Remind me - why 10T field?

[TallDave]

chris,

Try n*kBolt*Te = B**2/(2*mu0).

Assume B = 10 Tesla, mu0 =4.0e-7*pi, Te = 1.0e4 eV and kBolt = 1.6022e-19 Joules per eV.

Those are the edge conditions I linked above earlier (ITER comparison).

[MSimon]

Don't forget that Beta = 1 assumes the minimum field of the donut hole. The field goes higher away from the hole and is really strong around the coils.

[TallDave]

That's the "good magnetic curvature" part. Anywhere the electrons push, they encounter increasing resistance, thus avoiding various stability issues.

If Bussard really did discover a configuration with good curvature in which losses are

1) minimized at beta = 1
2) small enough for a reactor to work

then you can see why it would be such a big deal. They call economics the dismal science, but fusion hasn't been a whole lot better over the last 50 years. The only really useful advance was only good for killing millions of people at a time.

[TallDave]

Dan,

Interesting thoughts on D-He3. I would guess they're hoping to avoid doing D-He3. because the D-D side reactions mean the neutronicity would end up being high anyway. It might reduce the first wall problem a bit though, since you get a +1 14.7MeV proton and a +2 He at 3.6MeV out of the mix, which can swirl out the cusps and perhaps be direct-converted.

(Uh oh, I feel another calculation coming on... what would the first wall limit look like under those conditions, and what temp would be ideal to mitigate it, assuming we want to get to 1MW/cm^2, which I think was our material limit?)

You're right about the brem in toks of course, it may be insurmountable even in PWs. So who knows, they might end up stuck with D-He3.

[MSimon]

1 MW/m^2 is leading edge. Above that you start bleeding. For ITER diverter they are looking at 20 MW/m^2 and good luck to them.

[D Tibbets]

...
Interesting thoughts on D-He3. I would guess they're hoping to avoid doing D-He3. because the D-D side reactions mean the neutronicity would end up being high anyway. It might reduce the first wall problem a bit though, since you get a +1 14.7MeV proton and a +2 He at 3.6MeV out of the mix, which can swirl out the cusps and perhaps be direct-converted.
...

By using excess He3 , the neutronicity can be reduced to ~ 1 % instead of the ~ 50% of D-D. This would reduce the shielding needs and the thermal loads- at least on the magrids, and reduce the neutron damage to the superconductors, etc.

Diluting the deuterium present with excess He3 brings up another point. With P-B11, there needs to be an excess of protons because of bremsstrulung considerations. With D-He3, there needs to be an excess of He3 for neutron considerations. This means the He3 with a Z of two dominates the ion population, while the B11 with a Z of 5 is a minor portion of the ion population. This means the net energy of the ions in a given potential well will be calculated differently. The net energy vs potential well energy of D-He3, may come closer to the P-B11 example than I thought. A down side is that the fusion rates of both, at any given net energy, would be lower as there is not an optimal mixture (1:1?) of the elements (another factor to consider for the final compromise conditions).

Dan Tibbets

[chrismb]

chris,

Try n*kBolt*Te = B**2/(2*mu0).

Assume B = 10 Tesla, mu0 =4.0e-7*pi, Te = 1.0e4 eV and kBolt = 1.6022e-19 Joules per eV.

Those are the edge conditions I linked above earlier (ITER comparison).

Yeah.. I know what the bloomin' numbers are! My point is that from this you get a result that says n=3E22 at the edge. Now, I though there was supposed to be a x3000 'compression' betwee the outer edge and the inner reaction zone, so that'd make n= >STP density at the centre.

That's my point!!! That's what I've already said - if it is already beta=1 AND 10T AT THE EDGE then the central core is gonna be >STP density!

Does this not even raise the slightest concern with you that it is slightly far fetched!?

[TallDave]

Well, it's optimistic; Rick more or less says as much. It's certainly a very dense core, but any machine that does what PW is supposed to do would have to have such. I don't know why it would necessarily be far-fetched, though. It's just a consequence of the well. It's not like the density is running up against the Pauli exclusion principle or something really crazy like that.

Anyways, as Rick points out, 3e22 gives us 62,500 times the ITER output, so if the WB works we don't really need ion focus.

I'm having a little trouble following your overall point. You started off talking about MeV plasmas, I don't know why you think WB-8 would produce MW at beta=1 (we've calculated ~64W), and there's a lot of talk of ion pressure which shouldn't apply to beta (ions are confined by electrons).

