Screening and Coupling Currents Induced Fields on a Large-Scale Prototype REBCO Insulated Coil

The existence of screening currents is one large drawback coming from the use of REBa₂Cu₃O_7-x with RE standing for Rare Earth (REBCO) tapes to develop superconducting coils. It not only alters the quality of the magnetic field but also implies losses and additional mechanical stresses on the tapes. The High Field Laboratory for Superconducting Materials (HFLSM) from Tohoku University, Japan, together with Toshiba are now developing a 33 T all-superconducting magnet, cooled by conduction, with a 19T Robust REBCO Insert. One important element of the design is a turn made of two-tape bundle and a turn-to-turn insulation. In addition to the screening currents, magnetic field variations will induce coupling currents developing between these two REBCO tapes in direct contact. These coupling currents also affect the magnetic field quality. Recently, a large-scale prototype of this insert was manufactured to validate the mechanical design and the protection system in the event of local dissipation coming from defects in the tapes. This article presents detailed behaviors of the screening currents and coupling currents induced fields obtained over this prototype focusing on the impact of some operating conditions such as the current ramping rate, applied-field history, and temperature of operation. The magnetic field error, i.e., the difference between the measured magnetic field and the one obtained with a homogeneous current distribution, is the main tool used to carry out this study. It reached about 0.5 T and was significantly changed by the temperature of operation and the existence of previous magnetizations, as predicted by simulations results carried out by several groups.