Critical Current Measurements Research Articles

Design of a REBCO thin film superconducting undulator

Recent developments have shown that superconducting undulators, mainly NbTi- based, outperform the existing devices. However, cooling these undulators is a challenge. REBCO (RE = rare earth, barium copper oxide) coated conductors (CCs) have been found to be a promising alternative to these materials due to their larger temperature stability margin and high engineering current densities. Here, we have investigated the feasibility of building an undulator magnetic structure using REBCO coated conductors and conducted experiments to evaluate their performance. The undulator coil has been wound with no inter-layer insulation. The critical current measurements at 77K showed that the winding of the undulator does not noticeably deteriorate the performance of the tape. Transient behaviour of the undulator has also been investigated and found to be characterized by long magnetic field decay times that result from current sharing between the windings. Steady-state operation was found to be very suitable for most undulator applications.

Decrement in metastability with Zr nanoparticles inserted in Bi-2223 superconducting system and working principle of hybridization mechanism

This exhaustive study experimentally deals with the role of Zr foreign impurities on the electrical, superconducting and flux pinning properties of the bulk Bi-2223 superconducting compounds by the typical experimental characterization methods such as dc resistivity (ρ–T), transport critical current density (J c ) and powder X-ray diffraction (XRD) surveys. All the experimental findings show that the curial characteristics, being in charge of the varied attractive and feasible applications, retrograde significantly with the enhancement of the Zr nanoparticles in the Bi-2223 superconducting system due to the presence of two trap levels for mobile holes. This leads to the degradation of the metastability of the superconducting materials. In more detail, based on the dc resistivity and transport critical current density measurements, the Zr additives give rise to the localization problem as a consequence of the change in the dynamics of electron–electron interaction. Thus, the room state conductivity, $$T_{c}^{onset}$$ , $$T_{c}^{offset}$$ and J c values retrograde significantly. Similarly, the Zr foreign impurities embedded in the Bi-2223 crystal lattice make the artificial random defects, dislocations and grain boundary weak-interactions in the consecutively stacked layers enhance immediately, and the superconducting Cu–O2 layers distort especially. Moreover, the XRD investigations including the evidences about the decrement/increment in the c-axis/a-axis length verify the regression of the superconducting properties with the Zr dopant. The main differentiation of the lattice constant parameters stems from aliovalent substitutions (replacement of the divalent Cu by the tetravalent Zr impurties) along with the crystal structure. In other words, the reduction of the electronegativity leads to decrease the mobile hole concentration in the Cu–O2 consecutively stacked layers. Furthermore, all the decrement parameters are theoretically favored by hybridization mechanism.

The electric field jump—detection of damaged Nb barrier in MgB2 wires annealed under pressure

In this paper we present the results of critical current (Ic) measurements of MgB2 wires made with two different set-ups of the four-point probe method: current sweep type—constant magnetic field and increasing current, and field sweep type—constant current and rapidly increasing magnetic field. Results from magnet field sweep type measurements can be interpreted by a new physical concept—a jump of the electric field in low magnetic fields. This physical concept can be correlated with damages in the Nb-barrier existing in the MgB2 wire and be employed as a detection scheme. The damage in Nb barrier reduces critical current density (Jc) and complicates the study on critical temperature (Tc), upper critical field (Bc2), irreversible magnetic field (Birr), pinning force (Fp), and pinning centers in superconducting MgB2 wires. Our proposed method to detect damages in Nb barrier would benefit efforts in development and applications of MgB2 wires.

Calibration of Hall sensor array for critical current measurement of YBCO tape with ferromagnetic substrate

HAS (Hall sensor array) is a powerful tool to detect the uniformity of HTS (high temperature superconductor) tape through mapping the distribution of remanent or shielding field along the surface of the tape. However, measurement of HTS tape with ferromagnetic parts by HSA is still an issue because the ferromagnetic substrate has influence on the magnetic field around the HTS layer. In this work, a continuous HSA system has been designed to measure the critical current of the YBCO tape with ferromagnetic substrate. The relationship between the remanent field and critical current was calibrated by the finite element method. The result showed that the HSA is an effective method for evaluating the critical current of the HTS tape with ferromagnetic substrate.

