Characterization of ITER $\hbox{Nb}_{3}\hbox{Sn}$ Strands Under Strain-Applied Conditions

Abstract

Japan Atomic Energy Agency has developed four types of Nb 3 Sn strand which can be used in the ITER TF coils. One is a strand made by an internal tin process strand and the others are bronze process strands. The achieved critical current density is more than 790 A/mm 2 in the bronze process strands and more than 980 A/mm 2 in the internal tin process strand under 4.2 K temperature and 12 T magnetic field and there is hysteresis loss of less than 770 mJ/cc under 3 T cycle. Since these strands are utilized with an external strain, it is necessary to evaluate strain dependency to confirm the ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring to produce uniform axial compressive or tensile strain along the strand length, a strand being soldered on the outer surface of the ring. The detailed strand characteristics were investigated subjecting the developed strands to a magnetic field from 10 T to 13 T, a strain from about 0.8% to 0.5%, and a temperature from 4.2 K to the critical temperature. When the critical current is normalized to that under the conditions where strain is intrinsically zero, the bronze process strands exhibit better performance than the internal tin process strand. However, the three bronze process strands do not exhibit the same I -strain characteristics. Two types of scaling relations are applied to the data, and good expressions of strand performance were obtained by the least square method within 3 A as RMS.

Neutron Diffraction Measurements of Internal Strain in ${\rm Nb}_{3}{\rm Sn}$ Cable-In-Conduit Conductors

“…The superconducting properties of strands vary significantly, depending on the presence of internal strain [1], [2]. It is important to clarify the presence of internal strain in evaluating superconducting performance.…”

“…The superconducting performance of the strand as a function of strain, temperature and magnetic field was reported as the strand named bronze 3 in [2].…”

$J_{C}(B,T,\varepsilon)$ Parameterization for the ITER ${\rm Nb}_{3}{\rm Sn}$ Production

“…The whole envelope of data covers an intrinsic strain range from 1.5% to 0.4%, a temperature range from 2.35 to 16 K and a field range from 0.5 to 19 T. The first 12 datasets have been fitted with the proposed parameterization. For the other datasets, the results of the comparison are quoted from [9] (datasets 13 through 15) and [23] (datasets 16 through 20). The quality of the fit has been judged based on the r.m.s.…”

Performance of Japanese $\hbox{Nb}_{3}\hbox{Sn}$ Conductors for ITER Toroidal Field Coils

“…(1) where and are the operating current density and the strand scaling which was obtained in the critical current density measurement as a function of field , temperature and axial strain on the strand [6]. The Durham University type formula is used for the scaling, where the n-value and strain are fitting parameters.…”

“…The solid line in Fig. 8 is the resultant fitting curve using strand scaling [6]. Estimated Tcs, strain and n-value are 6.4 K, 0.73% and 3.5, respectively.…”