Polywell Parameters

[pstudier]

Can anyone give us parameters, such as dimensions of the coils, currents, magnetic fields, plasma density, electron and energy confinement times, plasma distributions, etc?

[jlumartinez]

The biggest source of details is Bussard´s IAC 2006 paper and the Google video. All theses technical data is only helpful is you are trying to reproduce the experiments. Are you investigating the WB6 results for that reason? It would be great!! What is your background?

[pstudier]

I studied plasma physics about 30 years ago. My degree is in Physics, but I spent my career doing engineering in an unrelated field. I was hoping to calculate things like energy and electron confinement time, whether this thing will have velocity space instabilities, etc. Polywell appears to be MHD stable, which is good because if it were not, the plasma would disrupt in about a microsecond. Fusion is easy, but breakeven is horrendous.

[jlumartinez]

There is a couple of more forums where you can find some technical answers about polywell: fusor.net and nasaspaceflight.com. Talk-polywell has just been born since a week. Soon there will be more members to answer your questions

[cuddihy]

I studied plasma physics about 30 years ago. My degree is in Physics, but I spent my career doing engineering in an unrelated field. I was hoping to calculate things like energy and electron confinement time, whether this thing will have velocity space instabilities, etc. Polywell appears to be MHD stable, which is good because if it were not, the plasma would disrupt in about a microsecond. Fusion is easy, but breakeven is horrendous.

Pstudier, I suggest you study the DTIC documents available by Bussard and Krall.

http://www.dtic.mil/dodsrch/

search for "Bussard polywell fusion"

also the links from Joe Strout's homepage under research articles:

http://www.strout.net/info/science/polywell/index.html

Hey, also, if you're interested, there's a Google Groups forum for engineers and physicists. Not much discussio there right now, but some files are available in the file section.

http://groups.google.com/group/polywell

[pstudier]

Pstudier, I suggest you study the DTIC documents available by Bussard and Krall.

http://www.dtic.mil/dodsrch/

search for "Bussard polywell fusion"

All those papers appear to be just theory, and the latest is 1992. I have seen very few actual numbers based on his experiments. If Polywell is to work, the key is the distribution of the electrons over space. I believe that most of the electrons will be outside the box made by the magnets. If this is true, then the ions are free to hit the grids.

[jlumartinez]

For electron confinement I suggest you to see the next youtube videos: http://youtube.com/watch?v=ao0Erhsnor4 ( www.mare.ee/indrek/ephi/ )
http://youtube.com/watch?v=jmp1cg3-WDY

As far as I have understood the "wiffeball effect" is based in that the more electron trapped the smaller space they have to escape because the magnetic field becomes distorted and seem to "close" the outlet paths.

The haven´t any public paper from 1992 till 2006 because they had a contract with the US Navy with publishing embargo. It´s a pity not having a paper in a well-known issue ...

[pstudier]

For electron confinement I suggest you to see the next youtube videos: http://youtube.com/watch?v=ao0Erhsnor4 ( www.mare.ee/indrek/ephi/ )
http://youtube.com/watch?v=jmp1cg3-WDY

As far as I have understood the "wiffeball effect" is based in that the more electron trapped the smaller space they have to escape because the magnetic field becomes distorted and seem to "close" the outlet paths.

The haven´t any public paper from 1992 till 2006 because they had a contract with the US Navy with publishing embargo. It´s a pity not having a paper in a well-known issue ...

The video at the first link is totally unrealistic. It shows electrons leaving the cube in the center of the coils going parallel to the field, and then coming back even after they leave the area of maximum magnetic field. Such an electron will go straight off into infinity, just like in a mirror machine.

The second link is very good at showing the magnetic field. It simulated a single electron which escaped after traveling only 11 meters. The coils were 0.6 meters in diameter. This is terrible confinement, especially because it neither modeled collisions or the repulsion of the electric field which would confine the ions. Both would make it easier for the electrons to escape. It would be nice to repeat this for a few randomly placed electrons.

