R2D2, the CEA Graded Nb$_3$Sn Research Racetrack Dipole Demonstrator Magnet

As a part of a collaborative effort with CERN on the long road to FCC-hh, CEA is developing a 1.4 m-long single aperture short dipole demonstrator. This Nb$_3$Sn block dipole, named F2D2, is foreseen to produce 15.5 T with a 14% margin at 1.9 K, and will use a shell-based support structure. In order to be compact, this magnet relies on the use of different cable grades in each coil. Among the milestones in this project, a single-layer graded racetrack dipole will be constructed in order to demonstrate grading in a simplified geometry while preserving key features of F2D2. This magnet, named Research Racetrack Dipole Demonstrator (R2D2), will permit the mastering of the fabrication methods foreseen for F2D2 and to confirm the behavior of Nb$_3$Sn cables representative of FCC. It should also validate some key technological aspects: compactness, operation at high-current (up to 16 kA), intermediate magnetic field (in the range of 11-12 T), and quench protection at very high energy/unit length (up to 650 kJ/m, corresponding to 168 MJ/m$^3$).