Study of AC losses in non-insulated REBCO stack and contact electric resistance measurement

In the field of magnetic‐confinement fusion, high‐temperature superconducting (HTS) magnets are attracting increasing attention because of their ability to carry very large currents at cryogenic temperatures. While Cable‐in‐Conduit Conductor (CICC) design has been the standard for toroidal and poloidal field coils in tokamak devices, the central solenoid (CS) must withstand rapidly changing magnetic fields and currents, posing distinct challenges. One promising approach under investigation is the use of non-insulated REBCO tapes to construct the CS, winding HTS tapes directly into coils rather than using traditional CICC cables. These tapes offer enhanced quench protection and improved thermal stability as a result of current sharing between layers. However, drawbacks include longer coil charging times and increased AC losses. This thesis aims to investigate AC losses in non-insulated REBCO tape stacks subjected to varying magnetic fields, identifying the key physical parameters that influence them. A comprehensive methodology was adopted, combining analytical modeling, finite element simulations, and experimental measurements. On the analytical side, AC losses were studied in a single superconducting layer, a REBCO tape, and in tape stacks, under both parallel and perpendicular magnetic field orientations. The analytical approach starts by using the Critical State Model and is then refined using Maxwell’s equations in conjunction with the E–J power law. These analytical results were validated through 2D simulations developed with COMSOL Multiphysics. Experimentally, the inter-tape electrical contact resistance, identified as a critical factor affecting AC losses, was measured through pressure-dependent tests carried out at Fermilab. Additional experiments examined the effect of mechanical reinforcement on contact resistance and transverse resistivity by inserting stainless steel layers of varying thicknesses between REBCO tapes. Finally, a 3D finite element model of a REBCO stack was developed to perform a parametric study on how factors such as stack length, width, number of layers, and transverse resistivity affect AC losses. The experimentally obtained resistivity values were used as input to ensure more realistic simulation conditions.