Enhanced magnetic anomaly detection using a nitrogen-cooled superconducting gradiometer

During the 1980's, the Superconducting Gradiometer/Magnetometer Sensor (SGMS) was demonstrated in the Magnetic and Acoustic Detection of Mines Advanced Technology Demonstration (MADOM ATD) to provide effective mine detection, localization, and classification capabilities, especially against buried mines, and to reduce significantly acoustic false alarms arising from bottom clutter. This sensor utilized Superconducting Quantum Interference Devices (SQUIDs) manufactured using the low critical temperature (low T(c)) superconductor niobium and liquid helium for sensor cooling. This sensor has most recently been integrated into the Mobile Underwater Debris Survey System (MUDSS) and has been demonstrated successfully in a survey to locate unexploded ordnance in coastal waters. Based on recent advances in material and device technology, a high T(c) superconducting gradiometer (HTSG) feasibility prototype has been developed and is being evaluated. In comparison to the SGMS, the nitrogen cooling for the HTSG in place of helium cooling makes it possible to substantially reduce package sizes and to effectively eliminate a cryogen logistics trail. Excellent motion performance has been demonstrated for the HTSG, with sensitivities better than that of conventional non-superconducting technology and approaching that of its helium-cooled counterpart, the SGMS. Based on ground testing, HTSG sensitivity in motion exceeds that of its low Tc counterpart for frequencies greater than 3 Hertz. Advanced concepts for sensor refrigeration using compact dewars and active cryocoolers, which have the appeal of turnkey operation totally eliminating any requirement for cryogen logistics and support, are now emerging. Development of a compact high Te gradiometer free of cryogenic logistics now appears achievable.