Nondestructive testing of niobium sheets for superconducting resonators

We have developed a LTS SQUID system for eddy-current testing of niobium sheets used to fabricate superconducting resonators for particle accelerators. Since the fabrication of superconducting resonators from planar niobium sheets is very costly, a measurement procedure is required which can test the niobium sheets before the resonator is made. Our system can detect relevant surface damage or inclusions of foreign material having a volume of as small as 10^-12 m³ in a test sample made from high-purity niobium. Due to the relatively high frequency of the eddy currents of up to 100 kHz, the system-although employing a magnetometer-can be operated in an unshielded environment. The SQUID readout uses 4 MHz AC-flux modulation; a peak-to-peak dynamic range of 15 fluxquanta is obtained at 100 kHz.