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−1) 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.