Critical Current Measurements Research Articles

DC performance of a Nb-Ti Rutherford cable: Measurements and model computations

High-current superconducting conductors are widely used in fusion magnets and high energy accelerators. In order to carry out experimental research on new high-current conductors, we have developed a new test facility for their critical current (Ic) measurements. We have used this facility to test a Nb-Ti Rutherford cable three times. Premature quenches were detected for the first two tests. They were mainly caused by small mechanical motion of the cable sample. After the mechanical support enhancement of sample, the critical current was successfully measured at the third time. The fixing structures of sample for these three tests are described and compared in consideration of transverse pressure applied to sample during different stages and the transverse displacement of cable after powering. The transverse pressure of first two tests reduced after cooling down and were not strong enough to withstand Lorentz force. Assuming uniform current distribution among strands and considering the inhomogeneous self-field, a model is developed to predict the V-I characteristics of the Rutherford cable using the measured properties of the strand. The expected Ic of cable under different background magnetic field are consistent with the measured results.

Effects of the initial axial strain state on the response to transverse stress of high-performance RRP Nb3Sn wires

High-performance Nb3Sn superconducting wires have become one of the key technologies for the development of next generation accelerator magnets. While their large critical current densities enable the design of compact accelerator-quality magnets for their operation above 10 T, the noticeable reduction of the conductor performance due to mechanical strain appears as a new essential characteristic in magnet design.In this work, we extensively investigate the effect of transverse loads, up to 250 MPa, in state-of-the-art Nb3Sn Restacked-Rod-Process round superconducting wires. The tests are performed using a compressive Walters spring device, where the force is applied to the resin-impregnated wire, and the critical current is measured under magnetic fields ranging from 16 to 19 T. As a complement, critical current measurements under axial strain are also performed using a standard Walters spring. Interestingly, the study shows that the wire’s electro-mechanical response under transverse stress depends on the initial axial strain condition. Nonetheless, when the main direction load becomes predominant, all tested wires converge to a common behavior. This observation allowed us to combine the results from critical current measurements under the loads exerted in both directions (axial and transverse), shedding some new light on the mechanisms behind critical current degradation.

Reverse advance internal magnesium diffusion process to produce dense MgB2 bulks and high JC wires through high pressure heat treatment

Here we report a new versatile technique to manufacture MgB2 massive samples, called reverse advance internal magnesium diffusion (r-AIMI). The idea focuses on the goal of obtaining dense bulk or wire samples depending on synthesis conditions. In respect to the traditional AIMI procedure, in which a central Mg rod is covered with a B corona, here a Mg tube is filled with B powder and clad in a Ti external sheath, which is quite similar to the traditional (powder in tube) technique. After cold deformation, during which several intermediate low temperature heat treatments are necessary in order to relax the Ti sheath and Mg tube, samples are reacted at high temperature and ambient pressure to form a dense MgB2 core. The MgB2 phase results are totally disconnected from metallic sheath, and can be easily extracted and characterized. Critical current density measurements show values exceeding 106 A cm−2 below 1.5 T at 20 K.In the last part of the paper, we show the effect of final heat-treatment performed under high pressure to eliminate the present void and connect the external sheath to the internal MgB2 core and so permitting the electric transfer necessary for power applications of wires.

Dynamic Transport Critical Current Measurements of MgB2 Superconductor

Dynamic Transport Critical Current Measurements of MgB₂ Superconductor

Weak emergence in the angular dependence of the critical current density of the high temperature superconductor coated conductor REBCO

Extensive critical current density measurements are reported as a function of magnetic field , temperature , angle between the applied field and the surface of the tape, and strain , on a REBCO coated conductor. The strain, , at which is maximised, is a function of , , and , which is consistent with weakly emergent behaviour. It is described by the chain model that considers competition between twinned domains with different crystallographic orientations and opposing responses under an applied uniaxial strain.Detailed effective upper critical field data are presented that show universal temperature and strain scaling. They lead to an accurate flux pinning relation for the volume pinning force, , where and and are constants, and are used to help parameterise the scaling behaviour of the angular data more accurately in those cases where cannot be measured directly. We derive approximate analytic in-field expressions that explain how the fraction, , of a-domains amongst the a- and b-domains affects the strain dependence of the critical parameters and conclude that in our tape, = 0.4, and the strain at which is the same in both domains is . We report a sharp peak in as the applied field approaches alignment with the ab-plane and the unusual result that with it, a suppression of the index of transition N also occurs. We find that the effective upper critical field increases as the field angle approaches the ab-plane significantly faster than any available theoretical model for the upper critical field .In addition, we conclude that a weak-emergence description is not limited to high temperature superconductors, but also describes some low-temperature superconductors.

