A method to evaluate cracks in a superconductive cylinder used to shield weak magnetic fields

To survey the local superconductivity and cracks in a large-size superconductor, a method for estimating the property from the difference (real part ΔR and imaginary part ΔX) in the complex impedance of a coil with a sample and another without one is proposed This method was applied to two samples which were superconductive and not superconductive at 77.3 K. It is understood that samples become superconductive when ΔR is under 0.0033 Ω and ΔX is negative. The relation between two superconductors, one without and the other with artificially induced cracks was investigated. It was confirmed that the property of the sample with cracks deviated from superconductivity. As for a superconductive cylinder of 500 mm in diameter and 1 m in length open at one end, ΔR at various points on the wall and at the closed end were measured at 77.3 K and the maximum values of ΔR were 0.001 Ω. The effect of the magnetic shielding measured along the axis of the cylinder agreed fairly well with the calculated values