A squid

Simulations about Eddy Current Distributions and Crack Detection Algorithms for a SQUID Based NDE System. 

A SQUID [2] provides two basic advantages for measuring small variations in the magnetic field caused by cracks [3-7]. First, its unsurpassed field sensitivity is independent of frequency and thus dc and ac fields can be measured with an resolution of better than IpT/VHz. Secondly, the operation of the SQUID in a flux locked loop can provide a more than sufficient dynamic range of up to 160 dB/VHz in a shielded environment, and about 140 dB/>/Hz in unshielded environment [8]. 

In this paper we present simulations and measurements of several types of excitation coils, which match the special requirements for a SQUID based eddy current NDE system. We note however that all calculations presented here on penetration depths, current distributions and crack-detecting algorithms are also useful for conventional eddy current testing systems. 

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. 

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. 

To realize an automatic evaluation system, it would be desirable to also suppress geometrically caused signals as well, so that only the actual defect signals are obtained. Several approaches have already been made which are also to be implemented as part of a SQUID research project (SQUID = Super Conducting Quantum Interference Device). 

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 modern approach to measuring magnetic properties is to use a superconducting quantum interference device (a SQUID), which is highly sensitive to small magnetic fields and can make very precise measurements on small samples. 

Figure 2.2 Ionic conductances underlying the action potential recorded from a squid axon. gNa = Na conductance gK = K+ conductance. (Adapted from Hodgkin, AL and Huxley, AF (1952) J. Physiol. 117 500-544)...

The measurement of the linear expansion coefficient can be carried out by several methods (see Chapter 13), for example, by means of an interferometric dilatometer [77], a capacitance dilatometer [78] or a SQUID dilatometer [74]. The latter can achieve resolutions as small as 2 x l(T14m. 

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

The superconducting quantum interference device (SQUID) is formed from a superconducting loop containing at least one Josephson junction. Basically, a SQUID amplifier converts an input current to an output voltage with a transresistance of the order of 107 V/A. The input noise is of the order of 10-11 A/(Hz)1/2. The bandwidth of the SQUID amplifier can be up to 80kHz. The dynamic range in 1 Hz bandwidth can be 150dB. 

In fact, with few exceptions [24], the resistance of TES is very low and the matching to a conventional FET amplifier is impossible. A SQUID amplifier (see Section 14.5) coupled to the TES by a superconducting transformer is the natural solution as schematically shown in Fig. 15.4. 

Fig. 10. H image of mineral oil phantom at 2 mT using a SQUID spectrometer (from website - http //waugh.cchem.berkeley.edu/noframes/research/).

Figure 15. Transmission scanning electron micrograph of two nanobridge SQUID s (Superconducting Quantum Interference Devices). A SQUID consists of a superconducting ring containing two weak-links . In this instance, the weak links are niobium wires 25 nm wide fabricated by electron beam.

The magnetic susceptibilities of dimer liquid crystals such as NC-Ph—Ph—O—(CH2>n—Ph—Ph—CN(n 9, 10) are measured by a SQUID magnetometer. The results obtained are interpreted within the framework of the RIS approximation, the effect arising from the conformational anisotropy of the flexible spacer being strictly taken into account. The order parameters of the mesogenic core axis thus estimated are found to be consistent with those directly observed at just below 7N) by the ZH NMR technique using mesogen-deuterated samples. 

The magnetic susceptibilities of some ether-type liquid-crystalline polymers are measured by a SQUID magnetometer. The Ax values estimated for the stable nematic state are analyzed according to a known RIS scheme. 

Using the information given in Table 2.1, calculate the potential difference (in mV) across a squid nerve cell. 

Fig. 3.11 Comparison of the correlation vectors of a SQUID COX-2 pharmacophore model and the CATS3D correlation vectors of the three molecules used for the calculation of the pharmacophore model.

Fig. 3.13 (a) SQUID fuzzy pharmacophore model of Tat-TAR interaction inhibitors, (b) Alignment of the best hit (colored by atom types) found with SQUID with the reference alignment (red, acetylpromazine green, fragment of CGP40336A). 

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