$\hbox {MgB}_{2}$ Coil Options for Fusion Poloidal Magnets

A hybrid solution is proposed for the IGNITOR research fusion machine by using of superconducting coils for some poloidal magnets, in association with high field copper magnets for the central solenoid and for the toroidal field coils. The choice to be made among the various superconductors is based on the materials performances in high magnetic field, on the mechanical strength of the conductors and on the cost of manufacturing large coils. In this study we analyze a coil based on MgB₂, a "medium temperature" superconducting material that we expect will avoid, in association with others high temperature superconductors, the use of liquid He in future machines. The external poloidal magnet, 5 m in diameter and subjected to a magnetic field of 5 T, represents a real test bench of the technical issues which should be addressed in the exploitation of the future fusion reactors. To fulfil the technical characteristics of the selected magnet we must optimize the fill factor of the superconducting MgB₂ wires, increasing the presently obtained 30% value. Accordingly, the effective current density in the superconducting wire should be of about 1500 A/mm² (@10 K, 5 T), a value which is compatible with the present best MgB₂ laboratory short wires, doped by C or SiC.