Is there a vacuum pumping technology availible that can...

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

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ohiovr
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Is there a vacuum pumping technology availible that can...

Post by ohiovr »

Is there a vacuum pumping technology that can maintain the vacuum conditions inside a polywell container to a satisfactory degree? It would seem to me that pumping out trace amounts of helium from a shell hundreds or even thousands of cubic meters to be a challenge to say the least. I mean for power production situationss where many grams of fuel are being consumed each hour....

D Tibbets
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Post by D Tibbets »

This has been discussed on other threads. A concern has been raised that it could be a show stoper. Beyond diffusion and or turbomolecular pumps, which may not have sufficient pumping speed there are getters(sp?) and electrostatic pumps. See Wikipedia article on vacuum pumps.

Problem with the last two is that they embed the particles into a substrate by electrostatic acelleration or chemical reaction, so 'fill up'. I don't know (ignorance is bliss) why an electrostatic collecter couldn't be riged to concentrate the charged fusion products into a smaller area / stream, effectively concentrating them to a point where the removal efficiency of a turbomolecular pump could finish the job. This would compete against the harvesting electrostatic grids, but I'm speculating that a compromise could be achieved, eg: a 5-10% loss in efficiency as the ions would not be decellerated to a standstill. It would be similar to a magnetic nozelle on a nuclear rocket engine.

Also, if the alphas leave the magrid mostly through the cusps there may be some geometry advantages- placing the vacuum ports in these paths. The electron guns and ion guns unfortionatly would also be in these areas.
A positively charged mag grid combined with the magnetic field outside the magrid might help a bit- helping to increase the concentration of alphas near the vacuum vessel wall (sort of an external confinement if you will). It would become a complex interation between all the magnetic and electrostic fields of the magrid, energy harvesting grids, and possibly electrostatic giuding / concentrating grids.

Removing the neutral atoms would be another challenge.

Dan Tibbets
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TallDave
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Post by TallDave »

I suspect if it can be done for the gigantic ITER vessel, it can also be done for a much smaller Polywell.

I think there's still some question what the actual vacuum requirements are.

KitemanSA
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Post by KitemanSA »

TallDave wrote:I suspect if it can be done for the gigantic ITER vessel, it can also be done for a much smaller Polywell.

I think there's still some question what the actual vacuum requirements are.
Does ITER require the VERY hard vacuum that Polywell seems to need?

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Post by MSimon »

KitemanSA wrote:
TallDave wrote:I suspect if it can be done for the gigantic ITER vessel, it can also be done for a much smaller Polywell.

I think there's still some question what the actual vacuum requirements are.
Does ITER require the VERY hard vacuum that Polywell seems to need?
No. But they have a larger volume to pump.

I was reading a paper from 1974 that posited turbo pumps capable of 1.5E5 liters a second. The pumps would have to be about 7 ft across. I do believe with magnetic bearings etc. we could make them bigger. The first one is going to cost a fortune.
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TallDave
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Post by TallDave »

KitemanSA wrote:
TallDave wrote:I suspect if it can be done for the gigantic ITER vessel, it can also be done for a much smaller Polywell.

I think there's still some question what the actual vacuum requirements are.
Does ITER require the VERY hard vacuum that Polywell seems to need?
Well, that's the thing. I'm not sure we know how hard a vacuum Polywell needs.

I'm assuming the need for a vacuum is driven by mostly by the need to avoid arcing, and Dr. Nebel has said they are still modeling that. Are there other factors? I assume even ITER-level vacuums are enough to avoid poisoning the reaction fuel.

choff
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Post by choff »

Not a problem for moon colonies or travel between planets.
CHoff

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Post by MSimon »

choff wrote:Not a problem for moon colonies or travel between planets.
If you have to recycle fuel because the fuel flow is 10X or 100X the rate fuel is burned ( to maintain a given vacuum pressure) then you will need a "closed" system. Space travel may not eliminate the need for pumps. With He scavenging required you may have another reason for pumps.
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chrismb
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Re: Is there a vacuum pumping technology availible that can.

Post by chrismb »

ohiovr wrote:Is there a vacuum pumping technology that can maintain the vacuum conditions inside a polywell container to a satisfactory degree?
See viewtopic.php?p=18484#18484 et seq.

