Keith Leslie

Field trials using HTS SQUID magnetometers for ground-based and airborne geophysical applications

Since December 1992, CSIRO and BHP have been field trialing rf HTS SQUID magnetometers for mineral prospecting applications. Ten field trials in widely varying environments(from -16/spl deg/C to +40/spl deg/C ambient temperatures) in mostly remote locations saw the development of a system which can be operated in many configurations including ground based and airborne Transient ElectroMagnetics (TEM). The magnetometer system has been developed to a point where, at late times in TEM applications, the SQUID system has a higher signal-to-noise level than the competing traditional coil technology. In some trials, a SQUID magnetometer detected anomalies at later times than were observed with the coil system, indicating an enhanced ability to detect highly conductive targets. This paper reviews development of our 3-axis SQUID magnetometer. SQUID systems as B field sensors have advantages over coils which are dB/dt type sensors. We will discuss the importance of these advantages for mineral prospecting in regions with a conducting soil cover or overburden typical of the Australian landscape.

Terahertz imaging at 77 K

Terahertz (THz) technology is receiving increasing attention around the world due to its important potential in many application areas. Novel compact solid-state sources and detectors are being sought for?THz radiation and detection. We report the realization of a?THz imager based on a high- Tc superconducting (HTS) Josephson detector working above liquid nitrogen temperature (77?K). The detector, made of a YBa2Cu3O7?x (YBCO) step-edge Josephson junction, is coupled to a thin-film ring-slot antenna and a hemispheric silicon lens. Images of high visual quality are obtained which demonstrate unique properties of?THz radiation such as the sensitivity to water content and the ability to penetrate packaging materials. The results should stimulate further research leading to the development of a HTS superconducting?THz imaging system.

Terahertz imaging using a high-Tc superconducting Josephson junction detector

A high-Tc superconducting (HTS) detector based on a YBa2Cu3O7−x (YBCO) step-edge Josephson junction has been developed and applied to terahertz (THz) detection. The detector was coupled to a ring-slot antenna designed for operation at 600 GHz, and used for THz imaging. The results suggest that the characteristic voltage and frequency of our HTS step-edge junctions can be readily optimized for the chosen THz frequency range at easily achievable temperatures. The images also clearly demonstrate some of the unique properties of THz radiation, including the sensitivity to water content and the ability to penetrate packaging materials. (Some figures in this article are in colour only in the electronic version)

A SQUID-based metal detector?comparison to coil and x-ray systems

The presence of foreign metal bodies and fragments in foodstuff and pharmaceutical products is of major concern to producers. Further, hidden metal objects can pose threats to security. In particular, stainless steel is difficult to detect by conventional coil metal detectors due to its low conductivity. We have employed an HTS SQUID magnetometer for the detection of stainless steel particles which is based on the measurement of the remanent magnetization of the particle. Our aim was to determine the detection limits of HTS SQUID-based remote magnetometry, especially for food inspection purposes, and to make a comparison of this technique to commonly used eddy current coil and x-ray inspection systems. We show that the SQUID systems sensitivity to stainless steel fragments is significantly higher than that of coil systems if the samples are magnetized in a 100 mT magnetic field prior to detection. Further, it has a higher sensitivity than x-ray systems, depending on the density distribution of the product under inspection. A 0.6 mg piece of grade-316 stainless steel (a fragment of a hypodermic needle 0.5 mm long and 0.65 mm diameter) represents the detection limit of our system with a 150 × 150 mm2 inspection orifice.

Operation of a geophysical HTS SQUID system in sub-Arctic environments

Transient ElectroMagnetic geophysical prospecting using SQUID sensors has demonstrated potential for improved target detection at late response times compared to conventional coil sensors. We have developed a three-axis, rf SQUID sensor system which has been extensively operated in sub-Arctic conditions by a geophysics contractor. Due to the harsh environmental and operating conditions, the system is designed to operate in sub-zero temperatures (as low as minus 40/spl deg/C) and is ruggedly packaged whilst still remaining quite portable. Auto-tuning of the rf electronics has been implemented by adjusting the rf SQUID control parameters via a microprocessor controller. After a small amount of training, regular field crews have operated two of these systems in the field continuously for periods of months at a time. An example, comparing SQUID B field data to coil dB/dt data, is presented in this paper.

