Evidence that filament fracture occurs in an ITER toroidal field conductor after cyclic Lorentz force loading in SULTAN

Abstract

We analyzed the ITER TFEU5 cable-in-conduit conductor (CICC) after the full SULTAN conductor qualification test in order to explore whether Lorentz force induced strand movement inside the CICC produces any fracture of the brittle Nb 3 Sn filaments. Metallographic image analysis was used to quantify the change in void fraction of each sub-cable (petal); strands move in the direction of the Lorentz force, increasing the void space on the low force side of the CICC and producing a densification on the high force side. Adjacent strand counting shows that local increases in void space result in lower local strand-strand support. Extensive metallographic sampling unambiguously confirms that Nb 3 Sn filament fracture occurred in the TFEU5 CICC, but the filament fracture was highly localized to strand sections with high local curvature (likely produced during cabling, where strands are pivoted around each other). More than 95% of the straighter strand sections were free of filament cracks, while less than 60% of the bent strand sections were crack free. The high concentration of filament fractures on the tensile side of the strand-strand pivot points indicates that these pivot points are responsible for the vast majority of filament fracture. Much lower crack densities were observed in CICC sections extracted from a lower, gradient-field region of the SULTAN-tested cable. We conclude that localized filament fracture is induced by high Lorentz forces during SULTAN testing of this prototype toroidal field CICC and that the strand sections with the most damage are located at the petal corners of the high field zone.

Comparisons of internal self-field magnetic flux densities between recent Nb3Sn fusion magnet CICC cable designs

“…From Fig. 2 and the data or information in [6], [20][21][22], [45] and [46], a cumulative reverse tensile strain (ε) of only 0.05% in the Nb 3 Sn strands can explain the total T cs increase in ITER CS CICC samples with STP cable, which is consistent with a finding in [16]. Calculation of the strand stress (σ) were attempted and using the relation…”

“…This is clear from destructive examination of CICC samples [4][5][6]20]. The jacket and central cooling spiral act as the most outer and inner layer barriers and boundary to the cable with compression of the strands from the rigid jacket or cooling spiral having detrimental effects.…”

Analysis of Nb3Sn Strand Microstructure After Full-size SULTAN Test of ITER TF Conductor Sample

“…We also attempted to compare our results with [6], where the strands from the TFEU5 conductor were examined. The drop of T cs after testing in SULTAN in TFEU5 was 0.35 K [6] against 0.1 K in TFRF3 [8]. We have converted our results into cracks/mm terms used in [6].…”

“…Among multiple effects that may contribute to the degradation, damage of the strands in form of filamentary cracks is considered as an important factor. Several works have addressed studying the cracks formation in the ITER type Nb 3 Sn strands [3,4,5,6]. In some of them cracks distribution in the strands taken from the SULTAN CICC samples after testing were investigated [6].…”

“…Several works have addressed studying the cracks formation in the ITER type Nb 3 Sn strands [3,4,5,6]. In some of them cracks distribution in the strands taken from the SULTAN CICC samples after testing were investigated [6]. There are several suppliers which produce TF conductors for the ITER.…”