Applications of high-temperature SQUIDs

Applied Superconductivity Vol. 3, No. 7-10, pp. 367-381, 1995 Copyright 0 1996 Else&r Science Ltd

Pergamon

APPLICATIONS

0964-1807(95)00053-4

Printed in

OF HIGH-TEMPERATURE

Great

Britain. All rights reserved 0964-1087195 $9.50 + 0.00

SQUIDS

Y. ZHANG, Y. TAVRIN, H. -J. KRAUSE, H. BOUSACK, A. I. BRAGINSKI, U. KALBERKAMP,’ U. MATZANDER,’ M. BURGHOFF and L. TRAHMS3 Institut fiir Schicht- und Ionentechnik, Forschungszentrum Jiilich GmbH (KFA), 52425 Jiilich, Germany; ‘Institut fir Angewandte Geophysik, Technische Universitiit Berlin, Ackerstr. 71-76, 13355 Berlin, Germany; ‘Metronix, Neue Knockenhauerstr. 5, 38100 Braunschweig, Germany; 3Physikalisch-Technische Bundesanstalt (PTB), 12587 Berlin, Germany Abstract-Using examples from our recent work, we describe the progress toward three types of applications of SQUID magnetometers and gradiometers operating at the temperature of liquid nitrogen, and using high temperature superconductor (HTS) YBa2Cu-,07 thin-film sensors. First, we demonstrated usefulness of our magnetocardiographic data for diagnosis of cardiac arrhythmia and determination of risk of sudden cardiac death by means of a new method. This was achieved in a shielded room using our sensitive HTS magnetometer, in a back-to-back comparison with a state-of-theart low-temperature (LTS) magnetometer. Second, using an eddy-current technique in conjunction with our HTS gradiometer in unshielded environment, we demonstrated detection of flaws located very deep, e.g. 36 mm, below the surfaces of stacks of aluminium sheets. This indicated a high potential for nondestructive material evaluation (NDE) of, e.g. aging airplane structures. Third, we showed the applicability of an HTS magnetometer for electromagnetic geological prospecting. A successful field test was conducted using the controlled-source audio magnetotellurics method, with reference to a commercial induction coil system. In these three areas of applications, HTS SQUID systems are expected to soon gain user and market acceptance.

INTRODUCTION

The SQUIDS (Superconducting Quantum Interference Devices) are the most sensitive detectors of magnetic flux, 0 (and field, B) known to mankind, especially useful at low frequencies of measured magnetic signal. They can be generally viewed as analog-to-digital converters of analog flux signals (quantized in units of the magnetic flux quantum, @, =2.0679 x lo-l5 Wb) to voltages. It is not our aim here to discuss the principles of such a SQUID sensor operation. An upto-date and comprehensive review of these can be found in ref. [ 13. Here, we view the SQUID as a complete system, which includes the sensor itself, the SQUID electronics, the mechanical support structure and cryogenic enclosure (dewar), and, the data acquisition/processing circuitry with installed software. We like to mention that the use of SQUID electronics with a magnetic flux feedback permits one to cancel the applied flux, and thus use the converter as a null detector. Such an analog device can resolve very small fractions of the flux quantum, down to 10e7 a0 in the case of operation of low-temperature-superconductor (LTS) SQUIDS at the liquid helium temperature. The purpose of this paper is to review the progress toward realistic applications of hightemperature-superconductor (HTS) SQUIDS using selected examples from our own current work. These examples will be limited to some of our results obtained in 1994, which did not appear in print at the time of this writing. Therefore, in a strict sense, it is not a review paper, as we did not intend to mention the work of others. A general survey of some LTS SQUID applications [2] is included in the same book as ref. [ 11. A most recent review of biomagnetic and materials testing applications, which include some considerations of applicability of HTS SQUIDS, can be found in ref. [3]. The principal advantages of HTS SQUIDS over their LTS counterpart are: the significant simplification of cryogenics with the ensuing reduction in investment and operating cost, and the simplification of handling and maintenance, including an increased portability. At present, all this 361

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is being achieved by replacing the liquid helium cryogen with liquid nitrogen, and thus operating at 77 K instead of 4.2 K. Small, reliable and inexpensive cryocoolers, which do not generate magnetic noise, remain a realistic hope for the future, especially in the case of portable SQUID applications. However, it must be clearly understood that the higher temperature of operation, and the properties of HTS materials, are and will be limiting the intrinsic and attainable sensitivity of the HTS sensor. The thermal (white) noise energy is at 77 K almost 20 times higher than at 4.2 K. One can thus only strive toward approaching at 77 K today’s sensitivities of typical LTS systems, which, fortunately, are far removed from the intrinsic noise barrier. At present, HTS SQUID sensors and systems are still in the phase of intensive development, in contrast to a relative maturity of LTS SQUIDS. The intrinsic and practical limits of HTS system performance are far from being attained. Our philosophy is to address in parallel the general SQUID magnetometer (gradiometer) systems development, and some of the specific applications for which such systems could be used. To make it possible, the KFA is teaming with groups and institutions vitally interested in using SQUID systems in one of three areas: biomagnetic diagnostics (medicine), nondestructive evaluation of materials (NDE), and geomagnetic exploration. Through such associations we hope to develop functional models and prototypes of systems, which will eventually gain a wide acceptance in the field, and thus in the marketplace. In this paper, we first introduce the few necessary system parameters, and describe the problem of magnetometry of very weak magnetic fields in a strong background field environment. We also briefly characterize the status of the SQUID performance attained to date, and define the system requirements (however, we do not describe the HTS SQUID design and properties). On this basis, we will describe examples of results attained in the three fields identified above. Finally, we present our conclusions, and an outlook for the future.

