magrid configuration brainstorming

Discuss how polywell fusion works; share theoretical questions and answers.

Moderators: tonybarry, MSimon

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Post by 93143 »

I'm all for brainstorming - two of my three undergraduate design group projects were abject failures (well, one was; the other didn't get built, thank goodness) because we rejected all the ideas except the 'right' one during the brainstorming phase.

For the project that never got built, the best way of doing it became obvious to me almost immediately after we had finished the project with a very wrong-headed approach that would have been useless in real life. The thing is, I think I had actually thought of the idea early on and rejected it...

As hanleyp has noted, the tokamak shell should provide neutralization in the centre, so line confinement should be possible for the ions. The question is, is that good enough? Are there enough head-on collisions, and can you force good resonant modes, and will there be stability problems with the charged plasma in a MHD-unstable field? Will the system Maxwellianize if the electron lifetime is too long, or if it takes too long for a given ion to fuse?

Most of these we can't even guess at without either a concerted numerical effort or (preferably) results from WB-7...

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

I think the MIT stuff with multiple grids has promise.

Solo
Posts: 261
Joined: Fri Jun 29, 2007 12:12 pm
Location: Wisconsin

Post by Solo »

I don't understand the electrostatic lens idea well enough, but it looks as though the catch in their work is to make sure that the scattered ions coming out of the core make it back into acceptable channels.

Since the polywell is confining electrons with magnetic fields, and the ions don't have to worry about grids, I think there's a pretty fundamental difference between the two approaches, so I don't think a hybrid is possible, unlike POPS and polywell, which seem complimentary.

As far as tokomak, is there some reason that electrons confined that way might be easier to handle than by a polywell? I know that a lot of the tokomak problems revolve around leakage issues, but might the electron confinement be better than the overall plasma confinement, so that it makes sense to use a tokomak config? I guess there's the fact that electron confinement is always easier due to the high e/m ratio.

When you are talking about tokomak confinement, are people envisioning having a toroidal vacuum chamber, or one that surrounds the entire thing, like a polywell? The ion losses would probably be lower if there wasn't a column in the center of the toroid. Wait, are we talking about putting the magnetic coils outside the vacuum chamber? I'm confused. I'd been thinking about just taking the polywell ring magnets and setting them up to make a donut instead of a sphere. Would that approach have any value?

EDIT: speaking of ramping up the voltage to increase fusion cross-section, what kind of dependence does fusion have on ion energies? What's stopping us from just setting the well potential to 500kV and slamming the heck out of the ions? Forget oscillating, fuse 'em all in one pass! I'm forgetting something, aren't I?! Brehms losses? Electron losses?

drmike
Posts: 825
Joined: Sat Jul 14, 2007 11:54 pm
Contact:

Post by drmike »

Check out http://www.bmbf.de/en/2270.php for pictures of what a tokamak vessel looks like and the plasma inside it.

You can go to higher energies, but then it costs more money in hardware. The idea is to get the biggest bang for the buck. So lower energy is better. It's just economics, not physics.

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

Solo,

If +ions get out of the core they are lost.

The positive charge on the anode (Magrid) keeps electrons circulating.

I was thinking of the grid structure as a way to confine electrons by channeling them. OTOH it may be how we set up the deceleration grids for direct conversion.

Or both.

Or it may be just another useless idea on the way to finding a good one.

I'm starting to get from my studies that there is no such thing as a stable plasma. What we need is more research on the various plasma instabilities and then an effort to put them to use instead of fighting them.

Edit: You are forgetting acceleration losses. At 200 KV grid voltage (around the peak of the p-B11 fusion curve) it takes 1.2 MeV to get the particles up to speed. The fusion reaction only produces 8.4 MeV. That is very close to the edge of feasible for a power producer.

I like the p-B11 resonance peak at 50 KV acceleration. At that level it costs you about 400 KeV to get fusion. However with a cross section of about 1/12th the peak, you may need better electron and ion confinement and possibly a reaction space with 12X the volume required at higher energies for a given power out. Which translates to a factor of 2.3X increase in linear dimensions.

The POPS experimenters have reported density increases of 6.7 if that could be increased to 12 or even 100 it would increase efficiency and reduce reactor size. The POPS guys think that getting up to 100X is doable. That would mean a reaction volume of 1 m radius operating at the resonance peak could produce the same output as a 2 m radius operating at the p-B11 peak.

hanelyp
Posts: 2255
Joined: Fri Oct 26, 2007 8:50 pm

My latest configuration brainstorm

Post by hanelyp »

A hybrid polywell simple mirror and tandem Penning trap.
Image
Electrons emitted just inside negatively charge reflector grids (light blue) at each end rush through opening in positive charged coils (red) and into core towards virtual electrode at the center. Scattered electrons are contained by magnetic mirror enhanced by wiffleball field inflation. Electrons which escape trough ends are reflected back by grids. I suspect the primary electron loss mechanism would be migration across the magnetic field lines.

