Polywell - Magnetic Electron Confinement or Magnetic Grid Shielding?

[Derek]

OK, I've decided to stop lurking for a while.

Precisely why is everyone assuming that magnetic shielding of the grid (to prevent electron collisions) is necessarily associated with magnetic confinement of the electrons to the centre of the machine? Surely these can be seen as separate considerations: the gain from the (potentially) increased concentration of -ve charge may not be worth the effort. If we can prevent electrons being lost to grid collisions whilst they circulate freely, the centre density issue will resolve itself ... no? There seem to have been several good points made about the relatively low efficiency of magnetic confinement systems and the tendency of their proponents to therefore 'go big': why, given that fusors are essentially electrostatic devices, are we looking for magnetic electron containment?

In other words, do we require a 'polywell', 'magrid' or simply a magnetically shielded grid (which might, for all I know, be what a 'Magrid' was supposed to be)?

[Richard Hull]

Magnetic confinement of STUFF, plasma, electrons, etc., has been the absolute watchword for fusion since Lyman Spitzer's first little group toyed with the concept. It is a tough cycle to break with so much rich history of big bucks being offered up for net fusion failures.

Magnetically bottling, mirroring and confining things just seems like a great mindset and you will always be in good company should you decide to join in on the action.

The magnetic bottlers have always reminded me of the perpetual motion folks. ......" If I only had this monster magnet... or....If the magnets I have on my wheel were only a tiny bit stronger, this sucker would spin on its own, forever.....WOW!"

What seemed like a good idea in 1950 is now a mantra.

Richard Hull

[Wilfried Heil]

>Why, given that fusors are essentially electrostatic devices, are we looking for magnetic electron containment?

The magnets confine electrons and let them recirculate, creating a negative potential well. This well then attracts and traps the ions, which are heavy and otherwise would need a much stronger magnetic field to be trapped. The same could be done with electrostatic means alone, like in Farnsworth's multipactor design.

We'll have to see what is technically feasible and also efficient.
The "Polywell" is not a bad idea. It would be interesting to see if it works.

M. Simon wrote:
> Magnetic confinement of electrons is 40X more effective vs. magnetic confinement of protons.

The magnetic field would have to be ~60x stronger to confine deuterium ions (with ~3700 electron masses) instead of electrons. Otherwise, will not be effective at all.

[Derek]

As I understand it Bussard's device is/was an EXL machine with the refinement of magnetic shielding of the anode grid and some degree of magnetic confinement of the 'virtual cathode' electrons effected by the same magnets. I understand the point about the electrostatic ion trap and indeed that a magnetic ion trap would require much stronger magnetic fields.
My point is that since an EXL machine forms a perfectly good virtual cathode from recirculating electrons why bother trying to contain the electrons at all: why not just magnetically shield the anode grid to discourage electron strikes from the recirculating electrons?

BTW the magnetic confinement only lets the electrons recirculate in the sense that it leaks at the cusps: the recirculating mechanism is electrostatic.

[Richard Hull]

I remember when Doc Bussard took me into his office with Tom and, having signed the non-dislosure agreement, he explained to me how his electron virtual cathode worked and how it would both obviate grid losses and sought to stop the bleed of the 50% electron losses encountered with the slamming into the walls in the simple fusor.

All well and good, but the other bleed is loss of non- productive ions through losses other than fusion. (recirculation is not and cannot be perfect in any real world device.)

My visit to work with Tom and Doc that day was very early in my fusor investigation with fusor II and was in 1998 just a few months before I actually did fusion. I was wide-eyed and hopeful as hell. In the years that followed, I became much jaundiced against fusion by magnetic schemes of any sort.

It seems that I had to actually do fusion to see just how crappy it was as a viable power source due to any number of natural barriers thrown up to keep the universe from going high order in seconds.

All we have to do is either totally overide natural urgings of heated matter or obtain total, intimate control over what we term gravitation.

Since those "anything is possible days", I have "groaned weary".

