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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MSimon
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Post by MSimon »

Chris,

I believe Dr. N. said on another thread that the average density of ITER and a BFR would be about the same but the reaction rate would be 62,500 times greater in a 10 T field BFR due to the increased density in the reaction volume.

If he is correct about such fields shielding from alphas (as opposed to the 1.5T fields Dr. B was envisioning) then a 20T field should work even better and give powers of 1.6 GW more or less.

If in fact magnetic shielding of a pBj BFR works there is no practical limit to reactor size. In addition intercept area then becomes a relatively free variable in terms of design. That would be handy. In addition design of the water cooling section of the coil also gets a lot easier. A single 5 to 7 cm thick water jacket would do. (and why so thick? - to thermalize and absorb with B10 - neutrons) In fact it may be possible to do away with the water jacket altogether if all the LN2 has to absorb was a ten KW or less. And for MgB low neutron fluxes actually improves superconductivity up to total dose of 1E18 n/sq. cm. And that is with natural Boron MgB. At a flux of 1E6 n/sq. cm. sec. it would take 30,000 years to gather a total flux of 1E18/sq cm. so shielding the superconductors from neutrons or even making them with B11 might not be worth the effort. Especially given that the neutron flux actually increases the critical field the superconductor can support.
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KitemanSA
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Post by KitemanSA »

MSimon wrote: Especially given that the neutron flux actually increases the critical field the superconductor can support.
Reference?

MSimon
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Post by MSimon »

KitemanSA wrote:
MSimon wrote: Especially given that the neutron flux actually increases the critical field the superconductor can support.
Reference?
http://fizika.hfd.hr/fizika_a/av01/a10p087.htm
http://www.iop.org/EJ/article/0953-2048 ... 03cc980c89

Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,α)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.
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 »

MSimon wrote: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.
Ah! no..no... I have not clarified my meanings.

This is important; *IF* I buy this argument of annealing [I'll run with it, but don't read too much into that] then this is an interaction with other particles high up in their potential field. That process would be fine and can be discounted from the 'MFP' that I am referring to. OK, yeah, 'MFP' is usually when a particle has any sort of interaction. This isn't so here, there are two populations a) the fuel ions, b) the background neutrals. Particles will thermalise with whateveritis that they bump into along their route, but as there are 2 types there are, effectively, 2 MFPs.

So, if the probability of fusing to Coulomb scattering in the core of the reactor is, say, a million then the MFP[fuel ions] is going to be a millionth of the distance required to travel to get a fusion event. In the notion of how the Polywell works, these Coluomb collision *are* , in a way, thermalising but the population of fuel ions is mono-energetic (for a given position within the potetial field). This is the process of annealing you are referring to at the edge, but it would also apply to an 'annealing' process in the reaction core also.

BUT the MFP with the background atoms is another set of thermalising collisions to worry about. Thermalising with fuel ions - no issue. Themalising with background - this is a big deal and will lead to an electric-field-screening thermalised plasma.

So my calcs compare two things; a) how far would a fuel ion need to travel to get a fusion IF THERE WERE NO BACKGROUND NEUTRALS AT ALL, and b) how far would a fuel ion need to travel BEFORE IT THERMALISES WITH THE BACKGROUND. If b<a then clearly the system is non-viable as a net energy producer (though it does not exclude 'sub-net' [inefficient] output reaction rates, like a fusor). And I calculate that to be when background pressure is >1E-9torr.

D Tibbets
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Post by D Tibbets »

chrismb wrote:...
So my calcs compare two things; a) how far would a fuel ion need to travel to get a fusion IF THERE WERE NO BACKGROUND NEUTRALS AT ALL, and b) how far would a fuel ion need to travel BEFORE IT THERMALISES WITH THE BACKGROUND. If b<a then clearly the system is non-viable as a net energy producer (though it does not exclude 'sub-net' [inefficient] output reaction rates, like a fusor). And I calculate that to be when background pressure is >1E-9torr.
Perhaps I'm reading this wrong, but it seems you are saying the density of neutrals can contribute only 1E-9 torr. If so the ions could then make up the other 99% to reach a total density of 1E-7 torr within the Magrid. In one of his papers Busard talked about the neutral density problem, though I believe he was talking mostly about the arcing problem. In the 30 cm diameter WB6 the neutrals (from the gas puffer) transit time was shorter than the near complete ionization time, so a greater than desired percentage was escaping without being ionized ( 10% unionized? - pure unfounded guess). He sugested ion guns to relieve this problem, and also stated that this problem would go away in larger machines because of the greater transit times for the neutrals, leading to a higher percentage being ionized by the electrical fields/ electron collisions (99.9-99.9999999% ?). I know this isn't what you are talking about but it illistrates that the neutral population within the magrid should be very low compared to the ion population- the neutral must be ionized on it's first pass or it bounces off something or exits the system (where it might then be ionized by escaping electrons and lead to arcing). The contained ions though would be transiting many thousands of times befor fusing or escaping. So, in this example 99.9% or more of the neutrals are ionized on the first pass and the ions then make 10,000 passes befor fusing or escaping. If the ion partial pressure was 1E-7 torr, the neutral partial pressure would be 1E-11 torr. I don't know how well my numbers would hold up but I think that it demonstrates that the neutral population must be very small if the system works and the effects of the neutrals on the MFP of the ions are trivial and can be ignorred.