MAGNETOMETRY

System parameters

OF WEAK

MAGNETIC

FIELDS

and their magnitudes

The magnetic field, B, sensitivity of any magnetometer in a noise-free environment is primarily limited by the equivalent flux noise of the device, defined by its spectral energy density &,(f, T), in units of @i/Hz. The field sensitivity is determined by both the &, and the field-to-flux transformation coefficient aB/%D in units of tesla&. That coefficient is the reciprocal of the effective flux pickup area, A (since

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of high-temperature

SQUIDS

369

3

,: maximum field

1000

f

i

E. 2 3

l@+

sl

i=m

I

typical environmental noise in a lab

m E

s : m 2

100

1000

Frequenz [Hz] Fig.

1.

Example of the dynamic range of a KFA HTS SQUID system.

The rf vs dc SQUID system

Until now, we did not distinguish between an rf and dc SQUID system. It is known that the developments of the past 15 years concentrated on LTS dc SQUID systems with planar, Nbl AlO, thin film sensors. The record low &, and BN values mentioned above are characteristic of such (usually multichannel) systems. The main arguments against LTS rf systems has been a much worse energy resolution, E, typically E = 5 x 10Pz9 J/Hz vs 10e31 to 1O-32 J/Hz in dc SQUID, and correspondingly higher SQ’S due to a dominating noise contribution of the rf electronics and rf tank circuit, cable, etc. Also, the dangers of crosstalks between multiple rf channels, and a possibly much higher cost of multichannel rf electronics served as deterrents. Consequently, optimization of planar LTS rf sensors and attending electronic circuitry has not been pursued. For HTS systems, the situation is different. First, the intrinsically higher SQUID thermal noise energy at 77 K makes the relative noise contribution of rf electronics less dominant. While best attained (white) &,‘s are still higher in rf SQUIDS, their ability to work with higher loop inductances results, thus far, in comparable B N values. Furthermore, the rf SQUID eliminates by itself the very high current fluctuation noise originating in HTS Josephson junctions. In dc SQUID, this noise contribution is dominant at lower signal frequencies below 100-1000 Hz, and a special bias reversal circuitry is required to eliminate it. Indeed, until most recently, the lowest low-frequency BN values on record were those of rf SQUIDS. At this juncture it is also technologically easier to fabricate single-junction rf SQUIDS and trim their parameters to attain an optimum performance at 77 K. For these and other reasons, KFA has been pursuing system applications studies using rf SQUIDS with one-three channels. Of course, some of the traditional arguments for dc and against rf SQUID remain valid. A precise comparative analysis of the two alternatives on a system level is not yet in hand. The future will tell whether HTS technology will follow the path of LTS, and whether HTS rf SQUID will remain a valid alternative or not.

Measurements

in the presence

of background fields

The task of measuring very weak fields is tremendously complicated by the existence of much stronger fields in the environment. The earth’s magnetic field alone is 9-10 orders of magnitude higher than the intrinsic SQUID noise, and its fluctuations up to five orders higher. A laboratory noise level of lo-100 pTl&& is typical over the low frequency range (Fig. 2). Man-generated “cultural” environmental signals can readily approach ,uT levels, especially at

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=

agnetlc field

~I5

=

-I,

of the Earth

s

2

lo*

2 s t

lo*

1

10 Frequency mz]

100

Fig. 2. Schematic plot of low-frequency dependencies of environmental signals and HTS SQUID noise. Superposed are ranges of human heart and brain signals.