So even if Bussard's system works perfectly, we are looking at a system that is 50% better, energy-wise, than the simple fusor; how is it an improvement beyond reducing power input for what was and is normal ionic electrostatic fusion of the stock, simple Farnsworth device?

For a given ionic energy delivered to the device, you should be doing no more fusions or, certainly, not many more. It is still the same old crap shoot at 1/2 power and lots more expense and infrastructure. Recombination and lost ions will still be there.

As always, myself and others await real quantitative data from an ostensibly working device which runs for an extended period, (minutes), without self destruction, fully involving Bussard's principle as he envisioned it. A tall order, I am sure.

Richard Hull

[Wilfried Heil]

Apparently someone will try to tackle that. A proof of principle and repeatable data is certainly not too much to ask for.

If the physics behind it allows Bussard's Polywell to be an energy source as intended is an entirely different matter.

[MSimon]

Magnetic confinement of electrons is 40X more effective vs. magnetic confinement of protons.

In any case physics simulations are being done plus the real thing is being tested in the lab as we speak.

http://powerandcontrol.blogspot.com/200 ... lasma.html

The requirement is to get electron losses down to the 1E-5 or 1E-6 range for net power.

Richard of course is convinced it is impossible. Men much smarter than I am are of a different opinion. We will have a pretty good idea who is right in about 70 to 120 days.

[Carl Willis]

Derek,

The point of the magnetic fields in these designs is to keep electron current from flowing to the anode, as you mentioned, but also to effect sufficient densification of charge in the intra-anode region by means of the so-called "Wiffle Ball" cusp confinement effect, essentially magnetic pinching-to-near-death of the electron leakage beams that exit the anode at its openings. These are complementary results produced by using the same magnets in the quasi-spherical anode. According to Bussard's papers, both effects--magnetic shielding of the anode AND the cusp-current-driven wiffle ball effect--are necessary to attain the purported revolutionary fusion efficiency of his design. This is my understanding. I hope I interpreted your question as you intended.

-Carl

[MSimon]

Carl,

That is my understanding as well. It is not just shielding but also raising the density in the reaction area vs the dead space. The density in the dead space is limited by arcing considerations so without higher density in the reaction space the net power out would be uneconomical.

[Derek]

Carl,

Yes, thanks, you have indeed interpreted the question as I intended. I have to admit that I must have missed the bit in Bussard's papers which stated that both effects were required. I can see that magnetic confinement would be helpful, I was interested to understand which of the two effects (screening of the anode or electron confinement) was felt to be most important ...

I guess I may have to give up on the old cars (yes Richard, I do rebuild old cars!) and build something to experiment with instead.

Derek

[Steven Sesselmann]

Bootstraps!

No you can't lift yourself by the bootstraps, no matter how hard you pull. The argument that it is easier to confine electrons than ions is flawed, because if you use magnets to confine electrons and then use the electrons to confine ions, then you may as well use the magnets to confine the ions in the first place.

For once I agree with Richard, it sound a lot like perpetual motion

This said, I still believe we are going to crack the Lawson criteria, and burn deuterium, and my feelings at the moment is that magnets is the hard way to do it.

Steven

[Richard Hull]

Derek I used to restore old Lincoln Continentals as a hobby in the late 70's and the early 80's. One of my many hobbies. I once had 10 restored Lincons registered to me, but once gas hit 75 cents per gallon they had to go with the 430's and quad carter guzzling 7-9 mpg. 75 cents per gallon was outrageous then. When I started on old cars, gas was 30 cents a gallon.

BUT, back to fusing ...er...or not fusing.

The electron loses are miniscule compared to the losses associated with the simple failure to fuse in the first place.

The difference between getting 300,000 fusions per second using 500 watts and getting the same number of fusion at only or 250 watts or even only 100 watts means zip in the grand scheme of things.

