An ac susceptometer for the characterization of large, bulk superconducting samples

The main purpose of this work was to design, develop and construct a simple, low-cost ac susceptometer to measure large, bulk superconducting samples (up to 32 mm in diameter) in the temperature range 78-120 K. The design incorporates a double heating system that enables a high heating rate (25 K h) while maintaining a small temperature gradient (<0.2 K) across the sample. The apparatus can be calibrated precisely using a copper coil connected in series with the primary coil. The system has been used successfully to measure the temperature dependence of the ac magnetic properties of entire RE-Ba-Cu-O [(RE)BCO] bulk superconducting domains. A typical ac susceptibility measurement run from 78 K to 95 K takes about 2 h, with excellent temperature resolution (temperature step ~ 4 mK) around the critical temperature, in particular.