Table of Contents ECNDT '98 Session: Power Generation Industry

Ultrasonic Testing of Aluminium Stabilised High-Current Superconducting Cables J. Neuenschwander and Th. Lüthi Swiss Federal Laboratories for Materials Testing and Research (EMPA) CH-8600 Dübendorf I.L. Horvath Swiss Federal Institute of Technology (ETH), CH-8093 Zürich V. Pasquer R/D Tech France, F-91893 Orsay Cedex Corresponding Author Contact: Juerg Neuenschwander Swiss Federal Laboratories for Materials Testing and Research (EMPA), Nondestructive Testing /Ropeway Technology, Ueberlandstrasse 129, CH-8600 Duebendorf (Switzerland), Phone: +41.1.823.4320, Fax: +41.1.823.4579 Email: juerg.neuenschwander@empa.ch , URL: http://www.empa.ch

TABLE OF CONTENTS

Introduction High - Current SuperConducting Cables Single Probe Ultrasonic Testing Phased Array Ultrasonic Testing Conclusion References

Introduction

Novel high-current superconducting cables have been developed for large scale magnets used in High Energy Physics [1] and Superconducting Magnetic Energy Storage (SMES) systems [2]. A superconducting Rutherford type flat-band-cable is coextruded with high-purity aluminium. Further mechanical reinforcement can be achieved by attaching high-strength aluminium alloy strips. For a satisfying service of the conductor a good bonding between the different constituents must be guaranteed. The main reasons are: cooling efficiency of the superconductor, current transfer to the high-purity aluminium in case of an emergency shut-down and mechanical stability. Modern ultrasonics is a well-known nondestructive technique to detect regions of disbonding. We have tested a large variety of aluminium stabilised superconducting cables in different development and production stages [3-5]. Our laboratories are involved in projects in Europe (CERN: CMS, ATLAS), in the USA (Fermilab DØ, SMES, SLAC-BABAR) and Japan (KEK-BELLE). This paper shall give an overview of the results of ultrasonic testing obtained over the last few years.

Extensive studies have been performed on short samples (typically 10-20 cm). Conventional mechanical C-scanning, applying the pulse-echo immersion technique, is appropriate to reveal detailed information on the bond-quality. But, for quality assurance reasons it is necessary to monitor the bond-quality continuously along the whole cable length of up to 80 km. Mechanical scanning of the complete bond-area, however, takes up too much time. In a first step we performed on the Fermilab DØ cable a partial bond-test with two fixed point-focused probes. The bond was checked on a width of 1 mm from both sides continuously along the complete length of 5 km. For the future, we propose the phased array technique for quality control. The sound beam is scanned electronically perpendicular to the direction of production. Hence, the testing speed is sufficient to allow a continuous inspection of the complete bond over the total length. For a feasibility study a probe with 128 elements was designed and manufactured. With the new probe in conjunction with the appropriate electronics and software a large number of test samples were investigated.

High - Current SuperConducting Cables

The magnets are expected to produce high fields in volumes up to several hundred cubic meters. For instance, the CERN-CMS solenoid requires a 50 km long cable, with a critical current of 60 kA at 5 Tesla and 4.2 Kelvin. The magnet coils are to be cooled with liquid helium. Figure 1 displays schematically the cross section of two typical high-current superconducting cables used in High Energy Physics.

Fig 1: Schematic cross section of two high-current superconducting cables: CERN-CMS and Fermilab DØ. Note: only approximate dimensions are given.

The magnets are expected to produce high fields in volumes up to several hundred cubic meters. For instance, the CERN-CMS solenoid requires a 50 km long cable, with a critical current of 60 kA at 5 Tesla and 4.2 Kelvin. The magnet coils are to be cooled with liquid helium. Figure 1 displays schematically the cross section of two typical high-current superconducting cables used in High Energy Physics. The principle current carrier is the central superconducting Rutherford type flat-band-cable. It consists of a twisted double-layer of 18-36 multifilament NbTi strands (Ø = 0.8-1.5 mm). The flat-band-cable is stabilised with high-purity aluminium (99.998%) in a continuous coextrusion process. The production speed is of the order of 2-6 m/min. Large cable lengths (up to 80 km) are produced in units of 1-12 km. In projects like CMS or SMES further mechanical stability is required. This is achieved by attaching high-strength aluminium alloy profiles with electron beam welding [2,6] or brazing.

The quality of bonding between the different constituents can be checked with ultrasonics. The pulse-echo immersion technique is applied with the sound beam being perpendicular to the inspected interface. Any disbonding will be indicated by an enhanced echo amplitude of the bond interface. However, in case of the high-purity aluminium to flat-band-cable bond, the testing is non-trivial. First, due to different acoustic impedances, also in case of perfect bonding part of the ultrasound will be reflected. The suitable gain has to be chosen carefully with calibration blocks. Second, high frequency broad-band probes have to be used, in order to separate the echoes from the surface and the back-side of the individual strands.