50% or even an 80% reduction in energy for a given output improvement is nothing when only a 99.999999999% improvement would make one really have a reason to smile. It's those order of magnitude improvements which pile on the trailing nines that turn up the smilometer

Now if you could get a billion fusions per second on 1,000 watts input, then that would be a nice improvement but still far, far distant of from any power device. (still 10,000X distant from simple breakeven, which even then will leave you with no real power to send down the wire).

In the ion-ion fusion biz we need a 10 or more order of magnitude improvement to be even remotely power ready. Reducing electron loses to zero is abysmally easy, given enough extra input energy and a much more complex infrasturcture.

The classic, simple fusor remains one of the best neutron sources and least expensive actual fusion systems in the under 500 watt total input power requirement range. It may forever remain one of the most interesting and cost effective of all the well polished fusion turds squeezed out since 1950.

The fusor still intrigues me and I tend to believe nothing will work to a grid level in the next century, unless the lucky donkey effect kicks in!

Richard Hull

[Derek]

Stephen,

Yes, that was much my point, although I can see that magnetic shielding to prevent electron losses to the anode might be a good thing.

Derek

[Derek]

Richard,

My particular vice is MK2 Jaguars but I only have four ranging from 42 to 46 years old ... gas price in England is currently running at around US$7.56 per US gal but I don't drive them much. My XJ8 uses about the same amount of fuel so I guess it doesn't matter anyway ...

Yes, I take your point about the inconvenient reluctance to fuse. Just have to drive the little perishers harder perhaps?

If one of the papers I read recently (God knows which one, I've read so many!) is correct, the average lifetime of an electron in the centre zone in a 'normal' fusor is less than one circuit: upping this to a few tens would be a start but, as you point out, a very small one.

Derek

[Carl Willis]

Steven,

You have made an interesting logical construction, but one that illustrates some misunderstanding of the physics behind Bussard's fusor.

>The argument that it is easier to confine electrons than ions is flawed, because if you use magnets to confine electrons and then use the electrons to confine ions, then you may as well use the magnets to confine the ions in the first place.<

It's a fact that electrons of some energy E are easier to spatially confine with a magnet than ions of energy E--to make the electrons stay within particular physical bounds, you need much, much weaker fields.

To imagine a fusor built with magnets that "confine the ions in the first place," as per the above, is to imagine a very different physical animal than Bussard's fusor, in important ways not at all analogous. There are magnetic ion-confinement fusion schemes out there, e.g. the Tokamak. Obviously, they're quite different from the fusor and they have some real issues particular to themselves.

In the Bussard fusor, electrons are being confined by electric and magnetic fields in order to (A) avoid the losses they cause when they slam into the anode, and (B) trap a sufficient number of them to form a virtual cathode that will attract a sufficient number of ions with kinetic energy to fuse. The magnetic field's effect on ions is not important to the idea. It's not perpetual motion, it's not absurd, it's not some flavor of "new" physics. It's understandable with the conceptual framework of classical electrodynamics.

Finally, a word about the Lawson criterion. This criterion applies when you're interested in the conditions for thermonuclear ignition--when you have established a dense enough thermalized collection of reactant and product particles to sustain fusion. To the best of my knowledge, this concept has not been considered practically relevant to the IEC devices we're talking about. These do not involve large, dense thermal plasmas that might potentially ignite. IEC people are more concerned about what it takes to do fusion efficiently enough to recover more energy than you put in--a much lower bar for success than the Lawson Criterion, and yet still oh so far beyond us now.

-Carl

[Steven Sesselmann]

Derek,

Just on your previous post, electron losses from the grid in a Hirsch fusor or a polywell is always going to be a problem, they naturally want to go to ground.
By immersing the whole guts of the machine in a vacuum chamber with a conductive gas, just makes it inevitable.
I believe that I have solved the electron loss problem with my S.T.A.R. device, where the cathode is surrounded by dielectric oil. If my inital four experiments are correct, and I believe that they are, S.T.A.R. is producing more neutrons with less input power.

