Critical Current Degradation in Wide Samples of High-Field Strip Superconductors

Abstract

Measurements of the magnetic anisotropy of critical current in samples wide enough for use in large magnets reveal a degradation effect in strip superconductors. The effect is manifested as a truncation or ``chopping off'' of the peaks of the anisotropy curves of these samples. With narrow samples the anisotropy curves (average critical current density plotted as a function of magnetic field direction) normally display a well‐defined peak centered about the direction parallel to the rolling plane of the strip. With wide samples, however, this peak appears progressively more truncated as the width is increased, and the truncation level (i.e., maximum current density) is roughly inversely proportional to the width, so that the total current tends to remain constant, independent of width. For Nb‐25% Zr strip 0.051 mm thick, this limiting current is approximately 125 A at a field of 4 Wb/m² for widths between 1 and 10 mm. 1.32‐mm‐wide samples display little or no truncation and have maximum current densities of 1.8×10⁵ A/cm², whereas 10‐mm‐wide samples show severe truncation and have maximum current densities of 2.6×10⁴ A/cm².