Influence of Mechanical Cutting Methods on Critical Current of 1 mm HTS Tapes

Over the past decade, the critical current of the tape with 4 mm width exceeds 200 A. Due to the high critical current, the narrowing process to 1 mm width for the HTS tape becomes a feasible technology, and the value of critical current for the 1 mm tape is able to above 50 A. In this paper, critical current loss of 1 mm HTS tape was observed after the mechanical narrowing process. This significantly affects the properties of the 1 mm HTS tape. To understand the influence of narrowing process on critical current, we proposed mechanical cutting methods to manufacture the 1 mm tapes. Meanwhile, the solder plating process was also considered in the manufacturing process of 1 mm tape. The critical current measurements were carried out in order to study the loss of critical current. The detailed results about the critical current measurement experiment, the microscope observation experiment, and the solder plating process were presented and discussed in this study.

Effect of Ni<sub>0.5</sub>Zn<sub>0.5</sub>Fe<sub>2</sub>O<sub>4</sub> Nanoparticles Addition on Tl-1212 High Temperature Superconductor

The effects of Ni0.5Zn0.5Fe2O4 nanoparticle addition on the superconducting and transport properties of (Tl0.85Cr0.15)Sr2CaCu2O7 (Tl-1212) superconductor were investigated in this paper. The Tl-1212 samples were produced by mixing high purity oxide powders through a solid-state reaction method. Nano Ni0.5Zn0.5Fe2O4 particles with compositions of 0.001 wt.%, 0.003 wt.%, 0.005 wt.%, 0.01 wt.% and 0.02wt.% with average size of 60 nm were added into the Tl-1212 powders. The transition temperatures (Tc-zero and Tc-onset) were measured using a four-point probe method. The highest Tc-zero recorded was 97 K which was exhibited by the pure Tl-1212 sample. The transport critical current, Ic, of the Tl-1212 samples were found through the 1 µV/cm criterion with temperature ranging from 30 K to 77 K. The sample with a composition of 0.003 wt.% displayed the highest value of Jc at 77 K with a value ranging up to 1780 mA/cm2. The Tl-1212 samples were characterised using scanning electron microscopy (SEM), powder X-ray diffraction method (XRD), energy dispersive X-Ray analysis (EDX), electrical resistance measurements and transport critical current density measurements. The Jc of the Tl-1212 superconductor has been improved through the addition of Ni0.5Zn0.5Fe2O4 nanoparticles but adding an excessive amount has caused its Jc to degrade.

Evidence of Kramer extrapolation inaccuracy for predicting high field Nb3Sn properties

Future applications requiring high magnetic fields, such as the proposed Future Circular Collider, demand a substantially higher critical current density, J c, at fields ≥16 T than is presently available in any commercial strand, so there is a strong effort to develop new routes to higher J c Nb3Sn. As a consequence, evaluating the irreversibility field (Hirr ) of any new conductor to ensure reliable performance at these higher magnetic fields becomes essential. To predict the irreversibility field for Nb3Sn wires, critical current measurements, I c, are commonly performed in the 12-15 T range and the Kramer extrapolation is used to predict higher field properties. The Kramer extrapolation typically models the contribution only for sparse grain boundary pinning, yet Nb3Sn wires rely on a high density of grain boundaries to provide the flux pinning that enables their high critical current density. However, whole-field range VSM measurements up to 30 T recently showed for Nb3Sn RRP® wires that the field dependence of the pinning force curve significantly deviates from the typical grain boundary shape, leading to a 1-2 T overestimation of Hirr when extrapolated from the typical mid-field data taken only up to about 15 T. In this work we characterized a variety of both RRP® and PIT Nb3Sn wires by transport measurements up to 29 T at the Laboratoire National des Champs Magnétiques Intenses (LNCMI), part of the European Magnetic Field Laboratory in Grenoble, to verify whether or not such overestimation is related to the measurement technique and whether or not it is a common feature across different designs. Indeed we also found that when measured in transport the 12-15 T Kramer extrapolation overestimates the actual Hirr in both types of conductor with an inaccuracy of up to 1.6 T, confirming that high field characterization is a necessary tool to evaluate the actual high field performance of each Nb3Sn wire.

