Talk-Polywell.org

Not sure if this was already discussed here, but it would be interesting to know what the upper limits of a polywell/BFR would be. If the seventh power scaling law turns out to be true, there shouldn't be a whole lot of difference between the smallest useful BFR and the largest. If power output is so great that no conceivable cooling system could keep the magnets going, or keep the rest of the thing from melting, that would establish an upper bound.

Just for laughs, I did a back-of-envelope calculation to find the radius needed for a BFR that would match the output of the Sun, and came up with something like 500 meters. Of course, on top of the cooling constraints and all, you'd have to feed the thing a rather enormous amount of D2 or pB-11.

I think materials stress will limit the upper bound. At some point the magnetic field from the coils pushes them apart hard enough that you can't find a material strong enough to hold them. Another bound would be the energy loading, you will have to absorb all the power the device procuces and materials can only take so much.

But build it in space with gravity to worry about and I bet it can be pretty darn large.

I did some back of the envelope calculations on this and it seems like Dr. Bussards choice of 100 MWth power for the test reactor was right at the edge thermally re: reactor size vs power production. The sweet spot.

That is to say it was barely within the bounds of non-exotic cooling strategies for the alpha particle heating loads on the coils.

It also turns out that this is a good size for shipboard reactors. Since he was soliciting Navy money, I'm sure he had that in mind.

it seems like Dr. Bussards choice of 100 MWth power for the test reactor was right at the edge thermally re: reactor size vs power production. The sweet spot.

Does that mean you think 100MW is "it"?

Don't those spacecraft/aircraft designs call for GW range? Were the cooling designs meant to be "an exercise left for the reader" ?

JohnP wrote:

it seems like Dr. Bussards choice of 100 MWth power for the test reactor was right at the edge thermally re: reactor size vs power production. The sweet spot.

Does that mean you think 100MW is "it"?

Don't those spacecraft/aircraft designs call for GW range? Were the cooling designs meant to be "an exercise left for the reader" ?

No. I think it is the ideal maximum size for an experimental unit. Then we go to work improving it.

In an experimental unit to prove the concept you don't want to be out past the leading edge on too many design aspects.

I remember reading M Simon's post on this. He pointed out that beyond a certain point, the radiative power inverse square law governs how powerful the machine can be, because of the limits on heat load.

Bussard gave a rough estimate of power scaling at r^7, the seventh power of the radius of the quasisphere (B^4 * r^3). Assuming that were to hold true, at some point you reach the maximum tolerable heat load on the components facing into our little homemade star and you have to make it bigger without making it more powerful than the corresponding dropoff in heat load from the increased distance from the fusing core.

So at that point, the bang for the buck from size drops from an astounding r^7 to a sad little r^2 -- while costs probably continue to rise as roughly r^3. So that's probably also the point at which the economics work best, though we may find out otherwise with more data.

Dave,

Thanks for reprising that. I can't remember if I did it here or over at NASA Spaceflight.

MSimon wrote:

JohnP wrote:

it seems like Dr. Bussards choice of 100 MWth power for the test reactor was right at the edge thermally re: reactor size vs power production. The sweet spot.

Does that mean you think 100MW is "it"?

Don't those spacecraft/aircraft designs call for GW range? Were the cooling designs meant to be "an exercise left for the reader" ?

No. I think it is the ideal maximum size for an experimental unit. Then we go to work improving it.

In an experimental unit to prove the concept you don't want to be out past the leading edge on too many design aspects.

So what are all the "low hanging fruit" of improvements? I can think of two off the bat
-adding more faces to make it more circular. I believe the do-decahedron polywell theoretically improves your confinement for the same power x 2.5.
-higher power superconducting coils with , well, okay, exotic but physically well-understood cooling methods.

Don't forget POPS! IIRC Nebel's paper (supported by real live experimental data) indicated ion density gains of 10^4 might be possible.

Tom,

The certain low hanging fruit is to go from a heat load of 1 MW/ sq meter to 3MW/sq meter.

That gets you up to 300 MWth. Or if you prefer 300 MWf - with the f standing for fusion. Now fu might be a better explanation but it has certain unfortunate connotations.

MSimon wrote:fu might be a better explanation but it has certain unfortunate connotations.

especially in CW: di-di-dah-dit di-di-dah

David_Jay wrote:

MSimon wrote:fu might be a better explanation but it has certain unfortunate connotations.

especially in CW: di-di-dah-dit di-di-dah

You are going to have to explain that to me. My amateur radio skills are severely atrophied.

It's Morse code, and it's almost Behtovan. So several meanings there.

drmike wrote:It's Morse code, and it's almost Behtovan. So several meanings there.
:D

So clue me. Or if the special meaning in Morse can't be mentioned in public send a private message.

Simon - ex K0ZMW ex K0NMR

Like you said the first time, I don't think there is anything "special". I bet it means the same on text messaging these days too!

I did a google on it, and most of the discussions were about cq and those origins. We may not use the telegraph and Morse code much directly, but we sure use a lot of it indirectly!

edit - Check out these shirts: FU Morse Code