Wouter Abbas

A device to investigate the axial strain dependence of the critical current density in superconductors

We have developed an instrument to study the behavior of the critical current density (Jc) in superconducting wires and tapes as a function of field (μ0H), temperature (T), and axial applied strain (ea). The apparatus is an improvement of similar devices that have been successfully used in our institute for over a decade. It encompasses specific advantages such as a simple sample layout, a well defined and homogeneous strain application, the possibility of investigating large compressive strains and the option of simple temperature variation, while improving the main drawback in our previous systems by increasing the investigated sample length by approximately a factor of 10. The increase in length is achieved via a design change from a straight beam section to an initially curved beam, placed perpendicular to the applied field axis in the limited diameter of a high field magnet bore. This article describes in detail the mechanical design of the device and its calibrations. Additionally initial Jc(ea) dat...

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

Performance of an ITER CS1 model coil conductor under transverse cyclic loading up to 40,000 cycles

The large currents in the cable-in-conduit conductors (CICC) destined for the high field magnets in the International Thermonuclear Experimental Reactor (ITER), cause huge transverse forces on the strands compressing the cable against one side of the conduit. This load causes transverse compressive strain in the strands at the crossovers contacts. Moreover, the strands are also subjected to bending and contact surfaces micro-sliding, which results into friction and anomalous contact resistance versus force behavior. Three Nb/sub 3/Sn central solenoid model coil (CSMC) conductors were tested previously in the Twente Cryogenic Cable Press up to 40 cycles with a transverse peak load of 650 kN/m. This press can transmit a variable (cyclic) transverse force directly to a cable section of 400 mm length at a temperature of 4.2 K (or higher). To explore life-time cycling, we tested a CSMC Nb/sub 3/Sn conductor up to 40,000 cycles. The coupling loss and the associated interstrand resistance between various strands and strand bundles are measured at various loads. The force on the cable and the displacement are monitored in order to determine the effective cable Youngs modulus and the mechanical heat generation. Some aspects of strand deformation in CICCs are discussed. The test results are discussed in view of previous press results and data extracted from the ITER model coil tests.

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 ...

Spatial Periodic Bending and Critical Current of Bronze and PIT

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.

{\rm Nb}_{3}{\rm Sn}

The Nb3Sn cable in conduit conductors (CICCs) for the International Thermonuclear Experimental Reactor (ITER) show a significant degradation in their performance with increasing electromagnetic load. Knowledge of the influence of bending and contact stress on the critical current (Ic) of the Nb3Sn strands is essential for the understanding of this reduction in performance. We have measured the Ic of Nb3Sn strands in the TARSIS facility, when subjected to spatial periodic bending using bending wavelengths from 5 to 10 mm, periodic contact stress and uni-axial strain on two strand types, manufactured by OST. These strands were used for the cables of the European TFPRO-2 sample tested in SULTAN. A priori predictions with the TEMLOP model led to the discovery that the severe Lorentz force response and degradation can be completely improved by increasing the pitch length in subsequent initial cabling stages. The TFPRO-2 /OST-II conductor, especially designed to examine this prediction, verified this significant enhancement. TEMLOP directly uses data describing the behavior of single strands under uni-axial stress and strain, periodic bending and contact loads. Here we give an overview of the TARSIS strand testing results.

Strands in a Steel Tube

In ITER CICCs, the strands experience varying mechanical strain levels due to thermal compression and electromagnetic loading, cumulating into severe periodic bending and contact strain. Depending on the choice of the cabling pattern, the strain may exceed the irreversibility limit leading to cracks in the Nb3Sn filaments and degraded performance of the conductors. We present a systematic microscopy study of filament fracture at gradually varied axial tensile strain levels in a bronze and internal tin processed type of Nb3Sn ITER strand. The axial stress-strain relation of the wires was measured at 4.2 K, up to a different peak load for each sample, successively increasing from 0.0% to 0.7%. Longitudinal cross sections were prepared from cut sections of the specimens. The crack pattern and distribution in the isolated filaments could thus be studied as a function of applied strain, revealing different fracture mechanisms. Electrical measurements were performed with applied strain with special attention for the tensile strain range. Finally, the microscopic information was correlated back to the observed macroscopic I c degradation.

Performance of ITER (EU-TFPRO-2)

The combination of current up to 50 kA and magnetic field of 13 T in the Cable-In-Conduit Conductors (CICC) for the coils in the International Thermonuclear Experimental Reactor (ITER), cause huge local transverse forces. This results in changes in the transport properties, friction and anomalous contact resistance versus force behavior. The latest design optimizations tend to go toward a lower void fraction (VF). This has an impact on the evolution of the coupling loss and on the possible degree of strand bending and deformation. Toroidal Field Model Coil (TFMC) type of conductors with VFs of 26%, 30% and 36% respectively, are tested in the Twente Cable Press, by which a variable (cyclic) transverse force of 650 kN/m is transferred directly to a cable section of 400 mm length at 4.2 K. The AC loss of the conductor, the inter-strand and strand-bundle resistance (R/sub c/) in the cable and the associated bundle deformation are examined during mechanical cycling. The test results are discussed in view of the previous results on Nb/sub 3/Sn ITER CICCs.

{\rm Nb}_{3}{\rm Sn}

A cryogenic cable press has been built and recently modified to investigate the life time properties and changes in the mechanical and electrical properties of full‐size ITER (International Thermonuclear Experimental Reactor) conductors. The press is a fully automatically controlled operating system designed to test Cable‐In‐Conduit Conductors (CICC) under transverse variable load along 40,000 cycles at temperatures from 4.2 to 300 K. The designed lifetime of the press, with moving parts and minimized friction at cryogenic temperatures, is at least a few millions of cycles. The maximum press load is 320 kN with a stroke of 5.5 mm. The maximum sample length amounts to 400 mm. The force on the cable and the compression are monitored simultaneously in order to determine the mechanical cable deformation properties and losses. A superconducting dipole coil integrated in the press surrounding the sample, provides the DC and AC magnetic field required to perform magnetization measurements with pick‐up coils. In ...

Strands Under Spatial Periodic Bending, Axial Strain and Contact Stress

In Nb3Sn cable-in-conduit conductors for ITER, the superconducting strands follow complex trajectories defined by the twist pitches of multi-stage cabling and the void fraction of the conductor. The conductor in operation suffers a Lorentz load which results in distributed strand deformations that alter the overall electromagnetic and mechanical behavior of the conductor. With the Twente cable press, a cable specimen is subjected to transverse load up to 30,000 cycles and the changes in the inter-strand contact resistances, the cable deformation, and the coupling loss are monitored. An ITER toroidal field coil conductor with option-II cabling scheme is tested for the first time in the press. In comparison with previously measured conductors, the long twist pitches of option-II result in a higher inter-strand contact resistance, a lower stiffness of the conductor, and initially a higher coupling loss time constant. Within 100 cycles, the time constant decays to a comparable level as the low void fraction conductor with short twist pitches.