Division of accelerators, cryogenics and magnetism

Coated conductors have high superconducting critical current density (J_c) under high field and high tolerance against high mechanical stress. The properties make coated conductors a promising candidate of a construction material of high-field superconducting magnets. On the other hand, coated conductors with high aspect ratios are magnetized by magnetic flux penetrating perpendicularly to the tape surface. Then screening current flows in the coated conductors. The screening current influences magnetic field generated by a magnet consisting of coated conductors in three following ways: reduction of magnetic flux density at center position, spatial homogeneity of magnetic field and temporal stability of magnetic field. The influence should be taken into design of magnets for Nuclear Magnetic Resonance (NMR) spectrometers and Magnetic Resonance Imaging (MRI) devices. This paper presents magnetic field distributions generated by screening current flowing in short coated conductors and coils.

We first measured magnetic field distributions generated by screening and transport currents flowing in a single short coated conductor in liquid helium under external magnetic field. A superconducting magnet applied the external magnetic field. Screening-current-induced field (SCF) increases at the external magnetic field from 0 T to a penetration field, where magnetic flux penetrates over the width of the coated conductor. Over the penetration field, the SCF decreases according to magnetic field dependence of J_c. The magnetic field distributions give current distributions of screening and transport currents via inverse problems. The measurement using a single coated conductor reveals that a ratio of a transport current relative to superconducting critical current determines a current distribution in a short coated conductor under external magnetic field higher than penetration field.

As numbers of superimposed coated conductors increase, intensities of SCF increase. Numbers of rows in which coated conductors were arranged varies distributions of SCF. Based on the current distributions of a single coated conductor, we estimated the magnetic field distributions by screening and transport currents flowing in multiple short coated conductors and then compared them with the experimental results. In addition, temporal variations of SCF for the multiple coated conductors were observed at a few kinds of temperatures. The variations were proportional to the logarithm of time.

Based on the current distributions in a single coated conductor under magnetic field, magnetic field distributions generated by pancake YBCO coils were estimated. Then, methods for reducing the SCF were suggested.