No, this is an idealized substance, doesn’t exist irl.

Heat would definitely kill fusion reactor magnets.

That’s why they’re so hard to build, plasma of a million degrees has to sit only a meters away from superconducting magnets which have to stay cryogenically cooled to near absolute zero.

Magnetism is used because it's long enough range that nothing has to actually touch the million degree plasmas and be instantly vaporized. If the materials actually generating the magnetic fields reached anything like the plasma temperatures, they would cease to exist.