SQUID systems

Abstract An Eddy current method applying a High Temperature Superconductor ( HTS ) DC SQUID sensor operating at Uquid nitrogen temperature (77K) is presented. The method is developed for the detection of surface or surface near defects. We compare the performance of the SQUID system with the performance gained from a commercial Eddy current system, while using identical probes. The experimental data are obtained on defects in gas turbine blades. The advantage of planar conformable probes for the use with the SQUID is discussed. 

We realized an Eddy current SQUID system of the high frequency type a room temperature Eddy current probe is connected to a SQUID sensor at hquid nitrogen temperature. Fig.3 gives an overview over the components of the system, fig, 5 shows a schematic diagram of the electronics. 

Several types of Eddy current probes were used with the SQUID system and the commercial system as well. High inductance wire wound probes with a ferritie eore and low induetance planar thick frhn coils were applied. The wire wound probe is the commonly used probe for high resolution conventional testing. The low inductance planar cod is more suited to be apphed in combination with the SQUID system. It is well adapted for surfaee defects and shallow defects. 

In order to maximize the excitation, precautions have to be taken to avoid cross-talk between excitation and signal. Therefore differential probes are commonly used with a SQUID system Nevertheless, for the discussed defects the SQUID system has a lower excitation field by a factor of about 100 compared with the commereial system This we must keep in mind, when we compare measured signal to noise ratios. There is a potential to improve for small defeets, when eross-talk is managed very well. 

HTS SQUID System with Joule-Thomson Cryocooler for Eddy Current Nondestructive... 

With the availability of such a SQUID system the question arises what it brings to nondestructive testing. One has to check if existing methods can be improved by using this systems. And, one has to check if this system allows for powerful new NDT methods. 

The latter values are not measurable by means of semiconductor amplifiers and a SQUID system is necessary. 

The CRESST II setup will consist of up to 33 modules with both light and heat detection, reaching up to 10 kg of active target mass. The system is read out by a 66-channel SQUID system, two readout channels for each module. 

Enpuku K, Hotta M, Nakahodo A (2001) High-U SQUID system for biological immunoassays. Physica C 357-360 1462-1465... 

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