The third website shows recirculating electrons. They spend more time outside the cube than inside. Similarly the ions will also be found outside the cube and will be free to collide with the coils. Later it shows the cusps closing. There is still the loss cone as in mirror machines through which the electrons will escape. In addition, if an electron collides and lands on a magnetic field, it will stay on it until another collision. The second collision will be as likely to knock the electron farther into the field as into the void in the center.

So my skepticism is two fold. First, the electron confinement will be so short that the energy required to replace them will exceed the fusion energy by many orders of magnitude. Second, the electrons will be at least as likely to be outside the polygon as inside, so the ions will hit the coils.

Isn't the embargo over? What prevents publishing some more technical detail.

[JoeStrout]

The video at the first link is totally unrealistic. It shows electrons leaving the cube in the center of the coils going parallel to the field, and then coming back even after they leave the area of maximum magnetic field. Such an electron will go straight off into infinity, just like in a mirror machine.

I don't know how accurate that video is — IIRC, it was created by an amateur, not by a physicist — but your comment about electrons going right off to infinity is incorrect. Electron recirculation is one of the key points that makes polywell fusion possible, and it has been both worked out in theory and demonstrated in experiment. Let me see if I can dig up some references...

OK, how about this one: p. 11 in the 2006 IAC paper:

Thus, in order for a Polywell to be driven in the mode described for the basic concept, open, recirculating MaGrid (MG) machines are essential. This, in turn, requires that the entire machine be mounted within an external container surrounding the entire machine, and that the machine be operated at a high positive potential/voltage (to attract electrons) relative to the surrounding walls.

So in short, yes, electrons exit the wiffle ball magnetic field all the time — but the coils themselves are held at a very high potential, which draws the electrons right back in (along the magnetic field lines).

This also makes the WB trapping factor simply a measure of electron density ratios (inside to outside) rather than a measure of "losses" to containing walls and structures. And, because of this, it is not necessary [to] achive Gwb values greater than, at moste, 1E4 — rather than the 1E6 required for non-recirculating machines.

This was one of the key breakthroughs made under the Navy contract.

The third website shows recirculating electrons. They spend more time outside the cube than inside. Similarly the ions will also be found outside the cube and will be free to collide with the coils.

No, ions don't have enough energy to reach the coils. The fusion products certainly do, and some of those fusion products will hit the coils; this is a loss mechanism to be minimized, but (at least AIUI) is not expected to be a major problem. The fuel ions, if they fail to fuse on one pass through the center, continue on "up" the potential well, turn around, and fall back down.

As for the electrons, it may be true that an individual one spends more time outside the wall than in — but the electron density (which is what matters) is much higher on the inside. (The outside, of course, is a lot bigger than the inside, and density is electrons/volume.)

Later it shows the cusps closing. There is still the loss cone as in mirror machines through which the electrons will escape.

Not escape; recirculate. The only electron losses are when they collide with something, which is why it's critical that all surfaces in the machine (including the coils themselves) be magnetically shielded.

So my skepticism is two fold. First, the electron confinement will be so short that the energy required to replace them will exceed the fusion energy by many orders of magnitude.

No — this was one of the first things they looked at. Even in a non-recirculating machine, sufficient confinement can be obtained in theory, as is described in mathematical detail in the 1991-1995 papers. But obtaining that Gwb is very hard. A key breakthrough was realizing that with recirculation, you can get much longer electron lifetimes with much easier engineering.

Second, the electrons will be at least as likely to be outside the polygon as inside, so the ions will hit the coils.

No, the electron density (and resulting electrostatic potential) has been calculated in detail, and verified by experiment. It's very deep in the center of the machine. Ions can't escape this well.

Isn't the embargo over? What prevents publishing some more technical detail.

