Wilhelm A.J. Wessel

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

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.

Axial tensile stress–strain characterization of ITER model coil type Nb3Sn strands in TARSIS

For a few years there has been an increasing effort to study the impact of (bending) strain on the transport properties of superconducting wires. As the stress distribution, originated by differences in the thermal expansion and electromagnetic load, is the driving factor for the final strains, the axial and transverse stiffness of the strand play a crucial role in the final performance. Since the strain state of the Nb3Sn filaments in strands determines the transport properties, basic experimental stress?strain data are required at the strand level for accurate modelling and analysis and eventually for optimizing cable and magnet design. We performed axial tensile stress?strain measurements on several types of Nb3Sn strands used for the manufacture of the International Experimental Thermonuclear Reactor (ITER) central solenoid and toroidal field model coils and a powder-in-tube processed wire. In total 48 wire samples were tested at boiling helium, boiling nitrogen and at room temperature. We present the computation of the stress?strain characteristic with a straightforward 1D model using an independent materials database, obtaining a good agreement with the experimental results. The details from the take-off origin of the measured stress?strain curves are discussed and the data are evaluated with respect to some commonly used functions for fitting stress?strain curves. The measurements are performed in the new setup TARSIS (test arrangement for strain influence on strands). A double extensometer connected to the sample enables us to determine the strain level whereas a load cell is used to monitor the stress level. For higher levels of applied stress (100?MPa), we found typically a higher strain for bronze route wires compared to a powder-in-tube and internal tin type of strand. Stress?strain results are essential to assess more accurately the impact of thermal and electromagnetic induced stress on the strain state of the Nb3Sn filaments for wires from various manufacturing processes.

2013 The effect of axial and transverse loading on the transport properties of ITER Nb3Sn strands

The differences in 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 axial, transverse contact and bending strains in the Nb3Sn filaments. These local loads cause distributed strain alterations, reducing the superconducting transport properties. The sensitivity of ITER strands to different strain loads is experimentally explored with dedicated probes. The starting point of the characterization is measurement of the critical current under axial compressive and tensile strain, determining the strain sensitivity and the irreversibility limit corresponding to the initiation of cracks in the Nb3Sn filaments for axial strain. The influence of spatial periodic bending and contact load is evaluated by using a wavelength of 5?mm. The strand axial tensile stress?strain characteristic is measured for comparison of the axial stiffness of the strands. Cyclic loading is applied for transverse loads following the evolution of the critical current, n-value and deformation. This involves a component representing a permanent (plastic) change and as well as a factor revealing reversible (elastic) behavior as a function of the applied load.The experimental results enable discrimination in performance reduction per specific load type and per strand type, which is in general different for each manufacturer involved. Metallographic filament fracture studies are compared to electromagnetic and mechanical load test results. A detailed multifilament strand model is applied to analyze the quantitative impact of strain sensitivity, intrastrand resistances and filament crack density on the performance reduction of strands and full-size ITER CICCs. Although a full-size conductor test is used for qualification of a strand manufacturer, the results presented here are part of the ITER strand verification program. In this paper, we present an overview of the results and comparisons.

Critical current and strand stiffness of three types of Nb3Sn strand subjected to spatial periodic bending

Knowledge of the influence of bending on the critical current (Ic) of Nb3Sn strands is essential for the understanding of the reduction in performance due to transverse electromagnetic load. In particular, for the large cable-in-conduit conductors (CICCs) meant for the international thermonuclear experimental reactor (ITER), we expect that bending is the dominant mechanism for this degradation. We have measured the Ic of a bronze, a powder-in-tube and an internal tin processed Nb3Sn strand when subjected to spatial periodic bending using bending wavelengths from 5 to 10 mm. Two of these strands were applied in model coils for the ITER. We found that the tested strands behave according to the so-called low interfilament resistivity limit, confirming full current transfer between the filaments. This is supported by AC coupling loss measurements giving an indication of the interfilament current transfer length. The reduction of Ic due to bending strain can then be simply derived from the bending amplitude and the Ic versus axial applied strain (e) relation. This Ic(e) sensitivity can vary for different strand types but since the electromagnetic force is the driving parameter for strand bending in a CICC, the stiffness of the strands definitively plays a key role, which is confirmed by the results presented

Powder-in-tube (PIT) Nb/sub 3/Sn conductors for high-field magnets

New Nb/sub 3/Sn conductors, based on the powder-in-tube (PIT) process, have been developed for application in accelerator magnets and high-field solenoids. For application in accelerator magnets, SMI has developed a binary 504 filament PIT conductor by optimizing the manufacturing process and adjustment of the conductor lay-out. It uniquely combines a non-copper current density of 2680 A/mm/sup 2/@10 T with an effective filament diameter of about 20 /spl mu/m. This binary conductor may be used in a 10 T, wide bore model separator dipole magnet for the LHC, which is being developed by a collaboration of the University of Twente and CERN. A ternary (Nb/7.5wt%Ta)/sub 3/Sn conductor containing 37 filaments is particularly suited for application in extremely high-field superconducting solenoids. This wire features a copper content of 43%, a non-copper current density of 217 A/mm/sup 2/@20 T and a B/sub c2/ of 25.6 T. The main issues and the experimental results of the development program of PIT Nb/sub 3/Sn conductors are presented and discussed in this paper.