Single Probe Ultrasonic Testing

Short samples were analysed in our laboratories with single probe ultrasonic C-scan imaging applying an Ultrasonic Sciences Ltd. equipment. Point-focused probes with nominal frequencies up to 50 MHz were used in conjunction with a high-precision mechanical scanner. The interface echo amplitudes are measured in millivolts (mV) and are colour- or grey-scale coded.
Figure 2 reveals 20 MHz C-scans of two different samples of the Fermilab DØ cable. In the upper sample (a) we perceive the regular structure of the individual strands of the flat-band-cable. The echo amplitude is relatively low: the bonding is homogeneously good. The lower sample (b), however, reveals enhanced echo amplitudes: the high-purity aluminium is disbonded from the flat-band-cable. For comparison, a test-block with flat bottom holes (representing complete delaminations) has been imaged (c). Further qualification of the ultrasonic results was obtained by polishing one end of these two Fermilab DØ samples to allow micrographic inspection (Figure 3). The images show part of the contact region between two strands and the aluminium. Figure 3a reveals the sample with the good bonding (Figure 2a): strands and aluminium are in tight contact. The sample with the enhanced echo amplitude (Figure 2b), however, has an obvious gap (Figure 3b).

Fig 2: Ultrasonic C-scans (20 MHz) of Fermilab DØ samples. a) Good bonding; b) disbonding; c) test-block with flat bottom holes (Ø = 5.5, 3, 2, 1 mm).

Fig 3: Micrographs of polished cuts of the two Fermilab DØ samples that were tested with ultrasonics. a) Good bonding (cf. Figure 2a); b) gap between aluminium and strands (cf. Figure 2b).

Fig 5: Micrographs of polished cuts of the electron beam welding test samples that were tested with ultrasonics (cf. Figure 4).

Fig 4: Ultrasonic C-scans (20 MHz) of two electron beam welding test samples. The sample on the right-hand side exhibits about 50% disbonding (black).

Fig 6: Continuous registration of the echo-dynamics of test-blocks. a) Flat bottom holes; b) short sample of the Fermilab DØ cable containing delaminations.

Fig 7: Continuous ultrasonic testing (10 MHz) of bond-quality of the Fermilab DØ cable.a) Good bonding; b) indications of defects.

Fig 8: Principle of electronic focusing in a phased array system.

Fig 9: Principle of electronic scanning in a phased array system.

Fig 10:Ultrasonic phased array C - scans (10 MHz) of Fermilab DØ samples.

Fig 11: Ultrasonic phased array C - scans (10 MHz) of CERN - ATLAS samples.

Electron beam welding has been successfully applied for joining high-strength aluminium alloy profiles to high-purity aluminium [2, 6]. Samples of tests with different production speeds have been imaged with ultrasound (Figure 4) and polished cuts were photographed (Figure 5). At 1.8 m/min practically the whole width (40 mm) is welded. At the higher production speed of 6.0 m/min still half of the profile is joined. Using two opposite electron guns a complete welding can be expected.

The above presented results clearly show that the ultrasonic testing of short samples give important information on the quality of the bonding. But only the beginning and the end of the unit lengths (1-12 km) can be checked in this way. However, for a satisfying service of the magnet, a good bonding must be guaranteed along the complete cable length (up to 80 km). Conventional mechanical scanning is too slow for such a testing. As a first step we performed a partial bond-test with 10 MHz point-focused immersion probes which were fixed on both sides of the cable. The echo amplitude of 1 mm bond-width was measured with two Krautkrämer USIP 12 units and recorded in function of position with a two-channel chart recorder (echo-dynamics). The appropriate parameters were determined in laboratory tests with test-blocks. Figure 6 demonstrates the clear detectability of flat bottom holes (a). Furthermore, good bonding and delaminations present in a short sample of the Fermilab DØ cable are readily discernible (b). The test equipment was then set up at the production site. At speeds up to 30 m/min we continuously tested the complete 5 km Fermilab DØ cable on both sides. The overwhelming part of the cable turned out to be without any indications (Figure 7a). Especially one section, however, revealed up to 0.5 m long disbondings (Figure 7b). Fortunately, they occur on one side only. The Fermilab DØ solenoid is now manufactured and provides the required field of 2 Tesla [7].

Phased Array Ultrasonic Testing

The continuous testing of a fraction of the bond-width presented in the previous section gives some information on the bond quality on the whole length. But there is a considerable chance that disbonds are outside the location of the sound beam. So-called "paintbrush-probes", having wide sound fields were found to be useless for the present testing problem. The reason was a lack of reproducibility. Another alternative would be the application of a large number of single probes. However, we propose the application of a modern phased array system. This technique requires multielement transducers and sophisticated electronics [8,9]. Phased array technology allows the generation of an ultrasonic beam with the possibility of setting the beam parameters such as angle, focal distance and focal point size through software. Two aspects that are essential for our applications shall be briefly mentioned: electronic focusing and linear scanning. Figure 8 displays the principle of electronic focusing. Each piezoelectric element of the phased array probe can be pulsed independently. Introducing characteristic delay times between the channels results in a focusing of the sound field. Electronic scanning (Figure 9) is achieved by successively firing groups of elements. The sound beam moves along the array probe. Since no mechanical motion is involved, very high scanning frequencies are possible. This shall allow a 100% testing of the bond-area.

