Participation in the development of a high field 30 T using REBaCuO magnet

Superconductors are divided into two categories, low critical temperature superconductors (LTS) and high critical temperature superconductors (HTS). With LTS, strict limitations are imposed on the operating temperature (<14 K) and the bearable magnetic field (<28 T). Higher cooling temperatures (up to 93 K for REBCO) and larger magnetic fields (> 100 T) are allowed for HTS superconductors. Recently, various R and D projects and applications at very high-field (RMN, IRM, Fusion..) and cryogenics temperature (4-20 K) have become possible with the significant drop in cost of HTS tapes. Different electromagnets have been built and tested to achieve high fields using REBCO tapes. The 'Nougat' magnet, for example, reached a record central field of 32.5 T: 14.5 T came from an internal REBCO winding alone (insert) and 18 T from a resistive external winding (outsert); this magnet is made up of a stack of REBCO metal-insulating (MI) 'pancakes'. MI windings offer passive protection against hot spots (quench) thanks to their ability to deviate the current in adjacent turns through this insulation. This tape made of high yield stress limit metal also serves as an additional mechanical reinforcement. Preliminary calculations have however shown that during nominal operation, the Laplace forces are so large that the radial stress which brings the turns into contact can become positive over a large portion of the coil. A positive radial stress indicates that the turns are in the process of separation, endangering the thermal conduction essential to ensure protection against hot spots. The purpose of this thesis is to provide a better understanding of the mechanical behavior of flat electromagnetic coils in MI REBCO subjected to different loads (pre-tensioning winding, cooling and energizing), thanks to the implementation of analytical modeling and to its experimental validation. The implementation of this model required developments in continuum mechanics, the identification of material parameters using tensile and diametrical compression tests assisted by digital image correlation (DIC), and the construction a cryogenic bench to allow in-situ measurements of the displacement field of a MI-REBCO pancake during cooling and energizing stages. (author)