Cored Rutherford cables for the GSI fast ramping synchrotron

The new heavy ion synchrotron facility proposed by GSI will have two superconducting magnet rings in the same tunnel, with rigidities of 200 T/spl middot/m and 100 T/spl middot/m. Fast ramp times are needed, which can cause significant problems for the magnets, particularly in the areas of ac loss and field distortion. This paper discusses the 200 T/spl middot/m ring, which will use Cos/spl theta/ magnets based on the RHIC dipole design. We discuss the reasons for choosing Rutherford cable with a resistive core and report loss measurements carried out on cable samples. These measurements are compared with theoretical calculations using measured values of inter-strand resistance. Reasonably good agreement is found, but there are indications of nonuniformity in the adjacent resistance R/sub a/. Using these measured parameters, losses and temperature rise are calculated for a RHIC dipole in the operating cycle of the accelerator. A novel insulation scheme designed to promote efficient cooling is described.

A Novel “Test Arrangement for Strain Influence on Strands” (TARSIS): Mechanical and Electrical Testing of ITER Nb3Sn Strands

Knowledge on the deformation state of the strands in International Thermonuclear Experimental Reactor (ITER) type Cable‐In‐Conduit Conductors (CICC) and its impact on both the transport properties and the superconducting transition is essential for optimal cable and magnet design. To date a relatively large part of the attention is concentrated on the influence of purely axial strain on the performance of Nb3Sn strands and sub‐size cables. However, local deformations such as strand‐to‐strand, strand to conduit contact pressure and strand bending will occur as well in multi strand CICC’s experiencing an electromagnetic load. More basic experimental verification is required on strand level from virgin state towards multi‐cyclic loading. Therefore a new test arrangement is developed in which the influence of various principle deformation states that occur in a CICC, i.e. local transverse deformations, transverse homogeneous loads, bending and axial load, can be studied separately or combined. Beside precise ...

Axial Tensile Stress-Strain Characterization of a 36

A newly built test apparatus for mechanical characterization of ITER (International Thermonuclear Experimental Reactor) relevant sub-size cables is described, and the first experimental results on a 36 strands cable are presented. The cable consisting of nonreacted Nb3 Sn strands was subjected to tensile axial load up to 10 kN at temperatures of 300 K and 4.2 K. The Young modulus and ultimate tensile strength of the cable specimen are compared with those measured in single strands, extracted from the cable. The cable contraction was monitored during the tensile stress-strain testing. An analytical model for wire ropes is used to predict the cable stiffness. The test results are aimed at interpretation and validation of the cable models being under development

rm Nb_3rm Sn

For high current superconductors in high magnet fields with currents in the order of 50 kA, single ReBCO coated conductors must be assembled in a cable. The geometry of such a cable is mostly such that combined torsion, axial and transverse loading states are anticipated in the tapes and tape joints. The resulting strain distribution, caused by different thermal contraction and electromagnetic forces, will affect the critical current of the tapes. Tape performance when subjected to torsion, tensile and transverse loading is the key to understanding limitations for the composite cable performance. The individual tape material components can be deformed, not only elastically but also plastically under these loads. A set of experimental setups, as well as a convenient and accurate method of stress–strain state modeling based on the finite element method have been developed. Systematic measurements on single ReBCO tapes are carried out combining axial tension and torsion as well as transverse loading. Then the behavior of a single tape subjected to the various applied loads is simulated in the model. This paper presents the results of experimental tests and detailed FE modeling of the 3D stress–strain state in a single ReBCO tape under different loads, taking into account the temperature dependence and the elastic-plastic properties of the tape materials, starting from the initial tape processing conditions during its manufacture up to magnet operating conditions. Furthermore a comparison of the simulations with experiments is presented with special attention for the critical force, the threshold where the tape performance becomes irreversibly degraded. We verified the influence of tape surface profile non-uniformity and copper stabilizer thickness on the critical force. The FE models appear to describe the tape experiments adequately and can thus be used as a solid basis for optimization of various cabling concepts.

Strands Cable

We measured the influence of spatial periodic bending on the critical current (Ic) of Nb3Sn strands that were tightly swaged in a steel tube, imposing a compressive strain, in attempt to simulate the conditions representative for an ITER (international thermonuclear experimental reactor) cable in conduit conductor (CICC). The huge electromagnetic transverse force causes both transverse compressive strain in the strand crossover contacts and bending strain on top of the strain induced by the differential thermal contraction of the various conductor components. For a CICC with stainless steel or Incoloy conduit, the effect of the thermal cool-down strain on the reduction in performance due to periodic bending strain is unknown. We performed periodic bending tests in the TARSIS facility (test arrangement for strain influence on strands) on three samples. The degradation of the Ic due to bending appears to be much more severe for a strand swaged in a stainless steel tube than straightforward simulated by available models assuming simply axial strain dependency. It is confirmed by AC loss measurements that the extra degradation can not be attributed to a change in matrix transverse resistivity, possibly affecting the current transfer length, so we believe this is related to the three-dimensional strain state of the Nb3Sn layer.