Longitudinal and Transverse Dimensional Changes During Heat Treatment of the Nb3Sn Cables for the Graded Research Racetrack Dipole Demonstrator (R2D2)

The Research Racetrack Dipole Demonstrator (R2D2) is a Nb₃Sn graded magnet developed in the framework of the CEA-CERN collaboration with the goal to assess key technologies for future high field accelerator magnets. This demonstrator magnet features two 1.7 m long racetrack coils, designed in the graded configuration needed for compact and efficient high field magnets. Each coil is made of two different Nb₃Sn cables wound around each other and submitted together to the high temperature heat treatment needed for Nb₃Sn compound formation. The experience on Nb₃Sn coils has taught us that the management of the deformations undergone by the Nb₃Sn cable during its formation must be a priority in the definition and the dimensioning of the tools and the manufacturing processes of the coils. This paper reports the results of the test campaigns carried out at CEA Saclay to study the behavior of R2D2 cables during heat treatment. The first campaign consisted of reduced-coils made of a few turns of each cable, with representative lengths. For each coil, the winding is performed with gaps in the components, and the gaps are measured before and after the heat treatment to quantify the coil length variations. A second campaign consisted in measuring the thickness variations of cable stacks in various configurations. The results allowed validating the choices made for the final R2D2 coils and heat treatment tooling to manage the longitudinal contraction and the transverse expansion.