High-transport current density up to 30 T in bulk YBa2Cu3O7 and the critical angle effect

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Abstract
Measurements of the dc transportcritical current of oriented‐grained YBa2Cu3O7 have been made using high quality Ag contacts and a high‐current sample mount. The critical‐ current densityJ c at 77 K for mutually perpendicular current and magnetic field B in the a,b plane is 8 kA/cm2 at 8 T, decreasing gradually to 3.7 kA/cm2 at 20 T, and remaining over 1 kA/cm2 out to 30 T. High magnetic field measurements of J c as a function of the angle θ of B with respect to the c axis are also reported. In contrast to earlier results at lower fields (<3 T) the measurements reported here in high fields reveal a J c vs θ curve with a head‐and‐shoulders shape, consisting of a sharp peak (‘‘head’’) <5° wide for B parallel to the CuO2 planes, and a wide (30° at 9 T, for example) shoulder region on either side of B⊥c, where the transportJ c remains high and constant. Beyond the shoulder region, however, the transportJ c decreases sharply, giving rise to the concept of a critical field angle for application design, defined by the minima in d 2 J c /dθ2 at the edge of the shoulders.