The prototype for a nitrogen-cooled high-T, SQUID gradiometer has been developed and is being evaluated for magnetic anomaly detection of underwater targets in mobile surveys. The prototype's design is based on the concept of the Three-Sensor Gradiometer (TSG). In the TSG approach, balance of two independent SQUID magnetometers is more difficult to attain than for conventional low-T, gradiometers in which signal subtraction occurs prior to a single SQUID stage. Experiments have been conducted using a platform-motion simulator to evaluate performance of this gradiometer for mobile operation. Sensor configuration, experimental procedures, approaches for improved performance, and empirical results are reported. Interesting results of predictions to estimate detection range obtained from matched-filter calculations are included. The paper concludes with a description of current preparations for a sea test of this sensor and a perspective of future developments.
Abstract. Passive magnetic sensors provide one means to conduct mobile area surveys and search operations, useful for a number of applications, including sea mine countermeasures and the detection of unexploded ordnance and packaged biological, chemical and radioactive waste for environmental cleanup. To date, the generally accepted method for such detection involves the generation of two-or three-dimensional magnetic anomaly field maps, using primarily total-field magnetometers. Sensor configurations measuring spatial gradients of magnetic field offer a new opportunity for better localization and classification. Sensors incorporating Superconducting Quantum Interference Devices. (SQUIDs) provide the greatest sensitivity available with current technology for magnetic anomaly detection. During the late 1970's and early 1980's, the Naval Surface Warfare Center Coastal Systems Station (CSS) developed the Superconducting GradiometerlMagnetometer Sensor (SGMS) specifically for mobile operations outside the laboratory environment This sensor technology utilized niobium superconducting components cooled by liquid helium. The SGMS has demonstrated rugged, reliable performance even onboard airborne and undersea towed platforms. In this article, a general perspective for the use of passive magnetic sensors for mobile operations will be established. The SGMS design will be described in some detail. General design principles underlying its mobile application, fundamental sensor and environmental noise issues, and approaches to compensate for them, will be presented. The magnetic sensor detection and classification concept developed for sea mine countermeasures and results from that demonstration will be discussed. Recent developments and future opportunities, especially encompassing the use of high temperature (high-Tc) superconducting components cooled by liquid nitrogen, will be addressed.
A dc superconducting quantum interference device (SQUID) modulation and feedback circuit operating at a modulation frequency of 16 MHz has been constructed. Using a novel wide band superconducting thin film transformer to impedance match the SQUID to a rf amplifier allows the system to operate at the SQUID noise level for most types of low-TC SQUIDs. This system has a closed loop bandwidth exceeding 2.5 MHz and a slew rate greater than 1×106Φ0/s at frequencies up to 1 MHz. This greatly improved performance compared to existing modulation methods can be obtained without enhancing the transfer function of the SQUID. The system allows low- and high-TC SQUID magnetometers and gradiometers to be operated totally unshielded without unlocking in the dc, 60 Hz, and radio frequency electromagnetic fields present in most SQUID applications.
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