Jc Measurements and the Observation of Flux Movements and Flux Jumps in Quench-and-Melt Growth YBCO by a Pulsed Magnetic Field

An observation of the flux motion in a ring-shaped quench-and-melt growth (QMG) YBCO has been attempted under a pulsed magnetic field in a temperature range from T c down to 25 K. The transport critical current density (J c) is estimated from the magnetic field where the flux just enters into the central hole of the sample ring. At low temperatures, the flux jump is clearly observed and its interpretation is made in terms of the traditional stabilization theory. In a thin sample, a peculiar flux motion suggesting the undulation of flux lines is observed.