This part of IEC 61788 covers a test method for the determination of the d.c. critical current of
either Cu/Nb-Ti composite superconductors that have a copper/superconductor ratio larger
than 1 or Cu/Cu-Ni/Nb-Ti wires that have a copper/superconductor ratio larger than 0,9 and a
copper alloy (Cu-Ni)/superconductor ratio larger than 0,2, where the diameter of Nb-Ti
superconducting filaments is larger than 1 μm. The changes for the Cu/Cu-Ni/Nb-Ti are
described in Annex C. The Cu-Ni uses all of the main part of the standard with the exceptions
listed in Annex C that replace (and in some cases are counter to) some of the steps in the
main text.
This method is intended for use with superconductors that have critical currents less than
1 000 A and n-values larger than 12, under standard test conditions and at magnetic fields
less than or equal to 0,7 of the upper critical magnetic field. The test specimen is immersed in
a liquid helium bath at a known temperature during testing. The test conductor has a
monolithic structure with a round or rectangular cross-sectional area that is less than 2 mm2.
The specimen geometry used in this test method is an inductively coiled specimen. Deviations
from this test method that are allowed for routine tests and other specific restrictions are
given in this standard.
Test conductors with critical currents above 1 000 A or cross-sectional areas greater than
2 mm2 could be measured with the present method with an anticipated increase in uncertainty
and a more significant self-field effect (see Annex B). Other, more specialized, specimen test
geometries may be more appropriate for larger conductor testing which have been omitted
from this present standard for simplicity and to retain a lower uncertainty.
The test method given in this standard is expected to apply to other superconducting
composite wires after some appropriate modifications.