Effects of Transverse Stress on the Current Carrying Capacity of Multifilamentary Wires

Differences in the thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER) in combination with electromagnetic charging cause significant axial, transverse and bending strains in the Nb3Sn layer. These high strain loads degrade the superconducting properties of a CICC. Here we report on the influence of periodic bending load, using different bending wavelengths from 5 to 10 mm on a Nb3Sn powder-in-tube processed strand. The strand axial tensile stress–strain curve, the critical current versus applied axial strain results, the influence of cyclic loading on the RRR and assessment of the current transfer length from AC loss measurements, required for the analysis, are presented as well. For the strand under investigation, we find an influence of bending strain on the Ic that corresponds well to the predictions obtained from the applied classical relations, distinguishing ultimate boundaries of high and low interfilament electrical resistance. The reduction versus applied bending strain is similar for all wavelengths and equivalent to the low transverse resistance model, which is consistent with the estimated current transfer length. The cyclic behaviour in terms of critical current and n-value involves a component representing a permanent reduction as well as a factor expressing reversible (elastic) behaviour as a function of the applied load. The results from the set-up enable a discrimination in performance reduction per specific load type and per strand type. In this paper, we discuss the results of the pure bending tests.

Section: Standard Strand Testssupporting

confidence: 75%

Impact of spatial periodic bending and load cycling on the critical current of a Nb₃Sn strand

Nijhuis

Eijnden

Ilyin

et al. 2005

“…Several experiments are known [35][36][37] but there is no generally applicable analytical expression available that describes the reduction of the critical current under increasing contact load. In addition, here, a polynomial fit through the experimentally obtained dependence gives the best accuracy.…”

Section: Periodic Contact Stressmentioning

confidence: 99%

A solution for transverse load degradation in ITER Nb₃Sn CICCs: verification of cabling effect on Lorentz force response

Nijhuis¹

2008

We present the latest results of the novel model for transverse electromagnetic load optimization (TEMLOP) especially developed for the ITER type of cable-in-conduit conductors (CICCs). The Nb3Sn CICCs for the International Thermonuclear Experimental Reactor (ITER) showed a substantial degradation in their performance correlated with increasing electromagnetic load. Not only do the differences in the thermal contraction of the composite materials affect the critical current (Ic) and temperature margin, but electromagnetic forces cause a significant transverse strand contact and bending strain in the Nb3Sn layers, resulting in localized filament cracking and permanent degradation. The most essential feature of the a priori TEMLOP predictions presented in May 2006 is that the severe degradation in CICCs can be improved greatly and straightforwardly by increasing the pitch length in subsequent cabling stages and by reducing the void fraction. These corrective measures give more support to the strands, sufficiently reduce the strain, and therefore avoid filament damage at the strand crossover points in the cables. It was the first time that an increase of the cable twist pitches has been proposed and no experimental evidence was available at that time. A full-size European prototype TF conductor sample (TFPRO-2), manufactured in autumn 2006, was adapted according to this new insight and tested in April 2007 in SULTAN for experimental validation of the predictions. The results were outstanding: for the first time an Nb3Sn CICC conductor achieved the performance that can be expected based on the single-strand properties, with high n value and no sign of degradation. As input, besides the cable properties, the model directly uses the measured data from single strands under uni-axial stress and strain, periodic bending and contact loads. The recent test results of the ITER OST strands used for the manufacture of the TFPRO-2 obtained with the TARSIS set-up are presented. With these most recent strand results, the model substantiates that not only strand bending is causing degradation but, depending on the strand and cable layout, the strand contact stress can also play a critical role. TEMLOP demonstrates that the twist pitch scheme and void fraction, of the proposed ITER reference TF conductor layout with a first-stage triplet twist pitch of 45 mm, turns out to be practically a worst-case scenario. It is also shown that shorter pitches can lead to an improvement but this requires more Nb3Sn material per metre composite conductor. However, it has been experimentally proven now that the proposed changes recover the ITER TF conductor operational margin up to the expected strand performance. The ITER TF conductor specification is being adapted now and it becomes possible to gain significant savings on the strand design, as degradation no longer needs to be compensated for.

“…Although several experiments are known [31][32][33][34], there is no analytical expression available that describes the reduction of the critical current under increasing contact load. On top of this, it is obvious that a polynomial fit through the experimentally obtained dependence has the best accuracy.…”

Section: Contact Stressmentioning

confidence: 99%

Transverse load optimization in Nb₃Sn CICC design; influence of cabling, void fraction and strand stiffness

Nijhuis¹,

Ilyin²

2006

132

134

We have developed a model that describes the transverse load degradation in Nb 3 Sn CICCs, based on strand and cable properties, and that is capable to predict how such degradation can be prevented. The Nb 3 Sn Cable In Conduit Conductors (CICC) for the International Thermonuclear Experimental Reactor (ITER) show a significant degradation in their performance with increasing electromagnetic load. Not only the differences in the thermal contraction of the composite materials affect the critical current and temperature margin, but mostly electromagnetic forces, cause significant transverse strand contact and bending strain in the Nb 3 Sn layers. Here, we present the model for Transverse Electro-Magnetic Load Optimisation (TEMLOP) and report the first results of computations for ITER type of conductors, based on the measured properties of the internal tin strand used for the Toroidal Field Model Coil (TFMC). As input, the model uses data describing the behaviour of single strands under periodic bending and contact loads, measured with the TARSIS setup, enabling a discrimination in performance reduction per specific load and strand type. The most important conclusion of the model computations is that the problem of the severe degradation of large CICCs can be drastically and straightforwardly improved by increasing the pitch length of subsequent cabling stages. It is for the first time that an increase of the pitches is proposed and no experimental data are available yet to confirm this beneficial outcome of the TEMLOP model. Larger pitch lengths will result in a more homogeneous distribution of the stresses and strains in the cable by significantly moderating the local peak stresses associated with the intermediate-length twist pitches. The twist pitch scheme of the present conductor layout turns out to be unfortunately close to a worst-case scenario. The model also makes clear that strand bending is the dominant mechanism causing degradation. The transverse load on strand crossings and line contacts, abbreviated as contact load, can reach locally 90 MPa but this occurs in the low field area of the conductor and does not play a significant role in the observed critical current degradation. The model gives an accurate description for the mechanical response of the strands to a transverse load, from layer to layer in the cable, in agreement with mechanical experiments performed on cables. It is possible to improve the ITER conductor design or the operation margin, by mainly a change in the cabling scheme. We also find that a lower cable void fraction and larger strand stiffness add to a further improvement of the conductor performance.