Application of superfluid helium cooling techniques to the toroidal field systems of tokamaks

The operational performance, during the last 10 years, of the toroidal field (TF) system of the Tokamak TORE SUPRA based at CEA Cadarache, has demonstrated that superconductivity associated to superfluid helium is a reliable and realistic option for fusion by magnetic confinement. This solution is now widely used in cryoelectricity. With superfluid helium associated to cable in conduit superconductor (CICS), new paths can also be opened for cryoelectricity and especially for fusion. The operation at FZK of the EURATOM LCT coil at 1.8 K and 11 T has also demonstrated, that the use of niobium-titanium can be extended up to very high fields. In the frame of the Large Hadron Collider program (LHC) at CERN, relevant commercial NbTi composites from different companies have been extensively characterized at 1.8 K between 9 and 11 teslas. Based on this characterization, practical considerations on conductor design criteria in superfluid helium are presented, discussed and compared to design criteria in supercritical helium. The operating temperature has an important impact on the cost of the system and can be determinant in the choice of the superconducting material. Some economical evaluations are given on the basis of a reference TF magnet concept.