Dan Tibbets
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chrismb
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Post by chrismb »

D Tibbets wrote:Perhaps I'm reading this wrong, but it seems you are saying the density of neutrals can contribute only 1E-9 torr. If so the ions could then make up the other 99% to reach a total density of 1E-7 torr within the Magrid.... If the ion partial pressure was 1E-7 torr, the neutral partial pressure would be 1E-11 torr.
Sure. You can set the 'fuel ion pressure' equivalent to anything you like, so long as they all collide/scatter at the centre and are otherwise under the control of, i.e. contained by, an electric field. And so, as you imply, the 'overall equivalent' pressure is something else, but that is a misinterpretation of the nature of 'pressure'.

The issue is the 'partial pressure', as you say, of the background neutrals, because this is what the vacuum pumps have to pump. Emitted alphas become neutralised (slowed down by venetian blinds, or just sputter the chamber wall, whatever you like to imagine), thus then become 'background' neutrals that need to be pumped. 500MW worth of these generated 'background neutrals' need to be pumped, and they need to be kept to this lower standard of pressure. You say 1E-11 torr - blimey, that'd make the pumping requirement 12 trillion litres/s!!

I do not understand what you are saying about the background becoming ionised. Polywell and IEC devices cannot afford for their fast fuel ions to waste their energy ionising backgrounds, all their energy must be devoted to getting a fusion event. If they ionise too many, you end up with a thermal plasma which screens the electric fields.

MSimon
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Post by MSimon »

BUT the MFP with the background atoms is another set of thermalising collisions to worry about.
Yep. However, that is dependent on the ionization rate in the core. 99% is probably enough to keep this from becoming a problem. If it is 99.9% or better it is down in the noise.

Out in the DC converter section a MFP longer than the converter (preferably much longer) will prevent arc break down.
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MSimon
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Post by MSimon »

Chris,

The background neutrals have a partial pressure of 1E-11 torr in the core.

But out in the DC collector area they have a density of 1E-6. So here is how you fix the problem - do not put your pump inlets inside the reaction volume. Fortunately that also avoids the problem of having to magnetically shield the pumps. I can see big savings there.
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chrismb
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Post by chrismb »

MSimon wrote:The background neutrals have a partial pressure of 1E-11 torr in the core. But out in the DC collector area they have a density of 1E-6.
Why should this be so? Why won't the neutrals' pressure not cause them to head straight into the magrids? If you've got a 1E-6 volume and you stick, say, a 1E-7 vacuum pump one side and you open up a 1E-11 volume the other, which way is the contents of that 1E-6 region going to go!!!??? You'd still need a 1E-11 or better vacuum pump to compete with that pressure imbalance!

MSimon
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Post by MSimon »

Ions do not waste their energy ionizing background atoms. They waste their energy ionizing fuel. Which needs to be ionized anyway. Ergo - no waste.
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MSimon
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Post by MSimon »

chrismb wrote:
MSimon wrote:The background neutrals have a partial pressure of 1E-11 torr in the core. But out in the DC collector area they have a density of 1E-6.
Why should this be so? Why won't the neutrals' pressure not cause them to head straight into the magrids? If you've got a 1E-6 volume and you stick, say, a 1E-7 vacuum pump one side and you open up a 1E-11 volume the other, which way is the contents of that 1E-6 region going to go!!!??? You'd still need a 1E-11 or better vacuum pump to compete with that pressure imbalance!
I think what this says is that you have to collect the ash on the first pass. Fuel atoms will not be a serious problem.
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chrismb
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Post by chrismb »

MSimon wrote: I think what this says is that you have to collect the ash on the first pass. Fuel atoms will not be a serious problem.
If you can catch the fusion products beaming out of the cusps, then maybe you'd be in with a chance. But you'd better be sure you can catch that selectively and first time - just 1% missed fusion ash will contaminate the reaction zone near instantly, at 500MW.

[say 1E21 alphas emitted per second, so 1% loss = 1E19/s. If the volume within the magrid is of the order of cubic metres then 1% loss of ash will contaminate the magrid volume to 1E-6 torr within a millisecond. A 0.001% loss of ash, unrecovered by the vaccum pumps, will cause a contamination to 1E-6 torr within 1 second. What percentage of the fusion ash do you think the vacuum pumps will recover, on the first pass?? Better than 99.999%?]

KitemanSA
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Post by KitemanSA »

I fail to see how there can be neutrals at all in the well. Given the large flow of high velocity electrons at the edge of the well, any neutrals flowing in should be ionized rapidly. So we may have ionized ash and ionized fuel, but how we have significant neutrals excapes me.

So in fact the way the neutral pressure differential between the well and the outside is maintained seems to be effectively an ion pump. POPS anyone?

Am I mixing metaphores?

chrismb
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Post by chrismb »

KitemanSA wrote:I fail to see how there can be neutrals at all in the well. Given the large flow of high velocity electrons at the edge of the well, any neutrals flowing in should be ionized rapidly.
Exactly the problem?! The mechanism cannot afford to ionise helium ash that wanders back into the well, but it will do this exactly if the pumps miss ANY of the ash. Once a neutralised helium molecule gets re-ionised by a particle in the well, it is destined to get stuck in the well. It cannot be upscattered to gain enough energy to leave the well - as the design is to keep all the recirculating ions monoenergetic. It cannot fuse, 4He just doesn't have any two body fusion reactions. So it's stuck, and as the 4He builds up so it contaminates.

I know there is a hope for POPS or some resonant scheme, but again we have to look at percentages here and what the equilibrium populations are. These discussions always seem to centre on 'all or nothing' but this is never a physical reality. Whatever system you pick to try to get 4He selectively out, it will leave something behind and an equilibrium population will build up until you get thermalisation and the electric fields can no longer confine.

icarus
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Post by icarus »

Solution: After however many days, hours operation it takes to contaminate reaction, shut everything off, suck it all down, restart. This thing can come up and down anytime by my reckoning ....

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