Best feature, it looks something like the sci-fi power core you expect to find in the middle of a huge empty chamber deep in the heart of your starbase. :wink:

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

Very pretty. I wonder if it would work.

Solo
Posts: 261
Joined: Fri Jun 29, 2007 12:12 pm
Location: Wisconsin

Post by Solo »

MSimon said: "I was thinking of the grid structure as a way to confine electrons by channeling them."

In other words, maybe we could use electrostatic lensing to protect the surface of the magrid next to the line cusp where the B-field sheilding is poor?

@Hanleyp: I like the look of that. But I don't know that it's technically a Penning trap; although there is some similarity. Could those coils be placed outside the vacuum chamber? That would be very handy.

It reminds me of this document I read about linear mirror machines with some kind of plasma injectors at either end. Let me see if I can look it up. Ah-hah: http://www.aps.org/meetings/unit/dpp/vp ... fusion.pdf

Nanos
Posts: 341
Joined: Thu Jul 05, 2007 8:57 pm
Location: Treasure Island

Post by Nanos »

I had pondered the rugby ball shape myself and wondering if using permanent magnets would be easier with such a shape, thoughts ?

hanelyp
Posts: 2255
Joined: Fri Oct 26, 2007 8:50 pm

Post by hanelyp »

Simon, I'm not trying to say it strictly is a Penning trap, but that it uses features from them, specifically to recapture the electrons that escape through the mirrors.

Thanks for the link, it does sound like it connects with my latest design. I wasn't previously aware of plasma drift in mirror machines, but the electron injection sounds similar to their solution.

I see no problem with building a vacuum chamber inside the coils except spoiling direct conversion.

I had the thought of electric potential shielding the linear cusp of a polywell myself last night. I expect it would greatly reduce electron losses. Need to keep the shielded area small or it thrashes ion containment. Having proper electron reflector grid surrounding the magrid could aid electron recirculation also.

Roger
Posts: 788
Joined: Fri Jul 06, 2007 2:03 am
Location: Metro NY

Post by Roger »

MSimon wrote:
I'm starting to get from my studies that there is no such thing as a stable plasma. .
I seem to recall in the 1970's that one of the ideas for Tokamaks was to have the toridial motion be not once, but a twice around the donut trip. IIRC the reasoning was to take advantage of an instablility in the once around scheme. the resulting twice around was felt to be more stable even with the twice around wobble.
MSimon wrote:
I like the p-B11 resonance peak at 50 KV acceleration.
I gotta get around to changing my sig. May I borrow this ?
I like the p-B11 resonance peak at 50 KV acceleration. In2 years we'll know.

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

Roger wrote:
MSimon wrote:
I'm starting to get from my studies that there is no such thing as a stable plasma. .
I seem to recall in the 1970's that one of the ideas for Tokamaks was to have the toridial motion be not once, but a twice around the donut trip. IIRC the reasoning was to take advantage of an instablility in the once around scheme. the resulting twice around was felt to be more stable even with the twice around wobble.
MSimon wrote:
I like the p-B11 resonance peak at 50 KV acceleration.
I gotta get around to changing my sig. May I borrow this ?
Sure.

hanelyp
Posts: 2255
Joined: Fri Oct 26, 2007 8:50 pm

electron reflector screen

Post by hanelyp »

Another idea to chew on, a negatively charged reflector screen surrounding the positively charged magrid. Electrons leaking out from the central well would be reflected back by the electric field rather than having to follow a magnetic field line around. Should open up volume for support structure around the magrid that would otherwise intercept recirculating electrons. I'm thinking that with such improved recirculation a leaky setup such as 2 magnet toroids like poles together might work.

Solo
Posts: 261
Joined: Fri Jun 29, 2007 12:12 pm
Location: Wisconsin

Post by Solo »

Gotta remember that those things are going to intercept alpha's like crazy if they aren't in the shadow of the coils, though. It'll be a trade-off, I guess.

jlumartinez
Posts: 143
Joined: Thu Jun 28, 2007 7:29 pm
Location: Spain

Post by jlumartinez »

Hanelyp, I think you are right. I figured exactly this last configuration, that you propos,e but my knowledge of electrical & magnetic fields and magrid are not any deep so I just skip to continue with it because it seem so simple that I told myself it is impossible that Bussard had not been implemented it yet if viable. But maybe is a fairly good design. ..who knows ...

Maybe this guys have a different configuration: http://www.fpgeneration.com/ I talked to them by mail and they said me that their reactor has opposite polarity to Polywell . They use POPS . I would like to have a look to their patent

Post Reply