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Electric Measurement Lab. of ENEA-Frascati Superconductivity section can carry out measurements of critical currents at variable magnetic field either at liquid helium temperatures (4.2K) or at variable temperatures.
This system can be characterized by wires at low Tc, either as single strands or as triplets (conductors 3x3 and sub-cables) winded up on ITER standard sample holders in Ti-Al-Va or in stainless steel depending on the magnetic field at the temperature of 4.2K.
The sample is subject to a constant magnetic field, with a 0.1% uniformity, with a value up to 14.5T per a sample holder with a diameter of 60mm or at a more intense magnetic field, up to 16T, using a sample holder of 35mm.
The sample, dipped in the same bath of liquid helium of the solenoid that produces the magnetic field of background, can be powered with currents up to 3.5kA.
Similar measures can be done with other power supplies up to reach 20kA on the sample. Samples of greater dimensions can be, however measured, but in such case the used cryostats are devoid of the magnet that produces the field of background .
The laboratories for electric characterisation are equipped with two facilities for transport measurements ,depending either on the temperature or on the magnetic field, called VTI(Variable
They are essentially constituted by two cryostats provided of superconducting magnets in NbTi with inserts in Nb3Sn that produces the field of background, cooled in helium bath. In both a chamber thermically isolated from the bath is inserted, that entertains the sample to be measured; through a needle valve the helium of the bath is introduced in the chamber and heated, so that to stabilize the sample at the desired temperature.
In the first VTI the temperature of the sample can be varied in the range 3K-100K and the magnetic field of background from 0T to 12T. In this system can be measured the critical currents of wound threads, that have a length up to 1m and of around 1mm of diameter, maintaining the external field perpendicular to the axle of the thread, or of portions of material bulk or film, long up to 3cm, for external fields perpendicular to the face of the sample. The bore of the cryostat is of 80mm and the maximum tide it’s equal to ~300A. At temperatures next to that of the liquid helium the oscillation of the temperature during the measurement is inferior to ±0.01K. The uniformity of the magnetic field is at least of the 0.1%.
In the second VTI, mainly used for samples at high Tc, the temperature of the sample can vary from 4.2K up to the room temperature and the field of background from 0T to 12T. The samples can be installed on with two different geometric configurations:
Dependence of the density of critical current from the angle between
the direction of the applied magnetic field and the normal to the
surface of the film measured at 77K on thin film of YBCO with
substitutions of Ca and BaZrO3.
in the first one the applied magnetic field is orthogonal either to the surface of the samples or to the current that flows in them, so that to realize the configuration of maximum strength of Lorentz; in the second it is possible to vary the angle of incidence of the magnetic field in comparison to the normal one at the surface of the sample, maintaining unchanged the condition of maximum strength of Lorentz. In both cases the bias current is limited among the 0.2A÷0.5A, according to the operational temperature. The bore of the cryostat is of 46.9mm.
In the laboratory is present also a close cycle Cryocooler of gas helium with cold finger commanded by a temperature controller type ITC503 for the acquisition of the resistance curves depending on the temperature.
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