Performance verification tests of JT-60SA CS model coil

As a final check of the coil manufacturing method of the JT-60 Super Advanced (JT-60SA) central solenoid (CS), we verified the performance of a CS model coil. The model coil comprised a quad-pancake wound with a Nb3Sn cable-in-conduit conductor. Measurements of the critical current, joint resistance, pressure drop, and magnetic field were conducted in the verification tests. In the critical-current measurement, the critical current of the model coil coincided with the estimation derived from a strain of −0.62% for the Nb3Sn strands. As a result, critical-current degradation caused by the coil manufacturing process was not observed. The results of the performance verification tests indicate that the model coil met the design requirements. Consequently, the manufacturing process of the JT-60SA CS was established.

Influence of YBa2HfO5.5 – ‘derived secondary phase’ on the critical current density and flux-Pinning force of YBa2Cu3O7−δ thick films

Enhancement in critical current density (Jc) and flux pinning force (Fp) in superconducting thick films of YBa2Cu3O7−δ (YBCO) added with small quantities of nanopowders of HfO2, BaHfO3 and YBa2HfO5.5, coated on YBa2ZrO5.5 substrate by dip-coating technique is reported. Critical current density measurements were done over an applied magnetic field using standard four probe technique and the results are compared with that of pure YBCO. High critical current density (Jc) of ∼4.84MA/cm2 at 77K in self-field was obtained for 2wt% of YBa2HfO5.5 added YBCO. A systematic increase in Jc observed in YBCO films prepared by the addition of nano HfO2, BaHfO3 and YBa2HfO5.5, attributed to the formation of a non-reacting ‘derived secondary phase’ YBa2HfO5.5 (YBHO) in the YBCO matrix. YBCO–YBa2HfO5.5 composite thick films have showed eightfold increases in Jc (3.29MA/cm2) at 77K and 0.4T compared to pure YBa2Cu3O7−δ film (0.37MA/cm2), while maintaining a high transition temperature (Tc). The development of effective pinning centers in nano particle added YBCO thick film have enhanced the flux pinning force from 1.8GN/m3 for pure YBCO to a maximum value of 13.15GN/m3 for YBCO–YBa2HfO5.5. X-ray diffraction and energy dispersive spectroscopic analysis confirmed the presence of secondary phase, derived in the matrix.

A New Experimental Setup to Measure Critical Current in Superconducting Strands Under Periodic Bending

Cable-in-conduit conductors made of Nb 3 Sn strands, as envisaged in high-field magnets for fusion applications (e.g., the International Thermonuclear Experimental Reactor (ITER) project), have been shown to be prone to degradation of their current transport capability when subject to high electromagnetic forces (coming from the combination of a high current and a high magnetic field), particularly under cyclic loading. Although some optimization of the cable layout can be envisaged to mitigate this effect, the knowledge of the electrical properties of Nb 3 Sn strands under periodic bending (simulating in-cable operating conditions) is generally considered a needed characteristic for magnet designers and users. The French Alternative Energies and Alternative Energies and Atomic Energy Commission (CEA)/Institute for Magnetic Fusion Research (IRFM) has developed a new Versailles Project on Advanced Materials and Standards (VAMAS)-like mandrel in order to test under relevant field and temperature conditions high-field superconducting strands subject to periodic bending provided by the strand deformation under its own Lorentz force. The advantage of this setup is its similarity with the setup used in critical current measurements on a VAMAS mandrel, which makes its implementation in existing facilities quite easy and rather cheap, as well as possible direct comparisons with unbent samples on a classical VAMAS mandrel. This paper presents the design layouts and carefully depicts the assembly of the different tested samples. The first test results obtained on Nb 3 Sn strands are given and discussed.

Fabrication of HTS Conductor Stacks for 20-kA Binary Current Leads for MagLab Series-Connected Hybrid and HFML Nijmegen Hybrid Magnets

The MagLab series-connected hybrid and the HFML Nijmegen hybrid outer superconducting coils each require a pair of binary current leads, comprised of a nitrogen vapor-cooled resistive section and a high-temperature superconducting (HTS) section, configured to minimize the heat load to the coils. The HTS sections of both pairs of leads are made with stacked and soldered Bi-2223 tapes supplied by Sumitomo. The MagLab fabricated and tested the tape stacks for both programs. The procedures and apparatus for fabricating the stacks and making the critical current measurements are described. Critical current and n-value measurement results are reported.