power line and harmonic frequencies. This situation is graphically illustrated in Fig. 2, where frequency dependencies of environmental fields, and the typical KFA HTS SQUID noise are schematically plotted between 0.1 and 1000 Hz. Superposed are the areas representing human heart and brain signals. For meaningful medical diagnostics it is necessary to resolve the lower limit of these signals with a high signal-to-noise ratio (SNR). In addition to low-frequency environmental signals, strong high-frequency signals of man-made sources (not shown) can also inhibit the SQUID system operation. For the measurement of biomagnetic and magnetic NDE signals in the frequency range between 1 and 1000 Hz, there are two standard ways to significantly suppress the environmental noise: (1) through the use of magnetic shielding, and (2) by gradiometry. In most applications, due to the large size of measured objects or subjects, shielding has to have the form of a magnetically shielded room (MSR). This is extremely expensive (typically between 0.1 and 1 million US$, and more, depending upon shielding performance), and represents one of the principal hurdles for SQUID acceptance in the field. For example, only in upper ranges of MSR performance and cost (low-frequency signal attenuation well in excess of 60 dB) can one hope to use a magnetometer for biomagnetic measurements. Many other applications are simply impractical in shielding. Consequently, the standard method of the SQUID trade (LTS and HTS) is to use gradiometry, even when magnetic field magnitudes rather than gradients are of interest. Since the useful signal sources are, as a rule, well localized, while most environmental signals are only very slowly varying in space, one can reject the undesired common mode by subtracting the flux picked up at two adjacent locations by nearly identical, rigidly mounted, and identically oriented antennae (pickup coils) [ 1, 21. First-order gradiometers are usually used in multi-channel LTS biomagnetic systems. In such LTS systems, differential gradiometric coils arrangements, axial or planar, with common mode rejection ratios (CMRR) of up to CMRR = 1O6have been employed. In the case of HTS, planar gradiometer coils patterned in multilayered epitaxial YBCO films can also be used effectively [9, lo]. As this is still technologically very difficult, we and others have been using axial arrangements of two or three SQUID sensors to attain CMRR between lo3 and lo4 in firstand second-order gradiometers [ll, 121. This is enough to perform diagnostic-quality biomagnetic measurements in an average-quality MSR. Narrow-bandwidth NDE measurements are possible even without any shielding (see the NDE section below). In the case of geophysical measurements, the desired frequency range of 10W4 to lo4 Hz must not be restricted by any shielding.

Applications COMPARATIVE

of high-temperature

SQUIDS

MAGNETOCARDIOGRAPHY

371

USING

HTS

SQUID

Sudden cardiac death risk stra@cation

Even though SQUIDS have been used to measure the biomagnetic fields of various human organs for about a quarter of a century, the technique has failed to become commonly accepted in medical diagnostics. There are at least two reasons. First, the advantage and/or complementarity of a biomagnetic method compared to commonly used methods, i.e. bioelectric, is in most cases until now not evident in the clinical practice and still has to be evaluated by a representative number of cardiological centers. Second, the measurements require liquid helium to cool the LTSSQUIDS, and a magnetically shielded room to suppress environmental magnetic noise. Both are not available in most hospitals and clinics. This situation might change due to availability of SQUIDS having sufficient sensitivity that require only the commonly available liquid nitrogen as a coolant. There is now a change to implement biomagnetic methods with possible advantages over and complementarity to other functional and imaging diagnostic methods. Each year in the US, 300,000400,000 people die because of arrhythmias (sudden cardiac death) [13]. An estimated 20% of the casualties in Europe is attributed to sudden cardiac death [ 141. Of these people, approximately 40% have no prior medical history indicating the risk [ 151. A simple detecting method for latent arrhythmias and an ensuing appropriate medical treatment can probably rescue a significant number of these people. There are strong indications that the plot of the biomagnetic field of the heart in time domain, a magnetocardiogram (MCG), offers additional or new information when compared to the electrocardiogram (ECG) [ 16-181. The biomagnetic laboratory of the Physikalisch-Technische Bundesanstalt (PTB) and the Benjamin Franklin Hospital in Berlin carried out studies with LTS-SQUIDS on arrythmia patients [ 191 and developed a method which has the potential for identifying the risk of a sudden cardiac death. Their MCG results to date appear more significant than those from the simultaneously recorded ECG. This method is based on a non-recursive binomial filtering of the so-called QRS-complex in the MCG (bandwidth 250 Hz) and quantifying the fragmentation of the filtered QRS-complex as a risk index for the danger of a sudden cardiac death. A confirmation by recording data from a very large pool of patients is still required. Comparative

A4CG and binomial

non-recursive

filtering

data

In our study, first attempts were made to utilize HTS-SQUIDS for testing this method. In the Berlin Magnetically Shielded Room (BMSR) of PTB [20], a one-channel HTS magnetometer 5or’ a

1

2

3 Time

[s]

Time

[s]

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2,4

2,6

Time

2,6

[s]

-lOO1 0

1

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2,2

2,4 Time

2,6 [s]

Fig. 3. Comparison of LTS (a, c) vs HTS (b, d) SQUID system in the BMSR, recording human real time MCG, (a, b) over several heart beats, (c, d) over one heart beat.

