Talk-Polywell.org

I would like to explore designing a standard Elmore-Tuck-Watson fusor using carbon fiber thread or even carbon fiber nanotube for the electrode.

BOE Calcs:
2 configurations: a 1.6M radius 100MW power reactor and a 0.154M radius (12” dia) test fusor:
Assume:
Carbon fiber thread 5 micron dia is commercially available.
The electrode consists of 6 great circles forming a truncated icosahedron ala mr-fusion.hellblazer.com
Then:
The 0.154M radius electrode has a spherical surface area of 0.29 m^2.
The electrode has a total cross section of 2.87e-5M.
That gives a transparency of 1/10,160.
This compares well to the 10,000 passes through the device required per fusion.
Assuming the small device has the same power density as the 1.6M radius 100MW polywell the electrode sees 8.51 watts of energy flux.
For the 1.6M radius version the transparency is 1/106,667 and it intercepts 938 watts.
These heat fluxes are quite low but the electrode is so small that it might still be a problem.
On the other hand carbon will take very high temperatures.
The electrostatic charge should help maintain the spherical shape of the electrode.
Carbon nanotube could be made much smaller therefore correspondingly more transparent and it would also intercept fewer alpha particles.
This might be very easy for someone with a working fusor to rig up. Macramé?

OK, guys shoot it down.
Does this idea have legs or is it DOA?

That's a fascinating idea. But, as you pointed out, the thread is very fine and will have to dissipate (by your calc's) a kilowatt. Because you have only radiation to transfer your heat, it will likely just get hotter and hotter till it breaks. Not only will it get hit by ions and electrons, it will also get hit by fusion products.

I was going to say it would have to support itself, but carbon nanotubes have extremely good tensile qualities (hence using them for a space elevator).

First, how would you cool it? Although again, the low cross-sectional area would reduce impact frequencyand thus the heating.

Secondly, I wonder whether the impact of a single fast ion would snap the wires if they are that small.

Some other thoughts:
Could a wire this fine support its own weight? In the same vein, could you have a loop of CNT that would support its own weight?

About the heat load: the surface area to volume ratio of the carbon wire is very high. Maybe radiation would be enough to keep the wire from burning up.

Carbon nanotubes have interesting properties. They have high heat conduction along the axis and low conduction perpendicular to it. So they won't radiate much, but you should be able to suck the heat out of them rapidly towards some support structure.

You also have to connect them to some kind of power supply, so that connection can have both thermal cooling as well as electrical supply.

The bond strength will be a problem. A 2 MeV alpha will break a nanotube with no trouble, so you'll need a way to have the tubes self repair. If enough of them are stacked next to each other, that may work.

An option would be to have several 100 microns of metal coating over the nanotubes. The coating would be radiation protection and the nanotubes would be the cooling. But that might defeat the whole purpose, so never mind!

A BOE on weight vs support strength would be interesting.

What if we used a several thousand KM length of nano tube and simple spooled it through the chamber. As the thread heats up we simply move a new section into the chamber and cool the superheated portion outside the chamber.

You could either do a spool to spool arrangement or do a closed loop.

Since a specific section of the thread would only be exposed for a short period of time the risk of overheating it is greatly reduced.

An arrangement like wire EDM was one fix I though of. (electric discharge machining)
It would also actively cool it against the pulley.
But moving parts in hard vacuum tend to shed particles.
Also it would have a lot of parts.
But I expect killer to be that the (charged) threads outside of the fusor sphere would also attract electrons and eliminate the potential well.
I.e. the electrons outside the ball really need to be attracted to just the ball.
The fibers would have to go straight to the wall where the pulleys are.
This might be fixable with shield tubes at the appropriate potential.

The 6 micron thread is, I am sure plenty strong & thick enough to self support and prevent breaking in an alpha strike. I had not thought about that.
The stuff comes on spools for weaving in high speed industrial looms so it has to be self supporting and then some. But I can't find any specs today.