Not everyone is convinced yet, but my next experiment will confirm it one way or another.

If I am wrong, and experiment #5 does not produce any neutrons, Richard will say "I told you so!", and if I am right, and S.T.A.R. produces lots of neutrons, Richard will call me "A lucky donkey.."

Just can't win, can I?


Steven

http://www.beeresearch.com.au

[DaveC]

Some points to consider further:

First - what Carl says about it being easier to magnetically confine an electron versus an ion of equivalent energy, is correct.

Forces generated by magnetic fields are proportional to the cross product of Velocity and Field, and scaled and directed by the magnitude and polarity of Charge, as in

F = Q (V x B). a vector equation.

It is clear from this, that only the velocity, charge and magnetic field determine the force. The effect of the particle's mass enters indirectly, since for equivalent energy U = 1/2 (mV^2 ) the heavier particle has lower velocity. Thus the deuteron will move much more slowly than the electron, being about 7200 times more massive, ( about 85 times slower, in fact).

Now when an electron cloud is contained by a magnetic field, so as to create a potential "well" as in the polywell scheme. things become a bit more complicated, when you begin to add positive ions.

The first thing that happens when positive deuterons are added to the negative well, is that the potential of the well decreases in direct proportion to the amount of positive charge entering. This can be countered by adding more electrons.

So that for some sort of steady state situation, one needs both an electron current and an ion current entering the well, just to maintain a constant well potential.

Since the deuteron has positive charge, it turns in opposite directions to the electron, at a radius of curvature some 85 times larger than that of the electron. Since the radius of curvature of either electron or ion increases as the energy (eV) increases, raising the ion energy and electron energy, requires a matching increase in magetic field to again maintain status quo.

Given the large disparity in mass, however, it NOT at all likely that ions will in fact circle within the magnet structure with a field intensity intended to confine electrons. A magnetic field sufficient to circle the ions, will keep the electron localized so far away that they will have zero effect on the ions. Circling ions have little or no likelihood of fusing.

But the forces between deuterons and electrons are not repulsive byt attractive. So that electrons and deuterons SHOULD seek each other and become nuetralized. Some have opined, that the ions are moving too fast to be neutralized by the electrons, a point that hardly makes sense, since the electrons which are moving about 90 times faster, are in fact contained by the magnetic fields of the polywell.

It does seem likely that the ions should pass right through the electron cloud. What the probability of "collision" will be depends on a pseudo mean free path calculation involving particles that are attracted to each other.

I haven't attempted this yet, but it seems plausible to me that if the electron cloud is dense enough to create a potential well that attracts the ions, it will be dense enough to ensure almost 100% probability of neutralization of the incoming ion.

Whether the neutral atoms will collide with one another and fuse more so than in other configurations, is the big question here.


Dave Cooper

[stob]

Neutralization doesn't seem to be a problem to me.
As far as I know, in a fusor the fuel usually is ionised by avalanche breakdown (EDIT: breakdown probably was the wrong word).
This means that a fast electron hitting an atom is more likely to knock out further electrons, than one hitting an ion is to be captured.
In a plasma dense enough for anything near net power fusion I doubt an atom could stay neutral for any significant time.

[Derek]

Steven,

I agree the electrons will tend to ground themselves when they can. I have looked at your design and follow your experiments with interest.

Derek

[Wilfried Heil]

It appears plausible to me that a "whiffle ball" of fast moving electrons could capture a cloud of hot ions in its center without excessive losses by recombination. The problem with the other loss mechanism, that of the electrons themselves from the magnetic trap, is what Bussard claimed to have solved by design.

This does look like a form of bootstrapping. The magnets shape electron beams which in turn capture ions. The electrons must not collide with the magnets, which apparently precludes the use of simple permanent magnets, e.g. NIB, where the field lines move through the magnet instead of around it. The "whiffle balls" use electromagnets.

Recombination should not be a problem as long as the electrons are fast and the ions relatively slow. In order to recombine, both particles need to have the same speed (or momentum in a center-of-mass system).