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from KFA with 40 fT/fi field resolution [21] and a single channel of a 37-channel LTS system with 9 fTI& field resolution [22] were used for comparative MCG measurement of a healthy person and an arrhythmia patient [23]. The MCGs of the healthy person (Fig. 3) showed a R-peak field intensity of 180 pT (HTS) and 82 pT (LTS). The signal-to-noise-ratio (S/N) was 30 (HTS) and 60 (LTS). Although the field resolutions of the systems differed by a factor of four, the S/N differed only by a factor of two. This was due to the reduced distance between the HTS-SQUID and the thorax, resulting from the thinner thermo-insulation of the HTS cryostat: 8 mm compared to 28 mm in the LTS cryostat. The fragmentation scores from recursive binomial filtering of averaged data agreed within an error margin of 20%, in part due to nonidentical magnetometer positions over the thorax. Recently, we again compared the same HTS system with a single channel LTS magnetometer from PTB, this one with only 10.7 mm distance between SQUID and room temperature, and a field resolution of 4 fflfi. With both systems, we conducted MCG measurements at nearly identical positions above the thorax of an arrhythmia patient and a healthy person. By signal averaging over 100 beats, the S/N ratio was increased to 200 (HTS) and 500 (LTS). On the basis of these data, the recursive binomial filtering was carried out (Fig. 4). The curve for the healthy person shows a relatively smooth pattern with less than 10 extrema. The fragmentation score defined in ref. [23] agreed with the data taken for HTS and LTS systems, with an error margin below 5%. For the arrhythmia patient, the curves in Fig. 4 are obviously more fragmented, again with less than 5% difference between HTS and LTS data. These results clearly demonstrate the usefulness of the HTS SQUID system as a diagnostic tool. The measurements described above were carried out in one of the world’s best magnetically shielded rooms. A standard shielded room used for clinical purposes offers insufficient damping of power line “noise” (Fig. 5) and thus requires a first-order gradiometer to make broadband recordings for the diagnostic method described above. The low-pass filtered magnetometer signal shown in Fig. 5 is not sufficient. It still has to be shown that a gradiometer in a standard shielded room gives results fully equivalent to those described. However, only the use of a less expensive shielded room together with a common mode rejection in the HTS system, can contribute to acceptance of MCG as a regular diagnostic tool, at least by larger cardiological centers. A much broader acceptance could be attained if magnetic shielding could be eliminated entirely.

0.2 F

0.3

0,4

0.5

2-

0,6

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= oz

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2-

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0,7

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0,3

1

0,4

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0.5

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0.6

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60.2 7

I I

0,3

Time [ms] Fig. 4. Time domain

I

0.4

0,5

r

0,6

0.7

Time [ms] representation

of the filtered

QRS

complex

of the MCG.

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50 25 4

0

ii El i7j

-25

-50 Bandwidth

0 - 47 Hz

-75

1.5

2.0

2.5

3.0

3s

4.0

Time [s] Fig. 5. Human MCG, recorded with a HTS SQUID magnetometer in a standard two layer mumetal shielded room (Biomagnetic Center, University of Jena), digitally low-pass filtered (47 Hz). The inset shows the raw data (with 50 Hz power line noise). NON-DESTRUCTIVE

Arguments

EVALUATION DEMONSTRATION

OF MATERIALS: EXPERIMENTS

EDDY-CURRENT

for SQUID application

Nondestructive evaluation (NDE) of metallic constructions being used in aircraft, ships, buildings and other areas is a fundamental problem today. A failure of structures and materials may have fatal consequences. Hidden flaws in thick metallic materials and structures (e.g. cracks, hidden corrosion) are hard to detect. Ultrasonic methods and X-ray testing give results that are often very difficult to interpret in terms of flaws, especially when the sample consists of several layers. Magnetic techniques are well suited for layered structures. However, deep flaws are not detectable by conventional NDE techniques such as the eddy current technique with conventional induction coils as sensors. Flaws in aluminum plates hidden 25 mm and deeper cannot be detected [24, 251. The magnetic field sensitivity of the induction coil sensor is not sufficient at the low excitation frequency required for these depths and skin depth limits the use of higher frequencies. It has been shown that liquid-helium-cooled LTS SQUID sensors can be used for eddy current detection of flaws in materials [26]. The LTS SQUID systems, however, are not well suited for practical use, because of the sophisticated and expensive cryogenics. The development of SQUIDS made from HTS thin films makes possible the development of a SQUID eddy current technique for practical testing purposes [27]. High sensitivity

gradiometric

measurements

The configuration of eddy current excitation coils is of great importance when HTS SQUIDS are to be used as sensors. On one hand, the maximum magnetic field and the maximum slew rate (dynamic range) of the SQUIDS and the flux locked loop electronics must not be exceeded by the excitation field. On the other hand, strong magnetic excitation fields are to be applied to the sample in test since the sensitivity of the technique is proportional to the excitation field. The configuration has to be chosen such that the field in the sample is maximized with minimum field at the location of the SQUID sensors [28]. The principle of operation is shown in Fig. 6. Figure 7 sketches the setup of the first testing experiment with the stationary electronic gradiometer system [ 1 I] and the attached excitation coils (two semicircular coils, radius 50 mm, four turns each, laterally adjustable at the bottom of the cryostat). With an excitation frequency of 10 Hz, currents of 1 A (peak-to-peak) could be used without negative effect on the SQUID operation. With these currents, fields of the order of 10 FT can be generated at the centers of the coils. The sample is scanned underneath the system to localize the faults. The gradiometer signal

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viz! 77

SQUID B SQUID A

eddy current frequency

eddy current excitation coils

Fig. 6. Principle of operation of the setup for eddy current testing with a HTS SQUID gradiometer. The subtraction of signals of SQUIDS A and B yields the gradiometric signal. After lock-in detection, the signal is filtered by an adjustable bandpass filter.

