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| The high field sensitivity of SQUIDs, especially at low frequencies,
makes them ideally suitable for applications in nondestructive evaluation
(NDE). Besides measuring remanent magnetic fields, a SQUID can also
be employed in an eddy-current NDE system, in which the SQUID measures
the secondary magnetic field generated by eddy currents in the material
under test.
For testing of conducting wires, such as aluminum bond wire, we developed
a special cryostat, which allows for pulling a wire of arbitrary length
(which is kept at room temperature) through a niobium flux transformer
connected to a dc SQUID. The wire is excited by either passing an
alternating current through it, or by exciting eddy-currents in the wire.
The system employs our NDE controller WBS KLI 250k and our 4-MHz dc SQUID
readout SQE 4M, and can be controlled manually, or by a standard personal
computer via the serial port. |
| Cryostat for testing
wires with a SQUID. The wire is pulled through a tube running through
the vacuum space of the cryostat. This tube in turn is situated inside
a flux transformer connected to a SQUID. |
 |
| Schematic drawing
of the cross section of the cryostat for wire testing. |
|
| The graph below shows results of a measurement on a 22-cm long piece
of 1.5-mm dia. copper wire. We drilled three 0.2-mm deep holes with
0.2 mm diameter at x0 = 10 cm, x1 = 12 cm and x2
=
14 cm, which could easily be detected. We could also detect scratches
at the surface of the wire, which were typically 0.05 mm deep and had an
area of about 0.1 mm by 0.1 mm. The excitation current through the
wire was 1 A, and the excitation frequency was 11 kHz. The ac field
produced by the wire was compensated at the location of the SQUID in a
similar way as for the metal sheet measurements.
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