Why does the Japanese Maglev train use superconducting magnets

[ryccoh]

My little home made air-core solenoid makes 1.5T, why is it that superconducting electromagnets often aren't stronger than one would expect.
Take the japanese maglev train for example:

http://www.supraconductivite.fr/en/index.php?p=applications-trains-maglev-more

According to this article the magnets are superconducting and have 700,000 amps going through them.
Let's assume they were to use only 250 turns per meter then B=u0(N)(I) which would already be 220 Teslas. But according to the article it's only 5 Teslas strong what am I missing here? Obviously you wouldn't want 220 Teslas because of the compressive forces but wouldn't you want more than 5? And if for design reason you only needed 5 then it really doesn't seem like you would need to be superconducting, 5 T can be achieved with only a few thousand amps. Also MRI machines are only 1.5T and they're superconducting as well. Please help clarify.

 

[f95toli]

Because superconductors are (nearly) lossless meaning a wire of relatively small cross section can conduct at LOT of current. The same amount of current in a copper wire would melt it.

Also, I am not at all familiar with the design they refer to in the article; but if they area really using 700 k (it sounds very high) the cross section must be large. The critical current for a typical NbTi wire is about 3000 A/mm2; meaning the "wire" used for for the train will be need to be at least 17mm in dimeter. Hence, they can't be using many turns.

Also, the cross section of the wires used for MRI magnets is tiny; typically you do not use more than about 100A to energize the magnet so you need lots of turns.