A-

Reference SQUID Signal SQUID

Flaw: lo&est plate cut across Fig. 7. Schematic of the experimental setup for eddy current testing with the HTS SQUID gradiometer. The stack of plates is moved laterally underneath the SQUID system.

is lock-in detected with a small bandwidth (3 Hz) to minimize noise and signals from unwanted environmental magnetic disturbances [28] (Fig. 6). For test measurements, the axial mechanical-electronic first order gradiometer with a baseline of 60 mm and an optimum field sensitivity of 170 fT/fi was used [ 111. It was the same analog system that had been successfully used for recording magnetocardiograms of the human heart in an unshielded environment [29]. Due to noisier SQUIDS, the noise level was at least twice as high in the experiments described here. As shown in Fig. 8, by comparing the noise level with and without the presence of the excitation field, it was verified that the system noise was not increased significantly by the excitation current. The sensitivity of the system was demonstrated by using samples with large volumes and thicknesses that were scanned underneath the SQUID system at large distances. To illustrate the potential of the technique with respect to the penetration depth, measurements were conducted on two thick samples (one with a flaw, one without). The samples consisted of a stack of 24 aluminum plates (material no. 3.0285.30, Al 99.8 F12, 0,~=35 . lo6 K’m-i), each of a thickness of 1.5 mm and dimensions 400 x 200 mm2. To simulate a flaw, the lowest plate was exchanged against a plate which had been cut in two pieces. The sample was moved underneath the system at a velocity of 10 mm/s for a distance of 150 mm. The vertical distance between SQUID sensor and sample was 60 mm. Figure 9 shows the variation of the gradiometer signal with time. The sample with a flaw showed a flaw signal of 160 pT at an excitation current of 1 A (peak-to-peak) at 10 Hz (Fig. 9a). In contrast, the sample without flaw yielded only a slowly

Applications

of high-temperature

time

SQUIDS

375

[s]

Fig. 8. Time trace of the noise of the SQUID gradiometer with and without the presence of the excitation field (bandwidth: 3 Hz).

time [s] Fig. 9. Test measurement with the SQUID eddy current system; (a) Gradiometer signal during movement of the faulted sample (24 aluminum plates: 23 flawless plates, lowest plate cut across). For comparison: (b) Signal from the reference sample (24 flawless aluminum plates)

varying signal due to inhomogeneities of the conductance (Fig. 9b). The offset between the signal on both sides of the flaw results from the increase of the signal while approaching the sample border. This effect becomes increasingly important at large sample-SQUID distances. The figures illustrate that our eddy current technique based on the HTS SQUID gradiometer is still far from its limit. A hidden flaw below 36 mm of aluminum can easily be detected with a signal to noise ratio greater than 100. Therefore, with this setup the detection of smaller flaws at even larger penetration depths is possible. The lateral resolution can be increased by reducing the size of the excitation coils and the coil-sample distance as well as the SQUID-sample distance. Further increases might require reduction in flux concentrator size. If the system is operated as a magnetometer by using the direct output of the lower SQUID, it becomes much more sensitive to environmental disturbances. Obviously, a magnetometer measurement is more noisy than a comparable measurement with the system operating as a gradiometer. Nevertheless, in some cases, when environmental noise is uncritical, a magnetometer system might be the system of choice because of simplicity.

Large dynamic

range magnetometer

measurements

Test measurements were also conducted with a SQUID magnetometer system in conjunction with the eddy current technique. The magnetometer was used with a digital electronics for fluxlocked loop operation [7]. Its principle is shown in Fig. 10. The advantage of this electronics is that the dynamic range is practically unlimited because of the introduction of controlled flux

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Transfer Function of SQUID Digital Control ( PC-Board DSP 16io ) .:’

Data Acquisition Processing

Flux Compensation By Coil _

and

Fig. 10. Schematic of the digital flux-locked loop electronics developed by Zimmermann et al. [7].