Fabrication and Testing of Racetrack-Shaped Double-Pancake Coil for Stator Windings of Induction-Synchronous Motor

We have fabricated a racetrack-shaped spliceless double-pancake test coil with GdBCO coated conductors and investigated its performance in air and magnetic cores. We have been developing a high-temperature superconductor induction/synchronous machine (HTS-ISM) for electric vehicles. To improve the specific power and torque density of the fully superconducting HTS-ISM, an increase in the current capacity of the HTS stator windings is crucial. The fabricated test coil was wound with two GdBCO conductors in parallel, which were transposed at a coil end. Thus, the self-inductances of the two conductors were equal, and the coil was free from splice resistances due to transposition. We confirmed the soundness of the transposition method by critical current measurements. We also investigated the critical currents of the test coil by placing it in a magnetic core and observed a decrease in the critical current compared with that in the air core. This is caused by the increase in the component of the magnetic field perpendicular to the coated conductors. To overcome this issue, magnetic materials were used for magnetic-field correction. We methodically compared the results of the critical current measurements and evaluated the results by numerical magnetic-field analyses.

Quench Propagation in <inline-formula> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn} $</tex-math></inline-formula> Rutherford Cables for the Hi-Lumi Quadrupole Magnets

The quadrupole magnets for the LHC High Luminosity (Hi-Lumi) upgrade will be based on Nb 3 Sn Rutherford cables that operate at 1.9 K and experience magnetic fields larger than 12 T. Because of the large stored energy, one of the major issues in the design of these magnets is their protection. To study the quench propagation in Nb 3 Sn Rutherford cables for the Hi-Lumi Quadrupole magnets, a campaign of measurements was carried out in the FRESCA test station at CERN. In addition to the critical and stability current measurements at 4.3 K and 1.9 K, we performed a series of tests where the quenches were induced by resistive heaters impregnated in the cable. The longitudinal and transversal speeds of the normal zone propagation were investigated at different magnetic fields and operating currents. In this paper, the numerical and experimental results are reported, compared and discussed.

Comparison of Critical Current Versus Axial Strain Measurements on Internal Tin <inline-formula> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex-math></inline-formula> Strand at ASIPP and University of Twente

In the last decades, various methods have been used to characterize superconducting behavior versus axial strain. In cooperation with the University of Twente (UT), the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) built a facility practically similar to the so-called “Pacman” spring device. The facility allows critical current measurements of an Nb3Sn strand at different temperature, magnetic field and applied axial strain. In the process of validating the new setup, an ITER Internal-Tin (IT) strand, which has also been measured at the UT, is tested with the ASIPP Pacman. The ITER IT wire has an irreversible strain limit around +0.3%. At ASIPP, the strand was characterized in a magnetic field from 10 to 13 T and applied axial strain from -0.7% to +0.5%. Similarities of the data obtained (IC(e), IC(B) and irreversible strain limit) from the two institutes were remarkable.

Measurement of critical currents of superconducting aluminum nanowires in external magnetic fields: evidence for a Weber blockade.

We have studied the critical current as a function of magnetic field in short and narrow superconducting aluminum nanowires. In the range of magnetic fields in which vortices can enter a nanowire in a single row, we find regular oscillations of the critical current as a function of magnetic field, with each oscillation corresponding to adding a single vortex to the nanowire. In this regime, the nanowires behave as quantum dots for vortices. As a function of current and magnetic field, we find diamond-shaped regions in which the resistance is zero and the number of vortices is fixed.

Flux pinning mechanism in SiC and nano-C doped MgB2: evidence for transformation from δTc to δℓ pinning

Magnetic and transport properties of 10 wt% SiC doped MgB2 and 5 wt% nano-C doped MgB2 were studied by resistance and critical current density measurements. The results showed improvement of the critical current density for the MgB2 superconductor doped with SiC in comparison with the nano-C doped sample. The flux pinning mechanisms of both doped MgB2 superconductors have been investigated based on the collective theory. It was found that the pinning mechanism in MgB2 was transformed by SiC doping from transition temperature fluctuation induced pinning, δTc pinning, to mean free path fluctuation induced pinning, δℓ pinning, while in the MgB2 doped with nano-C, δTc and δℓ pinning coexist. Their contributions are strongly temperature dependent, however. The δℓ pinning is dominant at low temperature, decreases with increasing temperature, and is suppressed completely at temperatures close to Tc. The δTc pinning mechanism shows the opposite trend.

