existing Fusor. Nor are there going to be, any time soon. So stop trying to confuse the topic with "singularities", please.

Next, round wires in a vacuum are generally poor electron emitters; a sharp point is much preferred by electrons for escaping a wire. When heated, sure, they will escape more easily than when the wire is cold.

Which leads directly to the next bit of silliness; your assumption that just because I used an incandescent-bulb filament as a starting point for describing a coiled wire, I somehow intended that that model had to be copied exactly, incandescence included, in the suggestion I was making. I most certainly did not! The coiled grid wire needs to be a good conductor, and we don't want to resistance-heat it any more than can be avoided.

The next bit of silliness on your part involves the ions impacting the wire and thereby heating it; why are they doing this if the magnetic field deflects them? Having the grid wire coiled so that it forms a deflecting the field is the whole purpose of this Topic Thread. If not-enough deflection occurs, fine, then the idea isn't so good, but you seem to be assuming none at all will be deflected.

The next bit of silliness on your part is a failure to read what I wrote. I am not at all talking about applying a bigger potential in a bigger vacuum. I am talking about lessening the electrostatic field on the coiled grid wire. This is workable because, in a larger vacuum, the ions have more room to accelerate under the influence of a weaker field; the net effect is that the same kinetic energy as before can be achieved. So, it logically follows that a big-enough vacuum chamber will allow a WEAK-enough electrostatic field on the grid wire, so that if the grid wire is also coiled and is conducting some DC, it will host a magnetic field that should be able to deflect the ions. The magnetic field can be "in balance" with the electric field. Or even overbalance it.
Why do you use the word 'silliness'? I don't understand. I may make points you disagree with (they may even be wrong) and you can make your points accordingly.

The bottom line is that 'within' the central grid of a fusor there is no field and most fusion goes on just around the outside of that grid, where the neutral density is highest and ion veolocity is fastest. Ions heading straight for a wire may well be deflected. But others that weren't originally heading towards the wire will also be deflected - into it. (Ions in the same direction cannot be defected in different directions by the same field.) This is what I was referring to regarding the balance of electric and magnetic fields. To avoid this 'magnetron' condition, you'd have to have an enormous field, thus an enormous current, thus a whitehot if not melting filament belching out electrons.

If you don't like someone questioning an idea you're not going to try yourself, then there's no need to post it! If you get around to building one, then good for you! But it sounds like it's an off-the-cuff idea that you'll hope someone else builds and makes work, while you bask in some glory. Doesn't work like that, especially as the idea of magnetic-field generating grids has been raised before.

Fusors won't ever improve their power-in to out efficiency because their ions' energies all (but a trillionth) degrade by coulomb collisions and thermalise, it's nothing at all to do with the grid - this is the same error of understanding Bussard made when he invented the polywell. In Polywell's case, if this 'annealing' process can work then maybe it's got a chance. I don't see that in fusors, so I don't know why it is expected to happen in a Polywell but, hey, anyone who builds an experiment is in a position to make any claims they like about it and if they come up with the goods then I'll be one of the first to congratulate them, just as I'm one of the first to support anyone who wants to do such an experiment, so just get on with it!
VernonNemitz wrote:So, it logically follows that a big-enough vacuum chamber will allow a WEAK-enough electrostatic field on the grid wire, so that if the grid wire is also coiled and is conducting some DC, it will host a magnetic field that should be able to deflect the ions. The magnetic field can be "in balance" with the electric field. Or even overbalance it.
You seem to be stating these as facts. Show me the calculations, and maybe you'll gain my support.

D Tibbets
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Post by D Tibbets »

Concerning the argument that a larger chamber will allow more acceleration with the same electrostatic potential on a central source. I've tried (not very hard) to get a handle on this. It seems that something starting from a further point will end up at a faster speed once it reaches the bottom of the well. But using gravity as an analogy that has the same inverse square law- The terminal velocity for the Earth is ~ 22,000 miles per hr. This will not be exceeded ( much?) even if you start out on the other side of the galexy. If you start out 1 foot above the surface, then obvously the final speed will be much less. So the range is presumably important, but only when the starting points are realitively close. I've never seen a qualification for the velocity of a charged particle in an electric field based on its range, so I'm guessing the significant ranges are tiny. I'm sure there is a formula somewhere describing this, any enlighteners?


Dan Tibbets
To error is human... and I'm very human.