A multi-wall larger-than minimum diameter CNT might hold together well enough to allow the broken ends to rejoin unless it actually removed a carbon atom.
That would be an interesting (read long and expensive and fun) materials science research project.

Are any conductors transparent to alpha particles? electrons?
CNT's may be thin enough to have very different absorption characteristics than bulk carbon.
i.e. What are the chances of actually hitting a carbon nucleus hard enough to break a bond? And there are only 2 to maybe 12 of them in a multiwall CNT.

BOE self supporting CNT:
From Wikipedia
63,000 MPa tensile strength, 1.4g/cc, 5nm dia, one 0.96 m circumference loop hanging from 1 thread, 2.58e-13 newtons, 0.013 MPa stress, safety factor = 480,000 good.
63,000 MPa tensile strength, 1.4g/cc, 5nm dia, one 10 m circumference loop hanging from 1 thread, 2.7e-12 Newtons, 0.138 MPa stress, safety factor = 456,000 good.
Some reports use 138 GPa strength which would be even better.
We also need to include static repulsion from itself, but with that kind of safety factor I’m not worried.

BOE self supporting Commercial Carbon Fiber (Tohotenax http://www.tohotenax.com/tenax/en/produ ... operty.php):
5800 MPa tensile strength, 1.8g/cc, 5 micron dia, one 0.96 m circumference loop hanging from 1 thread, 3.3e-7 Newtons, .017 MPa stress, safety factor 343,000 good.
5800 M Pa tensile strength, 1.8g/cc, 5 micron dia, one 10 m circumference loop hanging from 1 thread, 3.5e-6 Newtons, 0.177 MPa stress, safety factor 32,700 good.

The stuff is made by the kilometer, by the ton and is relatively cheap considering.
So, even if erosion requires frequent changes the solution is to automate the change outs, as long as the life is not too short.
I sort of see semiconductor fab type robots and load-locks.
(It sure beats gold hohlraums at $1200 a pop) (laser pellet fusion)

It can take very high temperatures (for a solid object).
But certainly the thread life is a make or break question [edit: pun not intended]

Nothing is transparent to electrons. The cross section for alpha impact is probably really small, but if you've got 100 MW worth of alphas, you are going to have to deal with the impacts.

If the structure is only 12 atoms, then the binding energy is on the order of 50 eV. A 2 MeV alpha has no trouble blasting that apart. But it would have to hit a nucleus, and that's really small, so the odds are low. The larger the diameter of the ball, the lower the total cross section will be (given the size of the wire does not change) so you might be able to find a balance between MTBF and run time.

It would definitely be interesting to do a materials study on it. If the carbon structure is similar to cell bonds, it will self repair. If it's similar to diamond, it probably won't. Definitely be interesting to know about.

I guess the specific question to answer is:
What is the cross-section around each carbon nucleus within which an alpha will transfer enough energy to break all 3 bonds?
Then I would multiply that by alpha flux, then multiply by the number of layers that need to be broken to cause fiber failure, get a life time and reduce by a safety factor.
The tensile SF is great enough to hope for a workable lifetime.

I did not mean a 12 atom structure I meant a 6 layer multiple wall CNT (i.e. MWNT).
The number of atoms in the whole molecule is huge but most are too far away to share in the energy dissipation.
The 6 layers was just a WAG as a large-ish number I remember hearing of for a multiple wall CNT.

I don’t expect a single missing atom to reduce the strength much.
Because of the hexagonal structure, the load would be rerouted through the remaining bonds on that circumference.
Look at the pictures in the Wikipedia article on CNT’s.
They show pictures of SWNT’s with ~10, 17 & 20 bond strings running lengthwise.
With the calculated safety factor the alpha flux would have to blast away well over half of the carbon atoms to affect the strength significantly.

CNT’s can be welded end to end to make longer ones using just pressure.
This implies a certain amount of ductility or malleability and possibly self healing.
(Does anyone here know the parameters of that process?)
Rensselaer Polytechnic uses heat and irradiation.
One hit used “mild electron beam environment”.
That sounds like our environment all right.
We might be hot enough to cause ongoing in situ annealing of broken bonds.