Winding Configurations and AC Loss of Superconducting Synchronous REBCO Motors

We investigate effects of armature winding configurations on properties of fully superconducting synchronous motors using REBCO tapes. The effect of the configurations on ac losses in the winding tapes due to the changing transvers magnetic fields applied to the tapes in direction of perpendicular to the flat face of the tapes is important. In generally, it is effective to close winding factor to 1 for improvement of properties of motors. For the purpose, the shape of armature windings is better saddle-shaped winding than race-track winding. In this paper, numerical calculations on the properties of fully superconducting synchronous motors using REBCO tapes with 500 kW class were described. Operating temperature was 65 K. In order to demonstrate the feasibility of saddle-shaped winding, test coil with 5 turns were wound with YBCO tapes and then critical current measurements were carried out. The degradation of the properties has not observed.

The magnetic field, temperature, strain and angular dependence of the critical current density for Nb-Ti

A scaling law for J c in commercial Nb-Ti wire is proposed that describes its magnetic field, temperature and strain dependence. The scaling law is used to fit extensive measurements of the total strand critical current density, J c,TS(B, T, ε), with the applied field orthogonal to the axis of the wire. We present critical current density, heat capacity and resistivity measurements to obtain , which shows clear angular anisotropy. At 4.2 K, the resistivity data show . We also discuss whether the fusion community should consider re-optimising standard commercial Nb-Ti wires that were developed for MRI applications at ~ 5 T, to produce higher J c at say 10T, and higher upper critical fields, perhaps using quaternary Nb-Ti alloys with artificial pinning centres.

Influence of local deformation on critical current of high temperature superconductor tape

In many applications, high temperature superconductor (HTS) tapes are placed on top of uneven surfaces. Because the electrical properties of HTS tapes are sensitive to mechanical loading, mechanical stress generated at such local imperfections can considerably decrease the critical current of a HTS tape. Then a method of predicting degradation of critical current from a numerical simulation would be helpful.We present the results of 3D mechanical finite element model analysis, using the software ANSYS for optimising the geometry of a round CORT (Conductor-On-Round-Tube) cable. In these simulations, the HTS tape is bent around a circular former of certain diameter with a surface imperfection. We used three parameters to characterize the imperfection geometry: diameter of the former, the height and the width of imperfection. In order to validate the mechanical simulation results, we performed in-situ measurement of critical current of HTS tape.

Heat treatment influence on microstructure and properties of MgB2 superconductor

Study results of 1 mm diameter MgB2 superconductors fabricated by “powder in tube” method are presented. Ti and Nb were used as diffuse barriers materials. The mixture of magnesium and boron, powder of MgB2, and also mixture of magnesium, boron and MgB2 were used to form superconducting compound. Heat treatments in temperature range of 650-700 °C and 750-900 °C for 1h were conducted. It is established that 1 h heat treatment at 700 °C or 1 h at 900 °C heat treatment are sufficient for formation of MgB2 superconducting phase (Tc=38 K) in superconductors with titanium barrier and powder mixture of magnesium and boron or magnesium, boron and magnesium diboride. The Tc of in-situ process Nb - sheated superconductors was 37.6 K, the same Tc were measured for ex-situ superconductors heat treated at 900 °C for 1 h. The critical current measurement results of fabricated superconductors were presented, optimal heat treatment modes were determined for fabricated superconductors.