MSimon
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Post by MSimon »

D Tibbets wrote:Concerning the argument that a larger chamber will allow more acceleration with the same electrostatic potential on a central source. I've tried (not very hard) to get a handle on this. It seems that something starting from a further point will end up at a faster speed once it reaches the bottom of the well. But using gravity as an analogy that has the same inverse square law- The terminal velocity for the Earth is ~ 22,000 miles per hr. This will not be exceeded ( much?) even if you start out on the other side of the galexy. If you start out 1 foot above the surface, then obvously the final speed will be much less. So the range is presumably important, but only when the starting points are realitively close. I've never seen a qualification for the velocity of a charged particle in an electric field based on its range, so I'm guessing the significant ranges are tiny. I'm sure there is a formula somewhere describing this, any enlighteners?


Dan Tibbets
Non-relativistically the velocity of a charged particle after passing through a field depends only on its initial velocity, its mass, its charge, and the voltage between the two points traversed. It doesn't matter if you traverse a 100 volt field in 10 meters or 1 cm. The particle only gains 100 volts times the particle charge of energy.
Engineering is the art of making what you want from what you can get at a profit.

VernonNemitz
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silliness

Post by VernonNemitz »

To chrismb: When it appears you are mistaking almost everything I write about, and, if I am not to assume that you are a complete ignoramus on these topics, then the only conclusion I can reach is that you are doing it deliberately --being silly, that is.

Your most recent message, though, contains one thing that is far from silly, the notion that ions not moving toward the grid might be deflected toward the grid. That could indeed be a huge flaw in the proposal at the basis of this Thread (and said at the start that it could be flawed).

Next, I quote:
Fusors won't ever improve their power-in to out efficiency because their ions' energies all (but a trillionth) degrade by coulomb collisions and thermalise
--could you repeat that in Plain English, please? I've already seen in other messages how you have used some words differently than others think they mean, so I don't want to read something into it that is not there. Because it is my understanding that, barring collisions with the walls or grid of a Fusor (or a Polywell), kinetic-energy/motion of ions will be preserved for the most part, since momentum must be conserved.

Finally, regarding a proof about relative field strengths, this should be so obviously logical that math would be redundant. When a vacuum can be made larger and larger, and an electrostatic field can be made weaker and weaker, with ions in the vacuum still able to acquire a particular kinetic energy, then at some point a weak steady magnetic field will be as strong as a sufficiently-diminished electric field.

chrismb
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Re: silliness

Post by chrismb »

VernonNemitz wrote: Next, I quote:
Fusors won't ever improve their power-in to out efficiency because their ions' energies all (but a trillionth) degrade by coulomb collisions and thermalise
--could you repeat that in Plain English, please?
A fusor is an example of a 'generally cold, locally hot' device - that is, a tiny fraction of ions are going at fusible velocities, the rest are as good as having no temp at all, fusionwise. So if the fast ions scatter off the large-majority cold moleculres/ion, so they loose their energy to them. It all becomes waste heat and useless, as far as fusion goes. This is the process of 'thermalisation'. It is a perfectly well-known concept and I use the term correctly. Unless the fast ions 'use' their energy for fusion rather than bouncing off colder stuff, then there's virtually no chance of fusion - and that's exactly what happens, virtually no chance of fusion!!

VernonNemitz wrote: Finally, regarding a proof about relative field strengths, this should be so obviously logical that math would be redundant. When a vacuum can be made larger and larger, and an electrostatic field can be made weaker and weaker, with ions in the vacuum still able to acquire a particular kinetic energy, then at some point a weak steady magnetic field will be as strong as a sufficiently-diminished electric field.
I regard a proof as a demonstration of 'viability' as well. I think you wil find that if you did the calcs (which you appear reluctant to do, in whcih case I can't be bothered either) you'll find that you need to make a reaction vessel the size of the planet earth, and your grid needs to generate 1000T. I don't know if that's true or not, it's for you to chew over because you're gonna end up with 1/r^2 singularities to contend with and I don't know what you're gonna do about that.

I suspect that, maybe, I typically just jump ahead two or three steps at the start to try to avoid going through those two or three steps with, but end up having to go through them AND somehow activating some alienation at the start as I try to avoid the a-round-about conversation that normally ensues.

VernonNemitz
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thermalization

Post by VernonNemitz »

OK, then, that is why a "high vacuum" is required in any sort of Fusor (including Polywell). You might note that if some remaining gas atom gets energized by a collision, it is thus encouraged to reach the walls of the container, where the vacuum pumps are. I would tend to think that the presence of the moving ions in the system would eventually work to help fully evacuate the device of all non-ions, after which your problem disappears.

About the magnetism thing, I never said it would be practical! I just said certain aspects of it would be possible.

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