Influence of Pb(BO2)2 Doping on Superconducting Properties of (Bi,Pb)-2223 HTS

The effect of lead borate Pb(BO2)2 doping on the phase formation and superconducting properties of (Bi,Pb)-2223 high-temperature superconductivity (HTS) has been studied. Samples with nominal composition Bi1.7Pb0.3−xSr2Ca2Cu3Oy [Pb(BO2)2]x, x = 0–0.3, were prepared by the standard solid-state processing. The appropriate mixtures were calcined at 850 °C for 30 h. The resulting materials were pressed into pellets and annealed at 840 °C for 30 h in air. X-ray diffraction, resistivity, and critical current density measurements were performed on the undoped (reference) and Pb(BO2)2-doped (Bi,Pb)-2223 compounds. The dominance of the low-Tc 2212 phase over the high-Tc 2223 phase was observed in the reference sample. With the increasing of doping level, the 2223 phase is significantly enhanced and its increase is associated with the decrease of the 2212 phase. Our results show that the sample with x = 0.075 is the best in the present study. The calculated volume fraction of (Bi,Pb)-2223 phase increases from ∼25 % for reference specimen to ∼85 % for x = 0.075 in a short sintering time (60 h). The (Bi,Pb)-2223 sample with x = 0.075 (0.15 wt% boron) reveals significant enhancement of critical current density (from 25 up to 360 A/cm2) compared to the reference sample. Obtained results could enable us to develop an accelerated and simple method for producing a high purity (Bi,Pb)-2223 superconducting materials by a proper amount of Pb(BO2)2 doping.

Nb3Sn Strand Verification for ITER TF Conductors of CNDA

China is charged with around 7.5% of the ITER toroidal field (TF) conductor fabrication. In total about 6000 km of Nb3Sn strands need to be supplied for Chinese Domestic Agency (CNDA), and now the TF procurement arrangement (PA) for CNDA is in the last phase, i.e., production. According to the PA, the Domestic Agency (DA) has to verify on a regular basis the room and low temperature measurements carried out by the strand suppliers. The verifications shall be performed by the reference laboratories on the strand verification samples adjacent to the samples used by the strand suppliers for critical current measurements. As the reference laboratory of CNDA, the superconducting strand test laboratory of Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) is undertaking the task of Nb3Sn strand verification. The verifications include diameter, chromium plating thickness, twist pitch direction and length, copper-to-non-copper volume ratio, residual resistance ratio (RRR), critical current (IC) and resistive transition index (n), and hysteresis loss. The verification level was based on the requirements of the ITER PA. From the results, the supplied strands for the TF conductors fulfill well the requirements of the ITER PA.

Critical Current Measurements of High-<formula formulatype="inline"><tex Notation="TeX">$J_{c}$</tex> </formula> <formula formulatype="inline"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> Rutherford Cables Under Transverse Compression

For the LHC upgrade, CERN has launched a large program to develop next generation accelerator magnets based on high- Jc Nb3 Sn Rutherford cables. These magnets are characterized by a magnetic field and/or an aperture significantly larger than that of current Nb-Ti LHC magnets. The increased field/aperture will require coil pre-stresses much larger than 100 MPa. Since Nb3Sn cables are extremely sensitive to strain, critical current measurements under traverse compression are essential to estimate the transport current properties of the conductor within the magnet. To this purpose CERN has developed a sample holder (to be used in the FRESCA test station) that allows testing Rutherford cables under a transverse force of up to 2 MN/m. The new holder can house cable samples up to 1.8 m long and 20 mm wide. The large transverse force is only applied over the sample high field region, which is 70 cm long and over which the FRESCA dipole magnet generates a homogeneous fields of up to 10 T. Recently the critical current of the first cable sample has been measured at different transversal loads ranging from 90 MPa to 155 MPa. The measurement was carried out at 4.3 K on a 10-mm-wide Rutherford cable based on eighteen Powder In Tube (PIT) wires with a diameter of 1.0 mm. In this paper, the results of the test are reported, discussed and compared with recently measured data of the same single wire (1.0-mm PIT) tested under transverse loads.

