new copper composite

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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prestonbarrows
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Re: new copper composite

Postby prestonbarrows » Sat Jul 04, 2015 3:49 am


Tyler Jordan
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Re: new copper composite

Postby Tyler Jordan » Sat Jul 04, 2015 7:42 am

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D Tibbets
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Re: new copper composite

Postby D Tibbets » Sat Jul 04, 2015 8:29 am

Better descriptions of practical considerations driving magnet design are appreciated. Bitter magnets have been discussed before, though I don't know if they lend themselves to the B field geometry desired.

The issue of electrical conductivity for this composite versus copper seems to be the confounding issue in this thread. Look at the charts with the article carefully.

One shows electrical conductivity versus temperature. Above 80 degrees C the composite may have a modest advantage. Below 80 a disadvantage. I don't know if this extends into the cryogenic range. Copper electrical conductivity may be ~ 6-8 times better at liquid nitrogen temperatures and ~ 20 times better at liquid helium temperatures. This does eat into the superconductor advantages and avoids quenching concerns, and as pointed out allows for rapid pulsating of magnet strength through small or large ranges that might have some advantage for managing plasma properties, facilitating electron injection, POPS effects, etc.

Another chart shows the electrical conductivity versus current in millions of Amps per cm of thickness. The important points here is that below a few million Amps per 1 cm thickness, the two materials are essentially the same. Only above this current density does the composite show it's advantage. The appreciation of what this means is to consider what the thickness ofthe wire is. A one cm wide wire (square wire) is 1 square cm in cross section. A 1 mm wire is 0.01 square cm in cross section. It is one tenth the diameter but the cross section is one hundredth of a 1 cm wire. This exponential relationship between diameter and cross sectional area is what leads to smaller and smaller wires having current carrying wire that decreases as the square of the diameter. Eventually, a tiny wire will exceed the current carrying capacity represented by the graph. Then the differences between the materials becomes significant.

This transition for copper is at ~ 3 million Amps in a 1 cm wire. In a one micrometer wire it is the square of the difference in diameter or (0.0001) ^2 less or 0.00000001 times less. This 1 micrometer diameter copper wire could carry only about ~ 3 * 10^-2 or about 10 milliamps. Any current higher than this results in the copper wire heating much faster because the electrical conductance is decreasing rapidily- the resistance is increasing rapidly. The cooling requirements quickly becomes unmanagable.
The composit with this transition occurring at ~ 100 times greater could carry about 1 Amp through a one micrometer wire before the cooling requirements ran away. Keep in mind that the resistive heating for both grow about the same below this critical temperature, only past these threasholds does the comparitive resistance diverge. It is almost like comparing a superconductor that works at 4 degrees K versus one that works at 80 degrees K. If you cannot keep the wire below quenching temperature it fails . It is easier with the high temperature superconductor. Compared to the copper and composite wire the situation is similar, only the baseline resistivity and the rapidity of slope change with quenching.

This comparison is between 1 cm and 1 micrometer wires. Shrinking the wires to 100 nanometers would decrease the current carrying capacity of the two wires further by 100 X. Now the composite wire could carry ~ 10 milliamps before heating concerns become tremendously more difficult. Hopefully my math has been reasonable. Just remember that the current carry capacity decreases as the square of the diameter. I think this applies to any wire weather superconducting or not. Practical human scale applications probably never reach current densities where the advantage of the composite is reached. Because of the exponential relationship this is not the case on the nano scale.

As the authors point out this nano scale realm is where this recipe us useful, perhaps very useful.

Can anyone give an example of an application where wire dimensions of perhaps 1 mm or greater require current densities where the electrical conductivity of this composite wire diverges from that of copper (ignore the mild high temperature advantage of this composite, that is a sort of different issue; just as the possible advantage of copper at cold temperatures is ignored)? Cooling considerations limits the current for both about the same in this macro realm, so there is no advantage. Questions of strength, rigidity, high temperature tolerance, cost, etc. are different engineering issues that may change the picture also.

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

prestonbarrows
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Re: new copper composite

Postby prestonbarrows » Sat Jul 04, 2015 3:49 pm


Tyler Jordan
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Joined: Sat Mar 29, 2008 2:34 pm
Location: Tasmania
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Re: new copper composite

Postby Tyler Jordan » Sun Jul 05, 2015 6:47 am

Thank you for the detailed explanation. It's filtered through my thick skull! lol Seriously though, I very much appreciate your time to explain.
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