600 450 300 lock-in signal

’ 2io

3&

Flaw 4c!10 &I

150 8&

760 0 -150

7-v

0

200

400

600

800

x [mm1 Fig. 11. Two-dimensional scan of the eddy current raw and lock-in signals from a stack of 15 aluminum plates. To simulate a flaw, the lowest plate had a groove of 60 x 1 mm in the center.

jumps when the range of the coil is exceeded. Of course, the noise performance, and therefore the sensitivity of the system, is limited by the environmental magnetic field noise and signals (cf. Fig. 1) of the laboratory. Therefore, the setup is about two orders of magnitude less sensitive than the gradiometer system described above. This disadvantage is compensated in part by the inherent digitalization of the signal, allowing one the application of powerful digital signal analysis tools. As a future improvement, the employment of a planar gradiometric SQUID sensor, flux-locked with digital electronics, is possible. The SQUID magnetometer was mounted above a two-dimensional scanning stage to move the sample underneath the magnetic field detector. With a setup similar to Fig. 7, a stack of 15 aluminum plates was scanned underneath the magnetometer. The two semicircular (radius 60 mm) eddy current excitation coils were affixed to the magnetometer cryostat. To simulate a flaw, the lowest plate had a groove of 60 mm long and 1 mm deep. Figure 11 shows the results of a twodimensional scan. The signal of the flaw can be easily identified by subsequent digital lock-in detection. This was achieved by multiplying the trace by a sine wave with the same frequency and phase as the excitation current, and filtering the result with a band-pass and a low-pass. The advantage of this technique is that optimization in phase angle and filter frequencies, as ell as application of more sophisticated signal processing can easily be done after the data acquisition,

Applications of high-temperature SQUIDS GEOMAGNETIC

Potential

advantages

MEASUREMENTS

311

IN FIELD

of HTS SQUID

For geophysical electromagnetic prospecting, the use of SQUIDS as extremely sensitive sensors of the electromagnetic response field of the earth has been demonstrated in the late 1970s [30]. However, because of the difficulty and cost of bringing liquid helium to remote sites up to thousands of miles away from the infrastructure required for this coolant, and the difficulties with liquid helium transfer on location, LTS SQUIDS have failed to replace magnetic field sensors such as conventional induction coils, fluxgates and others. The development of HTS SQUIDS that can operate in the earth’s magnetic field has renewed the interest in SQUID use, because of relatively easy handling of liquid nitrogen in a field measurement campaign. Advantages of a three-axis vector HTS SQUID magnetometer, in comparison to the commonly used conventional induction coils, should be: (1) Higher low-frequency sensitivity, especially below 1 Hz. (2) A compact, portable instrument smaller and easier to handle than an induction coil tripel. (3) One instrument sufficient for the entire frequency regime from 10e4 Hz to 3 . 1Om4Hz, which replaces several sets of induction coils, each optimized for a special part of that spectrum. (However, if highest sensitivity at higher frequencies, above 10 Hz, is required, induction coils remain superior to SQUIDSs. For instance, the sensitivity of the induction coil KIM 879 from Metronix is 1 fTI& at 100 Hz and 0.5 fTTI&% at f> 500 Hz). (4) Greater exploration depths, since in time-domain measurements the voltage induced in an induction coil decreases stronger (t -5’2) with time than the magnetic field itself, which is measured by SQUID (te3’*). Geomagnetic

field test of HTS rf SQUIDS near Lang Fang, China

In a field test near Lang Fang, China, two of our one-axis rf SQUID magnetometers were compared against conventional induction coil systems. The test was conducted in collaboration with Institute for Applied Geophysics, Technical University Berlin; Metronix, Braunschweig; Institut fur Physikalische Hochtechnologie, Jena; Department of Physics, Beijing University, China; Institute of Geophysical and Geochemical Exploration, Lang Fang, China, and Institute of Physics Academy of Science, Beijing, China. The objective of this test was, in addition to the valuable experience in handling a delicate device in a rugged environment, to identify problems that have to be solved when developing a three-axis SQUID magnetometer prototype for geomagnetic applications (see Appendix for explanation of geomagnetic methods and acronyms). Figure 12 shows the schematic setup and time plots from a CSAMT measurement comparing an induction coil (Metronix MFS 05) against a HTS rf SQUID with a 10 x 10 mm2 sensor having BN = 150 fITI&% down to 1 Hz, when measured in shielding. This system used an analog fluxlocked loop electronics. The transmitter, located 4.6 km from the measurement site, fed a 0.25 Hz square wave into a grounded dipole. Both magnetometers, the induction coil and the SQUID, resolved the signal well. The SQUID had the advantage of sensing the field directly, in contrast to the time derivative measured by the coil. Our system remained locked for hours of uninterrupted operation. The exponential decay of the signal-also present in the electric field-is attributed to a current drop at the transmitter, due to the build-up of contact resistance at the transmitter electrode. A TEM test measurement with a loop transmitter of L = 100 m, operated with a pulse excitation of 100 ps ramp edge and a period of 80 ms in the central loop configuration, was also used to compare an inductive coil with ferrite core (effective area 5800 m2) against the same SQUID. The slew rate of the SQUID restricted the transmitter current to 200 mA. Although the S/N ratios of both magnetometers were low due to the low transmitter current, the results shown in Fig. 13 indicated that the SQUID magnetometer can detect the signal at later times (7 ms) than the induction coil sensor (2 ms). Both types of measurements, the time domain TEM and the frequency domain CSAMT using a HTS SQUID sensor, yielded a data quality which gives the well-founded hope that HTS SQUID systems optimized for geophysical prospecting can replace induction coils in specific application APWP 3.IILO-E

