Application of Nb$_3$Sn superconductors in high-field accelerator magnets

Last year a record central field of 11 T at first excitation at 4.4 K has been achieved with the experimental LHC model dipole magnet MSUT by utilising a high J$_c$ powder-in-tube Nb$_3$Sn conductor. This is the first real breakthrough towards fields well above 10 T at 4 K. The clear influence of magnetisation and coupling currents on the field quality, the quench behaviour and the temperature development in the coils has been measured and is discussed. For application in high-field accelerator magnets (10-15 T dipoles, 300-400 T/m quadrupoles) these experimental results clearly reveal the potential, the present limitations and the necessary improvements of Nb$_3$Sn technology with respect to strand, cable and coil design and manufacturing. A brief review of developments in this field is presented. The focus is on accelerator dipole magnets but the key issues for quadrupole magnets are quite similar.