How a bout a destructive test of a cheap Polywell WB8 sized.

Discuss how polywell fusion works; share theoretical questions and answers.

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Nik
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Scary...

Post by Nik »

Much as it would prove a point, IMHO, you really, really do NOT want the very first WB break-even to be 'explosive'.

Okay, like early steam-boilers, some prototype WBs are likely to disassemble uncontrollably, but the basic concept is as far from nuclear fission and its weapon potential as you can get. Please, please, please keep it thus.

Er, there is one up-side-- Having uppity regimes sink their expertise and energies into WBs is *much* better than having them play with genuine nukes...

{eg NK ?? }

D Tibbets
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Post by D Tibbets »

I suppose you could take this approach for a proof of concept one shot deal. I'm not sure it would be cheaper or much faster. In a nuclear bomb you only have to hold things together for a few nanoseconds. The same for laser inertial confinement fusion, or a Z- pinch, etc (they are essentially bombs with the fission trigger replaced).
For a polywell you need to demonstrate that fusion conditions can be maintained for many microseconds at least. This could possibly demonstrate wiffleball formation, and perhaps some hint of electron confinement times. But longer times would probably be needed to demonstrate important electron recirculation, and ion confinement times that are long enough to get efficient fusion. Then there are concerns about demonstrating adiquate thermalization times. Now your into multiple millisecond time frames.

In a sense, the WB6,and WB7(?) machines were sort of intermediate devices, using rapid and poorly controlled capacitor discharges, poorly controlled gas feeds, temporary magnets that were severely limited in their operating time.
According to Bussard, this was long enough, (~ 250 microseconds) but actually demonstrating all of the physics- especially thermalization issues would presumably require 10's of milliseconds, if not longer (at least if your aim is a steady state machine). Again, I wonder how far Nebel, etel were able to push WB7 and WB7.1.

Building on these results, a one shot deal might demonstrate some scaling issues at significantly higher B fields, and be more reasonable as building a larger, more powerful survivable machine costs a lot more than the small machines.

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

MSimon
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Post by MSimon »

Selenium rectifiers? You got to be sh*tting me. Those things are poison if they vaporize. And Mercury arc rectifiers? Dude. You have to fill out reams of hazmat forms.

Silicon rectifiers are cheap enough. And you can easily buy replacements.

Plastic pipe for LHe? Do you know those things get brittle when cold? And how thick will they have to be to support the forces envisioned?

Vacuum tube emitters? Tubes that could handle the current are going to cost a fortune. Why not just use halogen automobile lights with the glass removed?

I don't think you want to be blowing up your diagnostic eqpt or smashing it into walls. That stuff ain't cheap.

And if you have to run 1,000 or 10,000 tests to get the conditions right? Costs are going to mount up.

Patience grasshopper.
Engineering is the art of making what you want from what you can get at a profit.

kunkmiester
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Post by kunkmiester »

I've been told one of the big advantages of fusion reactors is that they don't have enough energy contained at any one time to "melt down." Most likely, trying to overload a polywell like this will make it fail, but it'll not be explosive. It's a big advantage if your fusion reactor doesn't need much more than a concrete wall of standard thickness to protect the surroundings in the case of failure.
Evil is evil, no matter how small

KitemanSA
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Post by KitemanSA »

I was under the impression that the explosive part of a small scale fusion experiment was the HV drive system, and WB6 already completed that "explosive" experiment. Why re-invent the flat tire?

D Tibbets
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Post by D Tibbets »

True, the fusion output in most machines would not be the major contributor to explosive or thermal failure. I was thinking along the lines of passing so much current through copper wires so that you could achieve brief high B- fields, before the copper melted into slag. (or more precisely, the insulation melted and the coils shorted out, possibly leading to failure in a spectacular way- sort of a WB6 failure multiplied by ~ 100 (0.1 Tesla to 10 Tesla current differential). This would be far short of splattering things against the wall, but it would destroy the magrid, and possibly other equipment within the vacuum vessel. It would probably damage or at least contaminate the vacuum vessel wall to the extent that it would have to be scrapped or refurbished.