Deuterons are 3666x heavier than electrons, a proton has 1835 electron masses.

While it is fairly easy to bend electron beams with small magnets, much stronger fields would be needed to trap deuterons. Bussard’s “Polywell”, if it works, would make the extremely heavy and costly magnet structures of conventional fusion machines unnecessary.

If someone has an idea how permanent magnets could be used, that would make the device much more accessible, on a small experimental scale.

[stob]

I suppose permanent magnets could be used in something polywell like.
The magnets would have to be placed on the cusp axis behind the electron sources and had to be at ground potential or e-gun potential, so they'd be electrostatically shielded against the electrons.
In place of the magnetic grid there would need to be a normal wiregrid at a positive voltage.
However if the magnets were to far away from the grid, it wouldn't be shielded properly and would only allow a small plasma density. On the other hand if they were to close, the potential on the cusp axis would drop to low, allowing more ions to leak out of the well along the cusp axis.
So the question is whether there is a right spot at all.

Buildling that still wouldn't do it though. Like in a real polywell, there would need to be a way to insert a small, controlled amount of fuel on the inside of the wiregrid. If the fuel is ionised outside it, it goes to the chamber wall and produces a lose current. Also too much fuel would release more electrons than the magnetic field could hold.
A possiblity might be a second vacuum chamber for the fuel from where a pipe would lead to the grid.

[DaveC]

Sorry about the slip up on Deuteron to electron mass ratios. Been doing a lot with He ions, lately and absentmindedly used those ratios. So the velocity ratios and radii of curvature for the same energies and magnetic fields are about 60 to one instead of 85 to one.

Dave Cooper

[Richard Hull]

If someone is attempting to do something specific and succeeds at it They are not lucky donkeys. Lucky donkeys are those who, like Becquerel and Roentgen, were looking for something specific, but discovered something far off the beaten path. They both failed at their original quest and ideas, but unlocked totally new physics, (stumbled or blundered into it). That is the lucky donkey scenario.

The guy who breaks fusion might be looking for an ion gas piston design or some such dream.

Finally, the polywell should not trap or build up ions in the center of the polywell at all. The deuterons will ideally whiz through the electron knot (negative well) and recirculate. There should be zero ions in the center in the form of trapped ions. The verbage is important, I think. The ploywell accelerates ions towards the negative well where they either fuse or zip through to recirculate just as in a grided fusor. I got the distinct impression from some of the foregoing that some thought we were trapping ions somehow within the polywell.

Richard Hull

[stob]

Indeed my understanding of the polywell is, that ions are to be trapped inside of the magnetic grid.

Do I understand you right, that the ions would pass through the magnetic grid?

If that indeed is how a polywell is supposed to work, than I totally agree with you, it couldn't possibly be much of an improvement over a normal fusor. Ions passing to the outside of the grid would immediately be lost to the chamber wall.

However since the polywell seems to make so much less sense that way, than in the way I understand it at the moment, I don't think this is how it is supposed to work.
As I see it, the fuel is entered between the (positive charged) MA-grid and the electron cloud / 'wiffle ball', thus at a potential lower than the MA-grid potential.
Idealy the fuel is ionised by the fast electrons when it comes in contact with the 'wiffle ball', so the ions have nearly no kinetic energy at a potential which should be clearly below the MA-grid potential.
This leaves two ways for the ions to leave the well (as long as the electron trapping holds and the electrons don't cool to far down).
One way is upscattering, the other is the potential at the point where the axis crosses the grid being pulled down, either by electrons accumulating there or by something at low potential close to it (like an electrostatically shielded permanent magnet).

Stefan

[DaveC]

Stefan -

The whole point of my discussion above the correction post, was just that. Magnetic fields sufficient to confine electrons to make the well, are far too small to also confine the ions. Field strong enough to confine both, would leave them widely separated, so there would never be the density to fuse.


Dave Cooper