Summary, no, I don't think so. It would need these directional recovery techniques being discussed in this thread which might work, but as things stand must be presumed to represent a major project risk as there is no evidence that you can collect such fusion products in a highly selective way out of the cusps.

Further along are my presumptions that Polywell needs better than 1E-9 torr or so, so that the MFP is long enough to get a fusion event. Otherwise, it will thermalise.

Tokamaks run 1000 times higher pressure and also then have local cooling to 1/10000th of the plasma temperature at the divertors and thus have very high reduction of the gas volume. So the pump specs on JET/ITER should only be expected to pump a pressure millions (?) times higher than polywell and so millions times less volume. Given my 500MW polywell requirement of 120billion litre/s pumps at 1E-9torr, that'd make ITER's pumping demands in the order of 100,000litres/s if my presumptions are reasonable. I think 100m3/s is easily doo-able? Anyone know what ITER's pumps run at?

ohiovr
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Post by ohiovr »

Thanks for the link chris.

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Post by MSimon »

Chris,

Doc B. says 1E-7 torr. Or maybe it was 1E-6 torr.

The 1E-9 torr figure is the number for pre-operation conditioning. i.e. outgassing, leak testing, etc.

===

http://www.askmar.com/ConferenceNotes/2 ... 0Paper.pdf

p. 3
early work was limited to 3E-9 torr, while WB-5 ran at 3E-6 torr
p. 11
Final tests of WB-6 were made with the fast puff-gas/capdischarge system, starting at < 1E-7 torr tank pressure.
p.11
Typically, for no = 1E13 /cm3 (i.e. ptorr = 3E-4 torr), veo = 1E9 cm/sec (Ee = 100 eV), and sigmaizn = 1E-16 cm2, the cascade e- folds with a time constant of about 1E-6 sec (one usec).
The last one says that operating pressure will be about 1E-6 torr. Which says that pumping is doable. If you want to quote a difficult number then 1E-7 torr is probably a good place to start. And possibly as high as 3E-6 torr.
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Post by MSimon »

Tokamaks run 1000 times higher pressure.

Actually it appears that the Polywell and ITER vacuum pumps work at the same pressure 1E-6 torr. And that due to confinement Polywell has a factor of 300 increase of density in the reaction space. i.e a factor of ~10,000 increase in reaction rate. Dr. N. says that with a 10T magnet he can get reaction rates 60,000 times that of ITER.

He also suggested that by using POPS tuned to He he can add radial energy to the He making the cusps leakier for He.
Engineering is the art of making what you want from what you can get at a profit.

chrismb
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Post by chrismb »

I read what it says, but I don't buy it.

a) what is the MFP for a proton through a p11B mix at 1E-6 torr?
b) how far does a proton have to travel for a fusion with a 11B nucleus in 1E-6torr to become likely?


viewtopic.php?p=18512#18512

and

viewtopic.php?p=13258&#13258

are my answers, and I'm happy for those to be dissected, criticised and corrected/improved. Feel free. But I won't hang on numbers just 'cos someone has made an unsubstantiated claim.

Remember, if, say, WB-5 ran 3E-6 torr and let's even say it made neutrons - so what! The physics required is that the MFP be considerably longer than the distance required to get a fusion event. Sure, you can probabilistically get some fusion events of *some* of the population of ions within a distance of travel shorter than the MFP, just as a fusor does, but if the discussion is seriously about getting net power then MFP[outside reaction core]>>mean [total] distance of ion travel to get a fusion event within the reaction core.

So, pull my numbers apart and we'll then take it from there....

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Post by MSimon »

Chris,

The whole theory of the BFR is that the MFP is not a valid metric because of annealing. In fact if the annealing is quasi-Maxwellian a long MFP may be a bad thing.

So here is the ideal distribution: Short ion MFP near the grids. This promotes annealing (if it is quasi-Maxwellian). We can expect this part of the distribution scheme to be valid because the density of the magnetic field in the region. Long ion MFP from the high density grid area to the center. Short ion MFP in the center due to the confluence of the beams. This promotes fusion collisions.

Now is it like that? I have no data.
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choff
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Post by choff »

Would it do any good to use a multichamber approach, large outer chamber to take the vacuum down to _7, with filter pump inside for inner chamber, magrid in the center.
CHoff

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