Self-field ac loss of Bi-2223 superconducting tapes

The self-field ac loss of a monofilamentary Bi-2223/Ag tape has been measured using an improved lock-in amplifier technique. To investigate the dependence of the ac power loss on the phase-angle error of the lock-in amplifier, theoretical calculations of the ac power loss of a flat superconducting strip in a perpendicular ac magnetic field were performed. The expressions derived also predict the dependence of the apparent loss on the size of the lead extension of the voltage contacts. For zero phase-angle error and an infinite lead extension the Norris formula is obtained. The voltage wave-form for ac current amplitudes greater than the tapes critical current has been measured and calculated. Theoretical predictions agree well with the experimental data.

Airborne TEM surveying with a SQUID magnetometer sensor

Traditionally airborne time-domain electromagnetic (AEM) survey systems use induction coils as the sensor (receiver). We have replaced the induction coil in a transient electromagnetic (TEM) system with a liquid-nitrogen cooled superconducting quantum interference device (SQUID) magnetometer sensor. Using this prototype system, we aimed to improve performance in detecting conductive mineralization, particularly where the conductive mineralization of interest is covered by a conductive regolith. We successfully demonstrated one- and three-component SQUID sensors in airborne TEM surveying, and achieved performance comparable to the induction-coil systems.Implementation of the SQUID system required development of devices capable of operating in magnetically unshielded environments with low noise, high slew rate, and wide bandwidth. Operation of the SQUID sensor in the highly dynamic environment of a towed bird was also necessary, and this implies a high dynamic range and high level of noise associated with t...

Geophysical exploration using magnetic gradiometry based on HTS SQUIDs

Magnetic tensor gradiometry provides gradient components of true potential fields which enables unique depth estimates and improves analytic signal methods as well as providing a number of other advantages. A high temperature SQUID (HTS) gradiometer can provide measurements of the components of the earths field tensor creating a new tool for mineral exploration. A successful comparison between a HTS SQUID gradiometer and a Cs-vapour gradiometer under survey conditions has been conducted. Both instruments were configured vertically. The HTS gradiometer measured the B/sub zz/ component of the gradient tensor, while the Cs-vapor gradiometer measured the vertical gradient of the total magnetic intensity. The HTS gradient measurement was the difference in output between two coaxial SQUID sensors. Effective noise levels achieved were 0.16-0.3 nT/m RMS, compared with 0.1-0.5 nT/m RMS for the Cs-vapor system. The SQUID noise was dominated by vibration with additional contributions from the multiplexed sampling between the SQUIDs. This paper reports on the system development, design issues, trial results and the implications for geophysical exploration.

Axial high-temperature superconducting gradiometer with a flexibleflux transformer

An axial first-order gradiometer is formed by coupling the input coil of a flexible high-temperature superconducting flux transformer inductively to a directly coupled superconducting quantum interference device magnetometer. The transformer is patterned in a single-layer YBa2Cu3O7−x film on a flexible Hastelloy tape. The tape is bent such that the two outer pickup loops of the transformer are facing each other while perpendicular to the magnetometer plane resulting in a gradiometer baseline of 35mm. A superconducting shield is mechanically adjusted to reduce the gradiometer response to uniform fields applied perpendicularly to both the magnetometer plane and the plane of the transformer pickup loops, by a factor of typically 7000.

Application of high-temperature superconductor SQUIDs for ground-based TEM

Superconducting quantum interference devices (SQUIDs) are intrinsically very sensitive detectors of magnetic flux. Flux sensitivities of one millionth of a flux quantum per root Hz (1μφ0/√Hz) may typically be realized in low-temperature superconductor (LTS) materials, while sensitivities of ∼5μφ0/√Hz may be realized in high-temperature superconductor (HTS) materials. LTS devices are typically cooled with liquid helium (4 K) while HTS devices are typically cooled with liquid nitrogen (77 K). Coupling the magnetic field into a SQUID via a flux-transformer can result in a very sensitive magnetometer with, depending on the type of superconducting material used and the effective area of the flux-coupling transformer, achievable magnetic field sensitivities ranging from fT/√Hz to pT/√Hz over typical bandwidths that span hundreds of kHz. SQUID applications include NDE, biomagnetism and magnetic microscopes.