High-temperature superconductor critical current density problem

The scaling argument: pinning-site size scales inversely with critical temperature, so 40-70 K superconductors need pinning sites smaller than an electron — impossible.

There's a good correlation between T-sub-c and the critical magnetic field. You go above a certain field and you lose your superconductivity. When I was working on these, I was working on some of the highest-temperature superconductors then known — worked at 20 Kelvin. The niobium aluminum had critical fields of about 30 Tesla. Anybody have a feel for a Tesla? The strongest electromagnet with an iron core is about one Tesla. An Alnico magnet is about one Tesla. Iron neodymium boron might be two or three Tesla. The strength of the magnetic field basically goes as the square of the field. So 30 Tesla is a thousand times stronger than the magnetic field you have in a regular old motor.