A Critical Current Measurement Method for Strip Hard Superconductors Utilizing Pulsed Current

Long wire critical current (Ic) characterization is important for fabrication of superconducting magnets. Many inductive and transport Ic measurement methods have been developed for km scale rare earth cuprate (REBCO) based Coated conductors (CC), which is a very promising high temperature superconducting material for magnet applications. As a direct indication of the current carrying capability, transport measurement is favored by most tape users. However, the traditional transport measurement technique has its limitations for long tape characterization. The speed of transport measurement is usually not high enough for km scale tapes, and direct current (DC) measurement also has a certain risk of tape damage due to the Joule heating at current contacts, especially for some special CCs with higher contact resistance, e.g., stainless-steel encapsulated CCs. For long wire measurements, the risk of tape damage is generally higher than acceptable. A pulse measurement method based on induction principle is proposed and tested in this work, the feasibility of this method is verified by experiments. Where pulsed current is used to reduce Joule heating thus reduce the risk of tape damage. Moreover, the speed of measurement can also be improved by the technique compared with DC measurements.

A Transducer for Measuring the Field Quality in Superconducting Solenoids

The critical current of superconducting wires is measured at CERN in four test stations that use high-field superconducting solenoid magnets achieving up to 15 T. The reduction of the uncertainty in the critical-current measurements requires the field mapping of the solenoids in their operating condition. This paper presents the design and application of a solenoidal-field transducer based on a pair of nested induction coils, which are moved along the axis of the magnet. The system allows to extract the longitudinal and transverse field components as a function of the sensor's longitudinal position. The radial dependence of the magnetic flux density can be estimated directly from the nested coils of different diameters validated by applying the theory of pseudo-multipoles in solenoidal magnets. The results and uncertainties of the field mappings are presented.

Synthesis, characterization and electrical properties of hybrid mono-iron-substituted undecatungstosilicate/(Bi,Pb)-2223 phase superconductors

A conventional solid-state reaction technique was used to synthesize superconducting samples of (Bi1.8Pb0.4)Ca2Sr2Cu3O10+δ, (Bi,Pb)-2223, phase. The electrical properties of (Bi,Pb)-2223 samples were investigated upon the addition of different amounts of ferratoundecatungstosilicate (K5[SiFe(OH2)W11O39]·14H2O, {FeSiW11}) nanoparticles, x, ranging from 0.00 to 0.20 wt%. The optimum preparation conditions to obtain high quality superconducting bulk samples were considered. The average particle size of these antiferromagnetic nanoparticles was approximately 19.56 nm. Such Fe-containing polyoxometalates are a highly interesting class of inorganic nanomaterials, possessing outstanding properties of high oxidation states as well as thermal and oxidative stability. Consequently, the superconducting samples were characterized by powder x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) to study the structure, morphology and elemental composition of the prepared samples, respectively. The volume fraction of the (Bi,Pb)-2223 phase was improved by the nano-addition, while the lattice parameters showed unchangeable values. On the other hand, the superconducting electrical parameters were assessed via dc electrical resistivity and critical current density measurements. The obtained results indicated that the addition of nano {FeSiW11} enhanced the superconducting transition temperature (Tc) and the transport critical current density (Jc) up to x = 0.16 and 0.12 wt%, respectively. Conversely, Tc and Jc were suppressed with further increases of nanoparticle addition.

Test Facility for Critical Current Measurement of Superconducting Conductors

High-current superconducting conductors are widely used in fusion magnets, high-energy accelerators, and superconducting power transmission. In order to carry out experimental research on new high-current conductors such as Bi 2 Sr 2 CaCu 2 O x (Bi-2212) Cable-in-Conduit conductor (CICC), YBa 2 Cu 3 O 7-x (YBCO)-twisted stacked-tape cable (TSTC) conductor, and Rutherford cable, we have developed a new test facility for their critical current measurement. The key component of the facility is a 6-T dipole magnet with a useful bore of 80 mm, which is connected through a pair of 6-kA binary-current leads to the external dc power supply. The homogeneous high field length is 400 mm with uniformity better than 98%. Instead of the traditional expensive dc power supply and complicated current leads, a superconducting transformer is used to supply the test sample with current up to 30 kA. Quench detection and protection system are also used for both the primary coil of superconducting transformer and the 6-T dipole magnet. An NbTi Rutherford cable with ten strands has been tested to verify the test facility. The status of our test facility and the test results are described in this paper.