Critical Current Measurement of 30 kA-Class HTS Conductor Samples

Design activities on the helical-type fusion DEMO reactor, FFHR-d1, are progressing at NIFS. A 100 kA current-capacity is required for the helical coil conductors under the maximum magnetic field of ~ 13 T. High-temperature superconducting conductor has been proposed as one of the conductor options for the FFHR-d1 magnet. In this study, a 30 kA class HTS conductor sample has been fabricated and tested. The sample had no current feeders and the current was induced by changing the background magnetic field generated by the 9 T split coils in the cryostat. Rogowski coils and Hall probes were used for the measurement of the transport current of the sample. The critical current of the sample was measured at various temperatures and bias magnetic fields. To verify the self-field effect of the sample, a numerical analysis was performed by considering the current and magnetic field distribution among the tapes self-consistently. The analysis result was compared with the experimental observation.

Critical current measurement of HTS tape relating with cable structure for a DC power cable

In the 200 m high temperature superconducting (HTS) cable test facility at Chubu University, a coaxial power cable is used and composed of two BSCCO (Bi-2223) superconducting layers. The tapes are wound closely to reduce effects on the critical current of BSCCO at self-field. Accordingly, each superconducting layer has a different number of BSCCO tapes. Previously, we have investigated dependence of the critical current (Ic) on the gap in order to optimize the HTS DC cable design. We have been studying the effect on the performance of HTS tapes for the superconducting DC power cables by critical current measurements. In the present experiments several HTS tapes are used and set as a similar structure in the cable with a two-layer structure. The critical current of HTS tapes are measured against the gap between the tapes in the same layer. The experiments show the improvement of the critical current by optimizing the tape arrangements due to magnetic field interaction between the tapes. We will present the experimental results and discuss the design of the HTS DC cable.

First Model Power Cables Made of Russian 2G HTS Wires and their Test Results

Recently launched production of 2G wires by SuperOx Company permitted us to start tests of representative models of power cable made of their wires. In Russian Scientific R&D Cable Institute we have the test facility for extensive testing of heavily instrumented HTS cable models with full size cross-section and length up to 5 m. Our standard test program includes critical current measurements, current distribution measurements among layers, AC loss measurements, etc. In this paper we present details of first power cable models made of 4 mm width 2G wires from SuperOx and their test results. Model cables have two counter-wound layers with 18 tapes in each layer. Critical currents of cables were from 3 kA to ~4.5 kA. We also measured AC losses and compared them with losses in model cables made of 2G wires of other producers. AC losses measured were compared with calculated ones and demonstrated the good coincidence between experiments and calculations. First model cables tests permitted us to conclude that SuperOx wires have good parameters for production of 2G HTS power cables with low AC losses.

Epitaxial growth of ultra-thin NbN films on AlxGa1−xN buffer-layers

The suitability of AlxGa1−xN epilayers to deposit onto ultra-thin NbN films has been demonstrated for the first time. High quality single-crystal films with 5 nm thickness confirmed by high resolution transmission electron microscopy (HRTEM) have been deposited in a reproducible manner by means of reactive DC magnetron sputtering at elevated temperatures and exhibit critical temperatures (Tc) as high as 13.2 K and residual resistivity ratio (RRR) ∼1 on hexagonal GaN epilayers. On increasing the Al content x in the AlxGa1−xN epilayer above 20%, a gradual deterioration of Tc to 10 K was observed. Deposition of NbN on bare silicon substrates served as a reference and comparison. Excellent spatial homogeneity of the fabricated films was confirmed by R(T) measurements of patterned micro-bridges across the entire film area. The superconducting properties of these films were further characterized by critical magnetic field and critical current measurements. It is expected that the employment of GaN material as a buffer-layer for the deposition of ultra-thin NbN films will prospectively benefit terahertz electronics, particularly hot electron bolometer (HEB) mixers.