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[Nb.sub.3]Sn forced flow cooled cable in conduit superconductors for fusion applications can be optimised for combinations of field, peak operating temperature and coil winding layout (pancake or layer) to give either maximum cable space current density or minimum superconductor cost. Conductor design for ITER including design criteria, choice of conductor configurations and its optimisation are discussed.

Cable-in-conduit conductors in pulsed tokamak applications have design requirements imposed by the tokamak operation: typically an operating temperature dependant on the AC losses and pulse time, the field by the tokamak performance and a quench discharge time by the coil voltage limits. The void fraction of helium in the cable is determined by mechanical and thermal stability considerations. It is then possible to maximise the cable space current density for a given temperature margin and strand properties, using the procedure in the companion paper 'Parametric Study of Cost Functions in Cable-in-Conduit-Conductors'. [Nb.sub.3]Sn strands can typically offer high jc, high loss or low loss, low jc. The sensitivity of the cable design to the choice of strand is discussed with reference to the quantity of strand and the achievable current density.

The high field test facility SULTAN III in operation at PSI/Switzerland tests full size industrial prototype superconductors for fusion applications such as ITER. The facility provides a background field of up to 11 T over a length of 58 cm. A 50 kA superconducting transformer works as a very low noise current source which allows a criterion of 0.1 [micro]V/cm to determine the superconducting to normal transition. Three 3.6 m long cable-in-conduit conductors based on both NbTi and [Nb.sub.3]Sn, developed by different manufacturers, suitable for the central solenoid and toroidal field coils of ITER, have been tested so far. This paper presents the results of extensive measurements of critical current and current sharing temperature of the [Nb.sub.3]Sn conductors in the 8-11 T range for temperatures between 4.5 K and 11 K. Voltage versus current curves have been analyzed with respect to the n value. The manufacturing of a high quality joint between two [Nb.sub.3]Sn conductors after heat treatment is reported, together with some measurements of the joint resistance.

The superconductors for the ITER magnet systems will utilize the cable-in-conduit concept with [Nb.sub.3]Sn strands. The protection and stability requirements for the coils require that the operating currents will be at the 50kA level and that the cables are composed of hundreds of strands. The design features for the toroidal field coil and central solenoid conductors are given, together with their expected performance characteristics. The expected critical properties are given as well as the results of stability and quench computations. The status of the conductor development and testing program is summarised and conductor R&D plans for the future are outlined.