Useful service life for Polywell, esp. for rockets?

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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JohnP
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Useful service life for Polywell, esp. for rockets?

Postby JohnP » Mon Jan 19, 2009 4:33 pm

It appears that a practical Polywell will need periodic maintenance, to replace the coil casings and other parts after sputtering and neutrons have taken their toll. If we're talking stationary or earthbound applications, it's not a big deal, but what do you do on a spacecraft? Will this be a limiting factor for spaceflight?

clonan
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Postby clonan » Mon Jan 19, 2009 5:44 pm

My question is, who said they had to be "replaced"?

From everything I have read, the coils and walls will be coated with something to protect against sputtering etc. All that need to be replaced is this covering.

So, turn off the thing, seal the exhaust (for a rocket) and pump in some reserve slurry of whatever material you use. The whole thing is based on carefull control of electro magnets so you could set it up to essentially electroplate onto all the surfaces to whatever thickness you like.

Then you pump out the residual and reestablish the vacuum.


The big repairs will be if a coil goes...but that should be very rare assuming the sputtering is controlled.

Just my two cents

Jeff Peachman
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Postby Jeff Peachman » Wed Jan 21, 2009 12:47 am

clonan wrote:The whole thing is based on carefull control of electro magnets so you could set it up to essentially electroplate onto all the surfaces to whatever thickness you like.


That's a creative idea. Now, I could be wrong because I'm an aerospace engineer, not electrical or chemical, but you would have to fill the entire reaction chamber with a solution that has dissolved ions in it. Then you would need to put a potential on the protective cases of the coils, as if they were electrodes... I don't see any way how the electromagnets help you though, this would be an independent system.

And the weight of all the solution needed to fill the reactor sounds heavy. It would also make the reactor harder to design because now you have to consider if any components will fail if they're exposed to that solution.

Personally I'd rather just have some easily removable panels that an astronaut can replace every few years. But you still have an interesting idea.

-Jeff
- Jeff Peachman

KitemanSA
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Postby KitemanSA » Wed Jan 21, 2009 1:33 am

Greetings all,

I am not all that sure that "sputtering" will be the issue folks keep talking about.

The collisional cross section goes WAY down for high energy alphas. Thus the likelyhood that it will hit near the surface is small. How small? Well; after a bit of searching (no, I didn't keep the reference) I found that multi-MeV alphas and deuterons are shot at diamond to bring about a color change. The change takes place to depths of ~.2 millimeter. Given a bond length of ~154 picometer for diamond, this equates to a layer about 1.3 million atoms thick. With a 3MeV alpha, this is slightly over 2eV per atom. This does not look to provide enough energy.

No, I am NOT an expert on this, but I haven't seen anything yet by anyone that is an expert on alpha impingement on diamond coatings. Anyone know one?

clonan
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Postby clonan » Wed Jan 21, 2009 3:57 am

Jeff Peachman wrote:
That's a creative idea. Now, I could be wrong because I'm an aerospace engineer, not electrical or chemical, but you would have to fill the entire reaction chamber with a solution that has dissolved ions in it. Then you would need to put a potential on the protective cases of the coils, as if they were electrodes... I don't see any way how the electromagnets help you though, this would be an independent system.

And the weight of all the solution needed to fill the reactor sounds heavy. It would also make the reactor harder to design because now you have to consider if any components will fail if they're exposed to that solution.

Personally I'd rather just have some easily removable panels that an astronaut can replace every few years. But you still have an interesting idea.

-Jeff



For short trips, say a 5-10 years you are right. But what about truly long trips of decades plus?

To electroplate something you need enough material to conduct a current. It doesn't have to be fully pressurized since it will be in Zero-G. So while the material will probably be heavy you won't have to have 1000's of liters of it. Plus I bet you can make it as you need it from stock metal and a liquid that is used elsewhere on the ship.

As for a current, with pB11 the walls are already set up to hold a charge. For a thermal blanket it wouldn't be hard to wire the system from the opposite side. Same with the magnets.

clonan
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Postby clonan » Wed Jan 21, 2009 4:00 am

KitemanSA wrote:
No, I am NOT an expert on this, but I haven't seen anything yet by anyone that is an expert on alpha impingement on diamond coatings. Anyone know one?


But for rockets you would want the highest energy for the weight or p-p fusion. Neutron sputtering would become a problem since you can't slow them down first like you can with alphas.

tombo
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Postby tombo » Wed Jan 21, 2009 4:04 am

I would look at PVD (physical vapor deposition) or Vacuum Deposition.
No liquids needed.
Lots of vacuum available.
-Tom Boydston-
"If we knew what we were doing, it wouldn’t be called research, would it?" ~Albert Einstein

KitemanSA
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Postby KitemanSA » Wed Jan 21, 2009 4:06 pm

clonan wrote: But for rockets you would want the highest energy for the weight or p-p fusion. Neutron sputtering would become a problem since you can't slow them down first like you can with alphas.

I did not think neutrons lead to sputtering. They tend to deposit their energy WAY further in than alpha, and they tend to get absorbed, not collide and knock atoms out of their matrix, no?

D Tibbets
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Postby D Tibbets » Wed Jan 21, 2009 11:22 pm

As tombo, said, no liquid needed. vaper deposition, or sputtering in reverse would add layers- like coating telescope mirrors. I don't know how many layers/ thickness would need to be added to restore the surface.

And, does the surface need to be restored? So long as the shell is structurally sound, does a rough , pitted surface have any significance? Would it add to more contamination of the reaction space? Especially in space applications, vacuum is no problem. And, to protect the pressure vessel that containes your superconducting wires and liquid helium/ nitrogen you could have multiple spaced layers, limited only by the need to stay within the magnetic shield (and I supose the need to minimize the magrid surface area exposure) .

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

MSimon
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Postby MSimon » Thu Jan 22, 2009 1:42 am

pitted surface have any significance


Yes. You get field concentrations.
Engineering is the art of making what you want from what you can get at a profit.

D Tibbets
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Postby D Tibbets » Thu Jan 22, 2009 3:08 am

If local pits/ variations are introduced, so long as they are small ( how small are they?) and randomly distributed they should average out and not have much effect on the interior electrostatic field- no net electrostatic field contribution from the external 'spherical shell' magrid. And those areas that would contribute to the recirculation/ ossilation of the electrons are on the lateral surfaces and outer surfaces of the magrid and thus protected somewhat. Since the magrid is an anode, I'm guessing (without the least amount of knowledge) that the rough surface (peaks) would be less of a problem as an emitter, than if it was a cathode, with the emmision of much lighter electrons. I guess it is a matter of scale.

And if the pits, etc. are of a magitude to cause disruption, would thin coatings do more than mild smoothing? Sounds like polishing, rather than coating would be needed.


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

MSimon
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Postby MSimon » Thu Jan 22, 2009 3:18 am

It is a matter of radius.

I believe the coil housings for WB-6 were polished.
Engineering is the art of making what you want from what you can get at a profit.


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