L. Dörrer

Planar gradiometers with high- DC SQUIDs for non-destructive testing

We investigated planar gradiometers on the basis of galvanically coupled high- DC SQUIDs for application in non-destructive testing (NDT). The stability of the sensor of better than 1% permits NDT investigations in unshielded environments. The layout of the gradiometer sensors and a corresponding determination of their effective areas and baselines will be discussed as well as the properties of the Josephson junctions in the DC SQUID based on step-edge and bicrystal grain boundaries. We show measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as well as the influence of sensor position and experimental environment such as the dewar material on the performance of the sensor and the whole testing system. As an example first investigations of the permanent magnetization of hardened valves will be presented where we look for a correlation between hardening and magnetic field distribution.

High-Tc SQUID systems for practical use

Planar thin film DC-SQUID gradiometers with small base length (4...6 mm) were optimized for application in different measurement systems. The field gradient noise of these planar DC-SQUID gradiometers in unshielded environment is better than 5 pT/cm/spl radic/(Hz) (at 1 Hz, 77 K). Other components of these systems like electronics, cryostats, scanning equipment, and data analysis software are discussed. An industrial system for nondestructive evaluation and a clinical system for bedside investigations on cardiac infarction are demonstrated as examples of starting cooperation between university and small companies interested in application of superconductivity.

Improvement of sensor performance of high-T/sub C/ thin film planar SQUID gradiometers by ion beam etching

The sensor performance of galvanically coupled Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) dc SQUID gradiometers on 24/spl deg/ bicrystal substrates has been improved by thickness reduction in the region of the grain boundary Josephson junctions using ion beam etching. The prepared etching mask allows the reduction of the critical current by more than one order of magnitude while the SQUID inductance is slightly increased. This treatment shifts the SQUID parameter /spl beta//sub L/ from values above 10 to the proposed optimum around 1. The authors observed with decreasing critical current and increasing normal resistance a reduced I/sub C/R/sub N/ product with values between 300 and 400 /spl mu/V at 150-nm film thickness changing to values near 150 /spl mu/V at 50-nm film thickness. Despite this fact, the white flux noise level as well as the low-frequency noise is reduced. With their galvanically coupled 4/spl times/8 mm/sup 2/ dc SQUID gradiometer the authors obtained a white noise level of 4.2 /spl mu//spl Phi//sub 0///spl radic/Hz corresponding to a field gradient sensitivity of 430 fT/cm/spl radic/Hz at 77 K after the trimming process.

High-T/sub c/ DC-SQUID system for nondestructive evaluation

We developed different types of thin film high temperature superconductor (high-T/sub c/) DC-SQUID magnetometers and gradiometers for application in a nondestructive testing system. We used these sensors in a liquid nitrogen dewar mounted above a computer controlled x-y table within a compact /spl mu/-metal shielded chamber. The planar galvanically coupled gradiometers based on step-edge or bicrystal Josephson junctions were investigated in shielded and unshielded environment. We discuss the influence of junction type and SQUID parameters depending on the device geometry on the sensitivity of our complete testing system. Our interest is focused in particular on the noise properties and magnetic field resolution at 77 K. Measurements of ac and dc current distributions will be shown as well as measurements of magnetic moments of different samples.

Development and investigation of novel single-layer gradiometers using highly balanced gradiometric SQUIDs

The sensitivity of galvanically coupled single-layer gradiometers on bicrystal substrates for homogeneous magnetic fields can be minimized by an increase of the balance, which is the ratio of the effective area of the antennas and the effective area of the SQUID. In this paper, we describe possible realizations of new gradiometric SQUIDs (G-SQUIDs) on 10 × 10 mm2 STO-bicrystal substrates, which minimize the effective area of the SQUID, and therefore the parasitic area of the gradiometer, to 34 μm2. We integrate these gradiometric SQUIDs in well-defined gradiometer antennas. The G-SQUID layouts with coupling inductances between 20 pH and 100 pH do not show any dependence of the parasitic area from the SQUID inductance. With a gradient field resolution of 1.6 pT(cm√Hz)−1 (white, unshielded) at 24 pH and 1.8 pT(cm√Hz)−1 at 55 pH, these gradiometers possess outstanding noise characteristics. So the balance of comparable conventional gradiometers, which show values of about 100, can be increased by more than an order of magnitude to 1070 with the new layouts. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

