Investigation on Two Techniques for the Reduction of Screening Current-Induced Field Effects in REBCO HTS Coils

Abstract

Magnetization currents are known to be the cause of field disturbances in high-temperature superconducting REBCO magnets. The tape-like shape of REBCO-coated conductors implies that a magnetic field orthogonal to their flat surface can induce large screening current loops, which in turn generate a screening current-induced field (SCIF). This SCIF results in detrimental effects on the overall field generated by the magnet, lowering its amplitude and disturbing both the field quality and time stability. This paper focuses on the experimental investigation of two techniques for suppressing the detrimental SCIF effects. First, vortex shaking uses an auxiliary ac magnetic field of small amplitude to accelerate the relaxation of the current density in the superconductor, thus, erasing the SCIF in a short period of time. The second technique, the current overshoot, consists in charging the magnet to its nominal operating point with an initial overshoot, which creates a favorable initial current density distribution mitigating the time drift observed on the field. Those techniques were tested on small REBCO single pancake coils at 77 K with different kinds of insulation: full Kapton insulation and metallic cowinding.