[chrismb]

I'm having a little trouble following your overall point. You started off talking about MeV plasmas

If you don't follow it, then there is prob no point in me asking/saying anything [no offence intended]. I would've thought a realistic density for the outer edge might be n=10^19, as most stuff happens around there because higher and you get loadsa conduction (Paschen, &c.), but lower and it's all a bit too thin for much to happen. That's why fusors, tokamaks and the like run at that density. If you've got that density, around a micron pressure, and 10T, then its a 10MeV plasma. Pressure is electrons and ions. It doesn't matter if it is held electrostatically or magnetically, it is still a 'pressure' that is trying to make the thing push outwards. Unless the mix is seriously non-neutral, then that pressure will be equalised [equal and opposite] for any electric field forces on both ions and electrons in a space.

Or to think of it in another way, imagine if a given number ions and electrons are sharing a small volume in the outer edge. If the ions are feeling an electrostatic attraction INTO the centre by virtue of the electric field, then what are the electrons experiencing?

[D Tibbets]

I think (in my humble openion) that ChrisMB is confused about the numbers. The 3000 is a reasonable number for the Wiffleball traping number or ratio. This is a comparison of the density inside (Wiffleball edge region in the current discussion) and the density outside the magrid.
If there is any confluence with effective density increase towards the center, this may optomistically approach several orders of magnitude (~100X) according to what R. Nebel said in the quote TallDave linked to a few posts earlier. If the number quoted for the density increase over a Tokamak (at ~ 10^20) and a Polywell (~10^22) is ~ 250 as mentioned by TallDave, and a Wiffleball traping factor of ~ 1000-3000 applies, then the background density outside the magrid of the Polywell would be ~ an order of magnitude lower than in a Tokamak. That might have some bearing on the vacuum pumping, but I beleive it would be offset by the much smaller volume that has to be pumped.

Dan Tibbets

[chrismb]

I think (in my humble openion) that ChrisMB is confused about the numbers.

I didn't think I was, but I am after your post! I didn't understand any of that! So, what are you saying is the density at the edge in WB?? An order of mag lower than tokamak?

[TallDave]

I'm sorry, I guess that confused you. The reason I'm having trouble following is that you're being incoherent.

Unless the mix is seriously non-neutral, then that pressure will be equalised [equal and opposite] for any electric field forces on both ions and electrons in a space.

One part in a million should be enough that you only have to confine electrons. If you confine one species magnetically, the other must follow.

[chrismb]

If you don't follow it, then there is prob no point in me asking/saying anything [no offence intended].

I'm sorry, I guess that confused you. The reason I'm having trouble following is that you're being incoherent.

How so? I said that a 10T beta=1 plasma at n=10^19 was a 10MeV plasma and that would be unsustainable due to brems, and you said you had trouble following that. It is a statement of fact, and not even a point of debate, so if you can't follow that then there is no point progressing this.

Could you please requote [any] two statements of mine that are incoherent with respect to each other, so I understand why you say this?

[TallDave]

Yes... but why in God's name would you think we plan to have 10 MeV electron guns? That's what I don't follow. I mean, really, when has anyone suggested that? Holding the density constant makes no sense at all.

If you're going to make crazy assumptions, you're going to get crazy answers (like MW for WB-8 ). You sort of wandered from one crazy assumption to another, without any overall point I can discern. That's what I meant by "incoherent."

Have you read Valencia? It's a good place to start understanding PW theory.

[TallDave]

If the number quoted for the density increase over a Tokamak (at ~ 10^20) and a Polywell (~10^22) is ~ 250 as mentioned by TallDave, and a Wiffleball traping factor of ~ 1000-3000 applies, then the background density outside the magrid of the Polywell would be ~ an order of magnitude lower than in a Tokamak. That might have some bearing on the vacuum pumping, but I beleive it would be offset by the much smaller volume that has to be pumped.

This is something I'm not 100% clear on. Earlier on I'd assumed what you seem to -- exterior density is the same over the whole volume outside of the WB. But the more I think about it, the more I think the electrons stay by the Magrid and any ions follow the electrons, so pumping is just for fusion products and stray neutrals. I'm not sure this makes sense though. Bussard doesn't seem to really suggest one way or the other (but maybe he was thinking electrons only on the outside) -- though now that I think about it some more, it does sound a bit more like an ETW machine looked at that way, and Bussard did once say PW was most like an ETW...

I think I need to study ETWs some more.