Development of a heart monitoring system based on thin film high-T/sub c/ DC-SQUIDs

Our aim is to built up an one-channel heart monitoring system to achieve fast and reliable information about the patients condition as well in emergency cases as in serial examinations by a non-invasive and touchless method. Step-edge and bicrystal junctions were used to prepare planar galvanically coupled thin film gradiometers and magnetometers. These sensors were used to carry out measurements in well-shielded and weak disturbed environment and we compared the different noise properties and field gradient resolutions. The influence of antenna configuration, SQUID parameter, cooling conditions, and different kinds of signal analysis will be discussed.

Improvement of spatial and field resolution in NDE systems using superconducting sensors

We present a NDE scanning system for industrial purposes in magnetically and electrically unshielded environment. In this scanning system, planar galvanically coupled dc SQUID gradiometers with a field gradient resolution of 300 fT.(cmHz/sup 1/2 /) in the white noise region are used. The spatial resolution is above 1 mm in this case. On the other hand many applications in NDE do not require the extraordinary sensitivity of SQUID based sensors. For those purposes hybrid sensors, in which Hall sensors are connected with high-T/sub c/ superconducting antennas, are an alternative sensor concept less sensitive than SQUID sensors but exhibiting better performance compared to other commercially available magnetic field sensors.

Development of planar thin film HTSC–SQUID gradiometers for different applications

Abstract We investigated the main dependencies of the gradient resolution of planar galvanometer superconducting quantum interference device (SQUID) gradiometers made of YBa 2 Cu 3 O 7-x (YBCO). We focussed especially on the influence of antenna layout and the parameters of the galvanometer SQUID such as effective and parasitic areas on the performance of the gradiometer and the behavior in an unshielded environment. The efficiency (that is, the quotient of effective area to inductance) of different geometries of antennas will be compared. Some aspects of the layout of the galvanometer SQUIDs are discussed in terms of parasitic area and best current resolution. Special problems due to the use of bicrystal Josephson junctions in gradiometers for operation in highly disturbed environment are shown. Step-edge Josephson junctions can offer alternative concepts. Reached gradient sensitivity values in the white noise region are 0.46 pT/(cm/ H z) in the case of bicrystal junctions and 0.69 pT/(cm/ H z) for step-edge junctions.

Universal active dc biasing system for a high-T/sub c/ SQUID based on a liquid-nitrogen-cooled preamplifier

A system for the SQUID active dc biasing was developed and tested. This system provides an opportunity to change the output resistance of the biasing source over a wide range. The white flux-noise of a bicrystal SQUID-gradiometer has been measured in direct readout scheme with three different values of the biasing source output resistance. A maximum level of white noise of 11 /spl mu//spl Phi//sub 0//Hz/sup 1/2/@100 kHz was found in case of maximum output resistance (current-biasing). A white noise of 7,9 /spl mu//spl Phi//sub 0/Hz/sup 1/2/ was measured for the minimum of the biasing source resistance (voltage-biasing). The best level of the flux noise of 6 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ was found at a biasing source resistance close to SQUID normal resistance R/sub n/ (power-biasing). Significant reduction of the cut-off frequency (near 10 kHz instead of 100 kHz) was found for the tested bicrystal SQUID-gradiometers due to the active dc voltage-and power-biasing systems in comparison with the standard current-biasing circuit.

Development of a heart monitoring system with high-T c DC-SQUID gradiometers

We investigated different types of high-Tc DC-SQUID gradiometers with the aim to develop an one-channel heart monitoring system for non-invasive measurements in unshielded environment. This system could be used to obtain information about the patients condition in serial examinations and in emergency cases fast and stable. Step-edge and bicrystal Josephson junctions were used to prepare planar galvanically coupled thin film gradiometers and magnetometers. These sensors were used to carry out measurements in well-shielded and non-shielded environment and we compared the different noise properties and field gradient resolutions. With different types and sizes of high-Tc DC-SQUID sensors magnetocardiograms were measured in a magnetically shielded room. The influence of antenna configurations, SQUID parameters, and cooling conditions on the measurement of magnetocardiograms will be discussed. We investigated possibilities to suppress the earth magnetic field (with pairs of Helmholtz coils) and for noise field compensation in unshielded environment.