This paper presents comprehensive measurements on three advanced ITER internal-tin
Nb3Sn
strands manufactured by Oxford Superconducting Technology (OST), Outokumpu Superconductors
(OKSC) and Luvata Italy (OCSI) for fusion applications. The engineering critical current density
(JC) at
10 µV m−1 and the index
(n) characterized over
the range 10–100 µV m−1
are presented as a function of magnetic field
(B≤15 T in
Durham and B≤28 T at the European high-field laboratory in Grenoble), temperature
(2.35 K≤T≤14 K) and
intrinsic strain (−1.1%≤εI≤0.5%). Consistency tests show that the variable strain
JC data are
homogeneous (± 5%) along the length of the strand, and that there is a good agreement between different
samples measured in Durham and in other laboratories (at zero applied strain).
Limited strain cycling (fatigue) tests demonstrate that there is no significant
degradation in the critical current density in the strands due to cyclic mechanical loads.
JC
is accurately described by the scaling law that was derived
using microscopic and phenomenological theoretical analysis and
n
is described by the modified power law of the form
n = 1+rICs,
where r
and s
are approximately constant. Using variable strain high magnetic field data at 2.35 K for the
OCSI sample, it is demonstrated that these laws can be extended to describe data below
4.2 K. For these advanced strands, thirteen, nine and six free parameter fits to the data are
considered. When thirteen or nine free parameters are used, the scaling laws fit the data
very accurately. The accuracy with which the scaling law derived from fitting data taken at
4.2 K alone fits all the variable temperature data if calculated errors in fitting
JC
are shown to be primarily determined by uncertainties in
TC. It is shown that six free parameter fits can successfully be used when, as with these advanced
strands, the strain dependence of the normalized effective upper critical field at zero
temperature is accurately known—this approach may provide the basis for comparing partial
JC(B,T,ε)
data on other similar strands from different laboratories. The extensive data presented here
are also parametrized using an ITER scaling law recently proposed for characterizing
Nb3Sn
strands and the strengths and weaknesses of that approach are discussed.