Actually, I suspect that the actual Polywell costs are only a small part of the total cost - in money and time, for the experiments. WB6 was build quickly on a shoestring budget. It was the struggle with vacuum, power supplies and diagnostic equipment that probably contributed the most to cost, compromises, and frustration. I assume it was these peripheral (engineering) issues that where hopefully addressed with the WB7 experiments. They had the advantage of hindsight, and possibly some advantages in available interments.

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

Stoney3K
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Re: How a bout a destructive test of a cheap Polywell WB8 si

Post by Stoney3K »

DavidWillard wrote:And put diagnostic equipment all over it. Shoot the thing off in a mineshaft at the Nevada test site PROVING GROUNDS.
See what kinds of alphas and neutrons you get, just blow the thing up with extreme levels of power and minimal fuel to prevent vaporizing the diagnostic equipment with a 15 kiloton blast...
If you even manage to get a 15 kT blast out of an experimental Polywell setup (which I sincerely question, but still...), you've at least proven it can go past break-even.... once.

It does pose the interesting question of all possible failure modes, caused by either internal (malfunction) or external (power spikes, or even *eek* terrorist attack) causes.

Keep in mind that even without any fuel, the whole system holds a lot of stored energy in the cryogenics, capacitors, inductors and vacuum vessel. I'd hate to see some smart terrorist bomb a Polywell station in the future, causing quite the implosion and sending shrapnel all over the place.
Because we can.

TallDave
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Post by TallDave »

DavidWillard,

Too expensive due to B^4*r^3; it would cost about the same to do a few dozen non-destructive tests (the actual runs are in milliseconds anyway, followed by hours or days of cooling, so they sort of are like "shots"). Also, there's no runaway fusion effect like there is in fission. And it operates at beta=1, so it isn't just a matter of squeezing the plasma at high B.

What destroyed WB-6 was arcing unrelated to fusion, which Rick's team fixed rather elegantly with a giant water tank. If the machines fail, we want them to fail gracefully.

I think the force one gets from HE is much greater than from these magnets; that's why fusion bombs work (and they need a fission trigger anyway). I'd be interested to calculate exactly how far off the two are, though. I'm guessing the HE package packs the equivalent force of maybe ~100,000T.
n*kBolt*Te = B**2/(2*mu0) and B^.25 loss scaling? Or not so much? Hopefully we'll know soon...

D Tibbets
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Post by D Tibbets »

Lets see. If you are aiming for substantial power production, say 10^20 fusions per second (equivalent to ~ 100 MW) and your test runs for ~ 1 ms, then you would have ~ 10^17 fusions. That would consume ~ 10^-6 moles of deuterium, or ~ 2 micrograms. If atomic bombs produce ~ 10 million times as much energy as chemical explosives, then this 2 micrograms would be equivalent to ~ 20 grams of TNT. Perhaps the strength of a small hand gernade.

The energy stored in superconducting electromagnets could easily exceed this . If they are large and powerful like in a large tokamak, then if they quench, an explosive release of energy equivalent to a lot of TNT could occur if safety measures were inadequate. This has been discussed in other threads. There is a large range suggested, but at least several tons of TNT equivalent might occur. Since Polywell magnets are probably going to be a lot smaller,and perhaps a little weaker, the consequence of such an accident would be several orders of magnitude smaller.
ie: you could probably do about as much damage with several suitcases of explosives, rather than trying to short circuit the safety measures of a Polywell reactor, then triggering a failure.
Also, keep in mind that the magnets of a tokamak are on the outside, so the blast would be in air. In a Polywell the magnets are inside a strong vacuum vessel, so any blast effect would be substantially decreased.

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

KitemanSA
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Post by KitemanSA »

D Tibbets wrote: If atomic bombs produce ~ 10 million times as much energy as chemical explosives, then this 2 micrograms would be equivalent to ~ 20 grams of TNT. Perhaps the strength of a small hand gernade.
A VERY small hand grenade, indeed about the size of the booster in the hand grenade. HGs are more like 1.5 pounds, not .7 ounces.
What you'd have is a "quarter stick" fire-cracker.

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