High-temperature superconductor critical current measurement error

Papers in *Science* magazine around 1990 measuring critical current with field parallel to current (zero J-cross-B force), reporting fantastic values that would never apply to a wound magnet where J and B are perpendicular. Seven orders of magnitude difference.

I actually wrote an article around 1990. I was so tired of hearing all this bull about how superconductors were going to change the world, and I wrote an article and said, there's more than just T-sub-c, there's also critical current. People would have laughed at me when I was in the 1970s if I had published a paper where I measured the critical current with the magnetic field aligned in the same direction as the current. If J-cross-B is the force you're worried about, and you want to make the force go to zero, all you have to do is have the J and the B in the same direction. You would never put your sample vertical in the axis of the magnetic field over at the National Magnet Lab, because there's no force trying to destroy the superconductivity. But if you ever build a magnet, it's going to be wound, the current is going to be going in the circumference, and the field is going to be going axially. They're perpendicular, which is the maximum force to destroy the critical current. If I had gone to a superconductivity conference and presented a paper where I measured the critical current when the two were parallel rather than perpendicular, I would have been laughed out of the room.

*Science* magazine papers reporting critical currents two orders of magnitude above Tom's thesis values — measured with conductor parallel to the magnetic field, which is physically meaningless (J×B = 0). The peer reviewers didn't catch it. "One of the problems when there's lots of money, you bring out all the people who don't know what they're doing."

And it's because of some of the critical parameters. The first figure in my doctoral thesis — there's three parameters: high temperature, high magnetic field, and high critical current density. If you go to high current density, it loses its superconductivity. I had this little phase space with three axes, saying this is where you have superconductivity, in this little octant of space. They had improved one property, temperature, and they got high magnetic field along with that, but they hadn't improved critical current density, and they still haven't today, twenty-five years later. People were reporting all kinds of wonderful critical current densities parallel with the magnetic field. I did my thesis over at the Magnet Lab — anybody who tried to publish a paper measuring the critical current for a magnet with the conductor parallel with the magnetic field, you wouldn't have gotten your paper accepted. Because anybody knows you got J cross B forces which will destroy the critical current. If you have your conductor perpendicular — that's how you design a magnet. You can't design a magnet with the current parallel. J cross B, the vector products — the current has to be perpendicular to make the self-magnetic field. That's physics. That's Maxwell's equations.