I'm pretty sure the structure is more graphite-like than diamond-like.
I expect instant self healing as long as only 1or 2 of the 3 bonds are disrupted.
With MWNT the atom would be constrained (splinted if you will) by the adjacent layers, probably long enough to reattach even if all 3 bonds are broken as long as the atom is still in place while it cools down..

None of this fine tuning worry is needed for the 5 micron yarn which looks plenty transparent and plenty strong for the 1.6M radius electrode.
Also they have thousands of strands to share the loads and the damage.

Thermal load is still an issue.
For 100MW 1.6M radius electrode, energy flux at sphere = 3.11e6 W/m^2 for 5 micron fibers total heat intercepted = 937 W, total radiating surface = 9.47e-4, for emissivity 0.77 (from http://www.engineeringtoolbox.com/emiss ... d_447.html), Temperature required to radiate this much = 2182K Ouch
Same result for 5 nm fibers.
This is close their fabrication temperature.
I can’t imagine how they could conduct that heat longitudinally away from the hot zone due to the length to width ratio.
For a 12” dia electrode with same volumetric energy density (W/m^3) the Temp drops down to 1212 K
That is a lot more manageable but still very high.
http://www.sohim.by/en/catalog/carbon/ says theirs is good to 3000 C
That is a 1000 C safety margin which should be plenty given the T^4 power in the BB radiation equation.

I thought this might be a throw-away idea but it looks better and better.

50 Amps of grid current.

That's why I said 50 eV for the binding energy - you'd have to break at least 12 bonds at once. The odds of that are really small, but I don't know real numbers. If you figure elastic plus inelastic scattering cross section from nuclei then the combined transfer to lattice from an alpha is a very crude but BOE useful estimate. I haven't looked up the cross sections.

Lateral heat transfer is orders of magnitude better than transverse heat transfer, so it seems like something to take advantage of.

At least it sure would be fun to experiment with!

drmike wrote:Nothing is transparent to electrons. The cross section for alpha impact is probably really small, but if you've got 100 MW worth of alphas, you are going to have to deal with the impacts.

If the structure is only 12 atoms, then the binding energy is on the order of 50 eV. A 2 MeV alpha has no trouble blasting that apart. But it would have to hit a nucleus, and that's really small, so the odds are low. The larger the diameter of the ball, the lower the total cross section will be (given the size of the wire does not change) so you might be able to find a balance between MTBF and run time.

It would definitely be interesting to do a materials study on it. If the carbon structure is similar to cell bonds, it will self repair. If it's similar to diamond, it probably won't. Definitely be interesting to know about.

100MW?? I thought this was a fusor we were taking about. Fusors have to have debye lengths comapable to the machine to wall distance. If the radius is 0.154m the densities will be ridiculously low.

This could make a compact energy efficient portable neuron source, but a 100MW fusion reactor? Forget it.

I don't know how to calculate the optimal size so I just took a couple of commonly discussed sizes to bracket the problem.
I would be happier with a smaller machine.
Heck, any size that gets to above break even would make me ecstatic.
The Debye length issue could be a show stopper as it forces a trade-off between plasma density and device diameter.
I had been under the impression that the only show stopper for the fusors was the grid transparency.

50 amps grid current is tough.
But, the cross section is very small so it might be much lower than that especially if the return current came in via the ion gun.

Even though it is strong enough to self support, it is still very fragile in absolute terms and therefore hard to handle let alone to see.
That even goes for the 5 micron stuff which would have only half an ounce of tensile strength and would require a microscope to see.

Mike, where did you get that data?
The hits I get only point to abstracts to papers that I can’t read.
Or to hoods for sports cars or golf clubs, good grief.

on the carbon nano wiki page of course!

Might not be totally accurate, but it's probably close enough.

50 Amps is for 100 MW.

If you were doing a fusor 100 mA or less would do.