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-500

Induction

n

“C

1

L”

coil

. .

tim~[a] Fig. 12. CSAMT measurement with an excitation frequency of 0.25 Hz. Upper part shows schematically the spatial arrangement with the transmitter dipole near the left edge. Lower part shows time traces of the excitation current, of the electric field measured by a dipole antenna, the time derivative of the maenetic field measured bv an induction coil and the magnetic field directlv measured by a HTS rf SQUID. (b) SQUID

(a) Induction coil

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4

TimF[ms]

Fig. 13. Time traces of the TEM magnetic response field from an 80 ms pulsed excitation with 100 pts ramp edge in the central loop configuration. Shown are the signals from induction coil (a) and SQUID (b), averaged over approx. 1000 periods, in a linear and a double-logarithmic plot.

Applications of high-temperature SQUIDS

379

such as CSAMT and TEM. Especially in the late-time TEM measurements, the SQUID is likely to approach the performance of coils having large active area, while being easy to handle.

CONCLUSION The results presented in this paper illustrate the progress toward realistic applications of HTS SQUIDS operating with liquid nitrogen coolant. We limited ourselves to selected examples from our relatively current demonstrations, performed mostly in 1994, although some work by others is certainly of great interest too. Our demonstrations to date have been performed only with singleor two-channel small rf SQUID systems, but they have relevance for future HTS systems with possibly higher channel numbers, whether rf or dc. However, we were not especially concerned about the spatial resolution of signals in measured objects, and were thus using HTS flux concentrators of 25 mm in diameter (in MCG and NDE experiments). The issue of high spatial resolution, on the scale of the order of 1 mm and below, still remains open and should be addressed because of importance for biomagnetic and NDE applications. The presently attained magnetic field resolution, in our case of 25-40 fT/Hz”* over the required bandwidth (when in adequate shielding [6, 16]), is limiting currently possible clinical biomagnetic applications to magnetocardiography. At the 40 ff/I-I~l’~ performance level in good shielding, we could convincingly demonstrate the equivalence of LTS and HTS real-time and averaged MCG data, with a real-time signal to noise ratio only by a factor of two inferior for the HTS system. In case of nonrecursive binomial filtering of averaged MCG data, we showed the agreement of fragmentation scores (sudden cardiac death risk indices) within an error margin of less than 5%. HTS equipment with the presently available performance can thus be used in current clinical research studies. In nondestrucive materials evaluation, we concentrated largely on the eddy current method of deep flaws detection. We could show that a HTS first-order rf SQUID gradiometer having a narrow-band resolution of ca 350fT/H~“~ without magnetic shielding can detect, and with a signal to noise ratio of 100, flaws in aluminum sheets located deep, e.g. 36 mm, below the sheet surface. Such deep flaws are undetectable by the conventional eddy current induction coil technique. This demonstrated the potential of HTS SQUID for practical detection of deep cracks and corrosion, e.g. in the aging aircraft frames and other electrically conducting nonmagnetic structures. In the case of geomagnetic exploration methods, we showed in the field the general applicability of HTS SQUIDS for electromagnetic prospecting by comparing the CSAMT and TEM data simultaneously collected by SQUID and a commercial, dedicated induction coil system. In this case, we used a single-channel rf SQUID with a 10 x 10 mm2 sensor having a resolution of ca 150 ITTIm above 1 Hz (when in shielding). Practical application in the field should be possible upon constructing a reliable three-channel vector (XYZ) magnetometer with a bandwidth broader and response (slew rate) faster than those of the channel used. Increased depth of exploration will be possible by reducing the general noise figure to, e.g. 40 ffl& level, even with the earth’s magnetic field present. These improvements are within the state-of-the art capabilities. OUTLOOK In biomagnetism, work will continue toward further improvement of HTS SQUID system field resolution to attain 10 IT/Hz”~ over a bandwidth of at least 200 Hz, starting below 1 Hz. This performance level is desired without, or at least with significantly reduced magnetic shielding, and could lead also to future magnetoencephalography applications. The complete elimination of shielding will be a key to a very broad acceptance of any biomagnetic diagnostics. It will be attempted through the implementation of gradiometry and active compensation. However, some tradeoff between sensitivity deterioration and shielding reduction appears inevitable. We expect that titure HTS systems will eventually be capable of operating without shielding, but in not too noisy environments, and with sensitivity deteriorated in comparison to that attainable in shielding. The development of large multichannel HTS systems emulating today’s LTS systems will be

