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New Superconductor Found "Immune To Magnetism"
Lisandro sends in news that testing of the new class of superconductors we discussed a while back (compounds of iron, lanthanum, and rare earths) has turned up a major surprise: magnetism doesn't shut off the superconducting state. Magnetic fields represent one of three factors that limit expanded applications for superconductors (the others are current density and temperature dependence.) The research will appear in Nature; here's a preprint (PDF).
Ack - looks like caffeinated_bunsen beat me to the punch. But it bears repeating - this paper certainly says nothing like "this superconductor is immune to magnetism". This material has a very high critical magnetic field, and if they figure out how to improve the connectivity then it might even someday be able to carry a current density of engineering significance. But it certainly is not "immune" to magnetism in any qualitatively different way than any other type-II superconductor out there. Still...it's nice to see that high-temperature superconductivity can be observed outside the cuprate family, and this paper (showing that it also has a high critical magnetic field) should spur some serious R&D work outside the theoretical physics community.
Reasoning is never, like poetry, judged from the outside at all.
- 1.5 T is what's used in MRIs (people have died when metallic objects fly around in these fields)
- 16 T will levitate a frog
- 45 T is the strongest magetic field continuously produced in a laboratory.
- 10,000 T is instantly lethal to organic life
Basically we're fine levitating frogs, but probably won't be able to use it as part of an instant-death ray.Australia. :)
You're right that electric current creates a magnetic field. In a type-II superconductor (like the cuprates and these new FeAs materials), this is managed by introducing defects in the material (grain boundaries, inclusions, etc.) that "pin" the quantitized magnetic flux vortices and prevent them from moving through the material and destroying superconductivity. So it's not fair to say that you CAN'T use cuprates to carry large currents - it's just an engineering problem that has to be dealt with by clever manipulation of the structure of the materials.
So here's the short version:
Critical field = intrinsic property of the material.
Critical current = extrinsic property that depends on critical field, grain structure, presence of second phases, etc.
Reasoning is never, like poetry, judged from the outside at all.
When the generators which power those things lie still, their steel shafts warp under their own weight. Even when the generators aren't being spun to create the electricity, they have to spin (albeit more slowly) to make sure they don't bend. Wicked cool stuff.
A somewhat off-topic digression:
The conventional wisdom on Bussard Ramjets (included in the wikipedia article) is that they reach a terminal velocity due to the drag of collecting the fuel - and asymptotically approach their exhaust velocity. IMHO that's incorrect.
The bug is that the calculation assumes that they must accelerate the collected hydrogen to the velocity of the craft before fusing it, then depend on the fusion energy to re-accelerate it as exhaust.
However, as with the collected air in chemical ramjets, the momentum of the collected material does not need to be discarded. It can be fused on the fly through the ramjet, retaining its original momentum along the flight path (relative to the vessel). Thus the energy of fusion can be applied to accelerating the reaction products toward the rear. None is needed to replace the momentum allegedly lost capturing the fuel.
Now SOME of the axial momentum of the incoming fuel is traded for radial momentum to collect it. But the energy of that "lost" momentum is converted to pressure and temperature, compressing the material like any other gas. There is a drag on the scoop field from this. But when the exhaust expands again after the reaction there is a corresponding thrust against the nozzle field, reconverting the radial expansion of the reaction products to rearward velocity and recovering the "lost" momentum.
If this whole process were lossless there would be no top end to the kenetic energy the ramscoop could accumulate. With less than 100% efficiency in reapplying the compression energy to the mass (both from lost energy and lost mass) there is some drag from collection that is not recovered. (For instance: Mass lost as neutrinos is a non-trivial fraction.) So there may still be a speed limit. But it can be far higher than that calculated by assuming you "stopped" the gas when you "caught" it.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
A more important near term result would be a cheap Bussard Polywell fusion system.
A high temperature superconductor that is resistant to high magnetic fields would allow significant efficiency gains and eventually miniaturization.
Who knows in 40 years every new home might have it's own fusion reactor in the basement because of this material.