Development of a 15 T Nb$_3$Sn Accelerator Dipole Demonstrator at Fermilab

A 100-TeV-scale Hadron Collider (HC) with a nominal operation field of at least 15 T is being considered for the post-LHC era, which requires using the Nb$_3$Sn technology. Practical demonstration of this field level in an accelerator-quality magnet and substantial reduction of the magnet costs are important requirements for realization of such a machine. Fermilab has started the development of a 15 T Nb$_3$Sn dipole demonstrator for a 100-TeV-scale HC. The magnet design is based on optimized 60-mm-aperture four-layer shell-type coils, graded between the inner and outer layers to maximize the performance and reduce the cost. The experience gained during the Nb$_3$Sn magnet R&D; is applied to various aspects of the magnet design. This paper describes the magnetic and structural designs and parameters of the 15 T Nb$_3$Sn dipole demonstrator and the steps toward its fabrication.