Y. ZHANG et al.

380

required for localization diagnostics. The continuing development of software-intensive MCG diagnostic and risk stratification methodologies validated on very large pools of patients will be another key to a broad acceptance by the medical profession and insurance industry. The time scale of the next lo-15 years appears a necessary minimum for all that is to be attained. Once a reality, this will result in by far the largest market for SQUID systems. Limited use for some medical research purposes may occur much earlier, even at the present sensitivity level, but the research market is small. The use of stationary shielding-free HTS SQUID systems in NDE appears feasible even at the present field sensitivity level, but the improvement in spatial resolution will be essential in many cases. This will translate into development of smaller-size sensors, which must retain the same field resolution. Arrays of sensors will be desirable and are feasible. Portable and mobile NDE systems will require further developments in (a) reliable magnetic-noise-free cryocoolers, and (b) broadband, extremely high dynamic range and slew rate digital electronics, beyond the present state-of-the-art. Development of suitable testing methodologies and routines will also be essential. A successful completion of such developments will be the key to acceptance by users of conventional NDE test instruments. Indeed, we expect some market acceptance of stationary SQUID test equipment within five-seven years from today. The mobile equipment prototypes will require a longer development time, but should lead eventually to a broader penetration of the NDE equipment market. The introduction of HTS SQUID vector gradiometers as first-generation commercially available universal field instruments with a large frequency range for geomagnetic prospecting appears to be quite probable within the next three-five years. The need for several sensor systems, each optimized for a narrow frequency band, will be eliminated. The ease of handling is going to open new multi-receiver acquisition and interpolation techniques with enhanced 2D and 3D prospecting resolution. The necessary developments are straightforward and can be successfully completed using state-of-the-art approaches already in hand. In all HTS SQUID application areas, the issues of reliability, instrument ruggedness, sensor life time, user-friendly interface with extensive data-processing and expert-system capabilities, price and manufacturing cost will be critical for a future market success.

Acknowledgements-The KFA authors gratefully acknowledge the guidance and leadership of Ch. Heiden (Univ. Giessen), who initiated and directed their program until 1993. They are also acknowledging the earlier contributions of M. Muck (Univ. Giessen). Cardiac arrhythmia patients and essential cardiological advice were provided by A. Schirdewan (Franz-Volhard Clinic, Berlin). Geological field measurements in China were greatly facilitated by Y. -D. Dai and S. Wang (Beijing Univ.), and by C. Wang (Institute of Geophysical and Geochemical Exploration, Lang Fang, China) and L. Zhang (Chinese Academy of Sciences, Beijing). Last, but not least, we are grateful for the invaluable contributions of members of our teams: D. Drung and H. Koch (PhysikalischTechnische Bundesanstalt, Berlin), A. Chwala and V Schultze (Institut ftir Physikalische Hochtechnologie, Jena), H. Burkhardt and V. Rath (Technical University Berlin), J. Borgmann (University Bonn), E. Zimmermann, U. Clemens, G. Brandenburg, H. Halling (Zentrallabor fIir Elektronik, KFA Jtilich), M. Butzek (Zentralinstitut ftir Allgemeine Technologie, KFA Ji.ilich), M. Banzet, V Glyantsev, M. Grtineklee, A. Haller, K. -D. Husemann, R. Kutzner, D. Lomparski, G. Ockenfug, R. Otto, J. Schubert, H. Schtitt, F. Schmidt, M. Siegel, H. Soltner, R. Wordenweber, W. Wolf, N. Wolters, W. Zander (Institut fir Schicht- und Ionentechnik, KFA Jiilich), who made the described application demonstrations possible. This work was supported in part by the German BMBF (Project Numbers 13N633 1, 13N6332, 13N6527).

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APPENDIX:

SOME

ELECTROMAGNETIC

TECHNIQUES

FOR

PROSPECTING

TEM

The technique Transient Electra Magnetics (TEM) is based upon the registration of the time dependence of the secondary magnetic field, induced in the ground by a current transient in an excitation loop. The time range shortly after the pulse excitation contains information about shallow structures while late time signal represents the response from deep layers of the earth. The induced voltage in an induction coil decreases stronger (t -5’2) with time than the magnetic field itself, which is measured by the SQUID (t-3’2). Therefore, with a SQUID sensor the detection of late time signals ( > 1 ms) required for large penetration depths is possible with a larger S/N ratio compared to induction coils. AMT and CSAMT

The techniques Audio Magneto Tellurics (AMT) and Controlled Source Audio Magneto Tellurics (CSAMT) analyze the electromagnetic response of the earth in the frequency domain, using all three components of the magnetic field and the two horizontal components of the electric field [l]. In the case of AMT, the naturally occurring variations in the earth’s magnetic field, originating from micropulsations (interactions of the solar wind with the ionosphere) and thunderstorms (sferics) are used as sources. For CSAMT, the excitation is being generated by a dipole transmitter in the far field regime. The disadvantage of requiring a transmitter is often tolerated, as this technique offers a higher S/N ratio than AMT.