How to Computationally Determine the Maximum Stable Operation Current of an HTS Magnet

The short-sample critical current is only an indicative property for the maximum current a magnet can be continuously operated with. This was especially visible in the experiments of one of the world's first Roebel-cable-based high temperature superconductors dipole magnet prototype built and tested at CERN in 2017 where the thermal runaway developed very slowly in many cases. Consequently, the maximum stable operation current could be overstepped and stable operation could be recovered by lowering the current below the maximum of the stable range again. It is non-trivial to quantitatively predict this behavior from the critical current measurements which are observed under specific cooling conditions and based on an arbitrarily selected electric field criterion for the critical current. To make more rigorous predictions on the maximum stable operation current, one needs to consider in detail the interplay of cooling over the magnet surface and heat generation in the winding. This paper presents a methodology to determine the maximum stable operation current for a given magnet, as well as studies its mathematical background. Insight to this problem comes from the Roebel-cable-based dipole magnet studied at CERN during 2017.

Electro-Magnetic Performance of MgB2 Cables for the High Current Transmission Lines at CERN

Intense R&D work is ongoing at CERN in the framework of the High Luminosity LHC (HL-LHC) upgrade for the development of high current transmission lines, known as Superconducting Links (SC Links). The SC Link is an assembly of cables made from reacted MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> round wire. There are three different cable configurations, each formed from MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cables rated at different currents of up to 7 kA. Considering the different layouts, a compromise between the cabling parameters and the properties of the reacted MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires is essential to ensure the operational and functional requirements of the cables after the cabling process. In the present paper we report on the electromagnetic characterization of the three MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cables: 2 × 3 kA, 3 kA, and 7 kA. The cabling parameters were defined to fulfil the electromechanical characteristics of the reacted wires. The cabling process was experimentally assessed and its effect on the electrical performance of the cables was evaluated at 4.3 K. The validation of the electrical performance was performed by means of critical current measurements of short cables (2 m) in the FRESCA test facility at CERN. Finally, good agreement was found between the measured and the expected electrical performance of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cables.

Evidence for Longitudinal Homogeneity and No &lt;italic&gt;J&lt;sub&gt;e&lt;/sub&gt; &lt;/italic&gt; Degradation in Bi-2212 Wires Realized by the GDG Process

The GDG fabrication process developed at CNR-SPIN is a promising method for producing Bi-2212 wires without overpressure processing. In this paper, critical current measurements performed at ENEA on both short and long samples in magnetic fields up to 14 T are compared. The results show no significant difference in Je between short and long (∼1 m) samples, and suggest very good longitudinal uniformity, validating the effectiveness and scalability of the process. In addition, the suitability of Inconel 625 for reaction and test barrels is demonstrated, and the reasons for obtaining lower than expected absolute Je values are discussed.

Overcurrent Characteristics on a 22 kV/12 kA HTS Model Cable

Generally, electric power is transmitted from a generator through a step-up transformer to high-voltage transmission lines. The feeder from the generator to the step-up transformer is often installed with an aluminum bus line and so on because the current through the feeder is quite large; likely more than 10 kA. Even a superconducting cable can transmit a large current in compact size, so this type of feeder from a generator is expected to be one of the applications of superconducting cables. Therefore, together with Tokyo Electric Power Company, we have been developing HTS cables for feeder lines and constructed a 10-m model cable with 22-kV/12 kA capacity. Several performance tests were conducted on the model cable, including critical current measurement, nominal current application, ac loss measurements, and ac voltage withstanding tests. Overcurrent tests were also conducted. In the TEPCO grid of 22 kV, a fault current is estimated to be 63 kA for 0.6 s. Because the fault current is about five times larger than the nominal current of 12 kA, when such a large fault current goes through an HTS cable, the superconductor is expected to return to the normal state to make a large amount of heat generation. These phenomena may cause a variety of problems. Due to the limitations of the current supply apparatus, the overcurrent tests were conducted with dc current instead of ac current at an LN2 temperature of 77 K and pressure of 0.2 MPaG. The critical current of HTS cable was 19 kA, and the dc overcurrent was 38 kA for 5 s, with the generated energy estimated to be greater than that from 63 kA for 0.6 s. Test results showed the maximum pressure increase to be approximately 20 kPa, which is not so large as to cause problems. The critical current tests of the HTS cable found no degradation between before and after condition. However, it is not certain whether significant pressure increases might happen in a longer HTS cable system, such as an actual case 100 m long. Therefore, we have also been developing a simulation code and will compare the experimental result with the analytical one.