Superconducting shields

External fields must be shielded to about 10-7T, i.e. well below earth magnetic field ( 50 jlT). The maximum recommended field is 10 6T and the dTc/dH (about lOOK/Tesla) is supplied around H = 0. The maximum power which may be supplied to the coils is between 10-9 and 10-1°W. The sweep rate used to trace out the superconducting transitions should be less than O.lmK/min (for W and Be) to avoid hysteresis effects. Unfortunately, nowadays SRM-767a and SRM-768 are no longer produced. 

A very attractive feature of the SRD1000 device is the presence of the electronics to trace out the superconducting transitions of the samples, of the magnetic shields and of... 

In superconducting transitions, the purity of materials and the shielding from magnetic fields are fundamental to obtain steep transitions (see Figs 8.6-8.8). 

Figure 3 Shielding magnetization at 5 K versus increasing external field for a superconducting single crystal.

Figure 4 Flux exclusion (shielding) versus increasing temperature (solid triangles) and flux expulsion (Meissner effect) versus decreasing temperature (open triangles) in a 25 Oe external field for a superconducting YE Cu Oy.g single crystal. Exclusion and expulsion are equal for temperatures above the irreversibility point Tjrr (w90.5 K at 25 Oe) Tc is 92 K.

Figure 6 shows the exclusion/expulsion ratio versus applied field using the data from Figure 5. The inset to Figure 6 emphasizes the behavior at low fields as the measurement field is reduced below 10 Oe, the exclusion/expulsion ratio decreases smoothly to unity. This confirms that the entire crystal is superconducting, since the shielding fraction for this crystal is approximately 0.97 from the linear portion of the shielding M-H curve. There is a decided advantage to plotting the exclu-...

A. Ishiyama and H. Hirooka, Magnetic shielding for MRI superconducting magnets. IEEE Trans. Magn., 1991, 27(2), 1692-1695. 

Clay is one of the most important building materials conceived by humanity. Without clay, little from our past would remain preserved. Without clay, we would have fewer beautiful works of art to enjoy we would not have useful electrical insulators and other modern-day appliances. Recently, high -temperature (90°K or -183°C) ceramic superconducting materials have been developed. Ceramic materials are used to make heat shields for space vehicles, such as the space shuttle. Clay is a material of the past and of the future. We will always be playing with clay. 

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