My understanding is that Dr. Bussard's health isn't the best at the moment. And when I was in grad school, our lab would take a year to write up a decent paper on experiments conducted over the previous year (pipelined with ongoing experiments of course). We were young and in good health. Dr. Bussard is over 80, in possibly poor health, and has eleven years of results to write up. Let's give the guy a chance.

Best,

[Indrek]

The video at the first link is totally unrealistic. It shows electrons leaving the cube in the center of the coils going parallel to the field, and then coming back even after they leave the area of maximum magnetic field. Such an electron will go straight off into infinity, just like in a mirror machine.

That's what confused me too at the start. The key is that the coils are also positively charged (so inside is the magnet, the outside shell of a coil is charged using high voltage. So they work like accelerators and attract electrons back in and the magnetic field keeps the electrons from hitting the coils straight on.

Indrek

[jlumartinez]

For further technical details about WB6 operation follow the next links with some Bussard´s comments and final results: http://forums.randi.org/showthread.php?t=58665#27
http://www.emc2fusion.org/

From Wikipedia-Polywell page you can access all these links.

Bussard said that they have a 26 page document about WB6, but they are trying to find investors for their patents so they haven´t the intention of making public all the data. This is the reason why I think it is not so simple to replicate the machine unless is done again by experienced EMC2 people.

I think also that a peer-reviewed paper is needed to convince skeptics about the Polywell advantages and have more technical weight inside the scientific community. Although they have few data (only 4 runs with WB6) it is enough to write a paper and see the agreement between the simulation and results. In Google video, Bussard told that he was summarizing his results in the last 10 years into a 120 page technical document to be published in some scientific issue. Let´s wait ...

[pstudier]

For further technical details about WB6 operation follow the next links with some Bussard´s comments and final results: http://forums.randi.org/showthread.php?t=58665#27

This link seems to contradict itself. It states that electron life is about 0.1 microsec, but that the electrons recirculate about 100,000 times. Even at 10kev, an electron travels at 6e5 m/sec, which would only be 60 cm.

[pstudier]

Thus, in order for a Polywell to be driven in the mode described for the basic concept, open, recirculating MaGrid (MG) machines are essential. This, in turn, requires that the entire machine be mounted within an external container surrounding the entire machine, and that the machine be operated at a high positive potential/voltage (to attract electrons) relative to the surrounding walls.

So in short, yes, electrons exit the wiffle ball magnetic field all the time — but the coils themselves are held at a very high potential, which draws the electrons right back in (along the magnetic field lines).

Plasma shield electric fields in one Debye length, so this does not help. If somehow this does not hold in a polywell, then the field would repel the ions.

Second, the electrons will be at least as likely to be outside the polygon as inside, so the ions will hit the coils.

No, the electron density (and resulting electrostatic potential) has been calculated in detail, and verified by experiment. It's very deep in the center of the machine. Ions can't escape this well.

I guess we can argue theory forever, so why not show the experimental data. What is the electron density over space, and how was it measured?

[JoeStrout]

I guess we can argue theory forever, so why not show the experimental data. What is the electron density over space, and how was it measured?

Go get a copy of Krall et al. (1995). It will of course explain it in much better detail than I could summarize here.

MSimon over on the IEC Yahoo group was also quite impressed with a Japanese paper that experimentally measured the potential well in a different (and perhaps more thorough) manner. I haven't looked that one up myself yet, and I'm sorry to say that I can't find the reference at the moment. But according to Simon, the upshot of it was that it proved the formation and shape of the potential well, at least to his satisfaction.

[cuddihy]

Plasma shield electric fields in one Debye length, so this does not help. If somehow this does not hold in a polywell, then the field would repel the ions.

NEUTRAL plasma shields electric fields in one Debye length. A potential well by definition, would be a negatively charged plasma. not neutral.

During operation, electrons would have to continually be injected at a higher rate than ions. Several places Tom Ligon mentions this as when a polywell has too many neutrals in it, the reaction "chokes itself off". This is what would happen if you didn't maintain the potential well as you throw in ions.