Magnets, superconducting shielded

Low-magnetic-field applications. These applications include Josephson-effect devices, magnetic-flux shields, transmission fines, and resonant cavities, all of which require superconducting materials having a high critical temperature and a high critical magnetic field. 

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.

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

The consequent increase in the nuclear charge and reduction of the shielding of the 6d- and 7s-electrons lead to a contraction of the atomic radius, similar to that previously discussed for the ionic radius. In Am and Cm, the 5f-electrons are localized in the core, which causes an expansion of the atomic radius. The differences in localization of f-electrons between light and heavy actinides are also illustrated by their different superconductive and magnetic behavior. The Th, Pa, and Am metals are superconductors Tc of 1.37, 0.42, and 0.79 K, respectively), whereas the heavier actinide metals are not superconductors but have larger magnetic moments at low temperatures. 

Figure 6 Shielding current flow Ah in a surface layer of thickness X around a Type I superconducting cylinder in an axial applied magnetic field Sapp (from Ref. 8, p. 43)...

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