Ball-and-stick model of the unit cell of the high-temperature superconductor yttrium barium copper oxide.

Copper

Copper is a common material for magnet windings, but its electrical resistance causes significant ohmic heating and power consumption when strong magnetic fields must be maintained for long periods.

Superconductors

For this reason, superconducting coils have been used in tokamaks since the 1980s. Below their critical temperature, superconductors can carry electric current with essentially no electrical resistance. Superconducting magnets can therefore carry very high currents and generate strong magnetic fields with much lower resistive power losses than conventional copper magnets, although they require cryogenic cooling. Conventional low-temperature superconducting magnets such as NbTi and Nb₃Sn are typically operated at about 4 K (−269 °C), using helium cooling.

Niobium–Titanium

Niobium–titanium (NbTi) is a ductile superconductor that is relatively easy to manufacture into coils. At temperatures around 4 K, it is typically used for magnetic fields up to about 8 T, with superconductivity lost at fields of roughly 10 T.

Niobium–Tin

For higher magnetic fields, typically above about 8 T and into the 10–16 T range, niobium–tin (Nb₃Sn) is more suitable, but it is much more brittle than NbTi. Nb₃Sn coils are commonly wound before a high-temperature heat treatment forms the superconducting compound; after this treatment, the conductor must be handled carefully because of its brittleness.

ReBCO

Another option is the high-temperature superconductor ReBCO (rare-earth barium copper oxide). REBCO can remain superconducting at much higher temperatures than NbTi or Nb₃Sn and can also carry high currents in very strong magnetic fields. For fusion magnets it is typically operated well below its critical temperature, often around 20 K, and is manufactured as flexible coated-conductor tape.

REBCO coils were tested on the GOLEM educational tokamak in Prague and were later used for both toroidal and poloidal field coils in the small spherical tokamak ST25 HTS.

5 pictures

5 pictures