A detailed feasibility study about the application of the phased array technology for the present testing problem was performed. A new 10 MHz linear array probe with 128 elements was designed and manufactured. The elementary pitch was 0.6 mm and the mechanical focusing was 40 mm in water. The aperture consisted of 16 elements and the electronic scanning range was 67 mm. A large number of test samples were investigated with the new probe in conjunction with the appropriate electronics and software. The results are very encouraging. Figure 10 displays two 10 MHz phased array C-scans on Fermilab DØ samples. The scanning transverse to the conductor was performed electronically. The images were generated by moving the phased array probe once over the sample. The flat bottom holes in the aluminium test-block (b) are readily visible. In the Fermilab DØ test sample (a) the Rutherford flat-band-cable (width approx. 8 mm) can be seen. Disbonded areas (dark-grey) at both ends are clearly distinct from the correctly bonded section in the middle (light-grey).

A phased array image of a CERN-ATLAS type cable is shown in Figure 11a: the flat-band-cable (width approx. 21 mm) has a uniform low echo-amplitude indicating good bonding. The image of the test-block with flat bottom holes (b) demonstrates the ability of the system to detect small disbondings at any position in the bonding area.

Conclusion

Ultrasonic imaging of high-current superconducting cables allows a precise analysis of short samples in the laboratory. Production faults such as delaminations between the high-purity aluminium and the flat-band-cable or welding flaws in the aluminium alloy - high-purity aluminium junction are detectable. These laboratory tests are especially important during the development phase. During production this technique allows at least the testing of beginning and end of the unit lengths (1-12 km). However, in the final product, the quality of the bond along the whole cable length is of vital interest. We have performed in a first step such a continuous, but partial test with two point-focused probes. The mechanical scanning is too slow for the inspection of large length. We propose the modern ultrasonic phased array technology for a 100% testing (C-scan). Perpendicular to the direction of production the sound beam will be scanned electronically. That is why such a testing is sufficiently fast in order to allow a continuous analysis of the complete bond. An extended feasibility study demonstrated the imaging of disbondings with a phased array system. We are now on the way to set up such a system for testing long length of cables during manufacture.

References

1. I.L. Horvath, "Aluminium stabilised superconducting cable development for high energy physics detector magnets", Applied Superconductivity Conference, Oct. 16.-21, 1994, Boston, USA.
2. D. Fritz, I.L. Horvath, M. Harzenmoser, J. Neuenschwander, F. Wittgenstein, "Development of an aluminium stabilised reinforced superconducting conductor", 14 th International Conference on Magnet Technology, June 11-16, 1995, Tampere, Finland.
3. J. Neuenschwander, T. Lüthi, I.L. Horvath, "High-current superconducting cables: application of ultrasonic testing during development and production", in German, DACH Annual Conference on Nondestructive Testing of Materials, May 13-15, 1996, Lindau, Germany, DGZfP Vol. 52.1, p. 251.
4. R. Huwiler, Th. Lüthi, W.J. Muster, J. Neuenschwander, I.L. Horvath, "Qualification of advanced cryogenic components: Mechanical integrity of high-current superconducting cables", Recent Res. Devel. in Cryogen., Vol. 1 (1996) p. 7.
5. J. Neuenschwander, T. Lüthi, I.L. Horvath, V. Pasquer, "Continuous bond quality control of aluminium stabilised superconductors", 15th Int. Conf. on Magnet Technology, October 20-24, 1997, Beijing, China.
6. I.L. Horvath, F. Wittgenstein, F. Bertinelli, A. Desirelli, S. Sgobba, T. Tardy, D. Fritz, J. Neuenschwander, "Manufacture of aluminium stabilised and reinforced superconductors by electron beam welding technique", 15th Int. Conf. on Magnet Technology, October 20-24, 1997, Beijing, China.
7. Private communication by Toshiba Corporation, Yokohama, Japan.
8. G.P. Singh, J.W. Davies, "Multiple transducer ultrasonic techniques", in NDT Handbook Vol 7: Ultrasonic Testing, A.S. Birks et al (eds), ASNT, USA, 1991.
9. Y. Takishita, H. Kino, S. Yamaguchi, A. Iwasaki, N. Yamamoto, "Development of high-speed ultrasonic testing device using electronic scanning", Acoustical Imaging, Vol. 22, P. Tortoli and L. Masotti (eds.), Plenum Press, New York, USA, 1996, p. 799. 7th ECNDT, Copenhagen 1998