Advances in the Use of LTS and HTS SQUIDS in Electromagnetic NDE

Of the electromagnetic sensors currently under investigation for nondestructive evaluation (NDE), the superconducting quantum interference device (SQUID) arguably has the greatest potential. The characteristics [1] which make it suitable for eddy current NDE are: high sensitivity even in large ambient fields (detection of sub-nT signals); operation from very low frequencies (a few Hz or less) to very high frequencies (potentially MHz) permitting detection of surface and subsurface flaws; and high spatial resolution. Spatial resolution is related to the physical size of the device, which is often less than 1 mm square, even when the need to maintain its other properties is taken into account. This often allows the SQUID to be treated theoretically and practically as an ideal point sensor. However, it must be operated in a cryogenic environment: low temperature superconductor (LTS) SQUIDs need liquid helium and liquid nitrogen (LN2) is needed even for high temperature superconductor (HTS) SQUIDs. This makes it difficult to reduce the specimen-to-sensor stand-off below approximately 1 mm.</p

Magnetoencephalography Using High Temperature rf SQUIDs

We have developed high-critical-temperature radio-frequency Super conducting QUantum Interference Devices (SQUIDs) with step-edge grain-boundary Josephson junctions and large flux focusers. These planar devices were fabricated from epitaxial YBa2Cu3O7 films and operated in the magnetometer and first-order gradiometer configurations while immersed in liquid nitrogen. At the temperature of 77K, we have attained a magnetic field resolution for the magnetometer better than 200 fT/Hz1/2 down to less than 1 Hz, i.e., over the low signal frequency range important for medical diagnostics. The results to date show a high promise for biomagnetic diagnostics. For the first time, we recorded the evoked responses from human brains using a high-temperature magnetometer and a first-order electronic gradiometer channel simultaneously. These results were obtained in a magnetically shielded room. An improvement in the magnetic field resolution by another order of magnitude is possible and probable