A SQUID NDE system for the investigation of pinch welds

HTS SQUID NDE systems attempt to achieve both high spatial resolution and high signal sensitivity, though often the more important parameter is determined by the sample of interest. SQUID microscopes offer micron-sized resolution by either bringing the sample into very close contact with a small SQUID sensor or by using a 'flux guide' that is located in the tail of the cryostat. Conventional SQUID NDE systems typically have a larger sensor cooled by a liquid cryogen (or possibly a cryocooler) and place more import on signal resolution. Our system is a hybrid of the above ideas, developed specifically to permit the characterisation of small, irregular-shaped stainless steel samples containing pinch welds. We have investigated using a ferromagnetic flux guide with our system, but for convenience have simply attached it to the tail of the cryostat 4 mm below a washer SQUID magnetometer. To increase the signal resolution we have chosen to employ local electromagnetic shielding around the measurement system. We detail the characteristics of the system and present experimental NDE data as individual time series traces of the SQUID output for a set of samples that were fabricated using a broad range of weld parameters