A SQUID-Based Nondestructive Evaluation System for Testing Wires of Arbitrary Length

The high field sensitivity of superconducting quantum interference devices (SQUIDs), especially at low frequencies, makes them ideally suited for applications in nondestructive evaluation. For testing of conducting wires, such as aluminum bond wire, we developed a special cryostat, which allows for pulling a wire of arbitrary length (which is kept at room temperature) through a niobium flux transformer connected to a niobium dc SQUID. The wire is excited by either passing an alternating current through it, or by exciting eddy-currents in the wire. The cryostat is made from a stainless steel inner vessel; the outer tube is from fiberglass. The gradiometric pickup loops are wound on a German-silver tube. As the wire under test is at room temperature, thermal noise produced by the wire is limiting the sensitivity of the system, rather than thermal noise produced by the stainless steel dewar.