JT-60SA Magnets now superconducting!
In the last 24 hours the magnets reached a low enough temperature to become superconducting. First the resistance of the niobium-tin windings of the central solenoid could be seen to drop to zero and then, about 12 hours later, the toroidal field and equilibrium field coils made from niobium-titanium followed.
The electrical resistance of the magnet windings is monitored primarily as an additional means to measure the average temperature of the winding, since their resistance depends on their temperature. However, below a certain cryogenic temperature the resistance of the materials drops to zero and they can carry current without consuming any power. This happens at about 18K for Nb3Sn and at about 10 K for NbTi. It is no longer possible to derive their temperature from that measurement – but it confirms their most important property!
Exploiting this phenomenon is a key feature of the JT-60SA tokamak that will enable it to generate strong magnetic fields during long experiments.
You can see the current magnet temperature here.