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In our article, we show the n value determined from measurements using the magnetic field sweep method (curves of E from B/Bc) and the current sweep method (curves of E from I/Ic). The results from these two methods allowed for a scientific n value analysis. Moreover, measurements using these two methods allow for a better understanding of the behavior of superconducting wires after the transition from the superconducting state to the normal (resistive) state. Measurements were made for the NbTi wire at 4.2 K, MgB2 at the temperature range from 4.2 to 30 K, and high-temperature superconducting tape at 77 K. The magnetic field sweep method results show that the n value has a constant value independent of an increase in the magnetic field, number of filaments, type of boron, diameter of the wire, and small amount of the dopant. Moreover, research studies point out that the n value in the magnetic field sweep method depends on the type of superconductor material, a large amount of doping at high temperatures, and the low density of pinning centers. Based on the results obtained, it can be indicated that each superconducting material will have a constant n value. Measurements carried out using the current sweep method showed that the n value decreases significantly with an increase in the magnetic field. Moreover, research studies have shown that the n value depends on the number of filaments, type of boron, dopant, wire diameter, and temperature. On this basis, it can be indicated that the n value for the current sweep method depends on the morphology and structure of the superconducting wires. In addition, based on the samples measured in this research, the current sweep method allows for higher n values than the magnetic field sweep method in low magnetic fields. However, the magnetic field sweep method allows obtaining higher n values than the current sweep method in middle and high magnetic fields. [ABSTRACT FROM AUTHOR]

Our research showed that a physical phenomenon appears in MgB2 wires, which has not been reported in previous studies. We have found that the flow of transport current in the current penetration depth of normal metal areas leads to the creation of voltage in magnetic fields from 0 to Bc1. Scanning electron microscope (SEM) images showed that normal areas can be formed at the junction between the superconducting material and the diffusion barrier. SEM photos indicate that this surface is very non-homogeneous and porous. This new physical phenomenon, that is, the voltage created by these normal areas at the current penetration depth is called, in this paper, the interfacial topological effect. Further measurements showed that above Bc1, the voltage disappears and becomes unmeasurable for magnetic fields above 120 mT. This is due to the appearance of a mixed state and current flow at a deeper depth of the superconducting material. From the interfacial topological effect, transport current flows on the outside surface of the superconducting MgB2 wires not only in magnetic fields from 0 to Bc1 but also in the mixed state and disappears in magnetic fields above 120 mT. This indicates that it is related to the superconducting state. The measurements performed by using a physical property measurement system (PPMS) for the low current (100 mA) indicate that the interfacial topological effect is on the boundary of two superconducting materials: the Nb diffusion barrier and the MgB2 material. In addition, the PPMS measurement results point out that the magnetic field eliminates the interfacial topological effect. Further results indicate that the appearance of voltage (the interfacial topological effect) in a MgB2 coil does not mean that the coil is not superconducting and cannot conduct the transport current without loss. Our results indicate that test procedures for MgB2 coils should assume exceeding the magnetic field of 120 mT because in this field, the voltage can disappear to zero and the transport current flows without loss. This is a very important factor for the future production of superconducting coils made of MgB2 wires with the Nb barrier on an industrial scale. [ABSTRACT FROM AUTHOR]

The use of a low annealing temperature during the production of coils made from superconducting materials is very important because it reduces the production costs. In this study, the morphology, transport critical-current density (Jc), irreversible magnetic field (Birr), and critical temperature (Tc) of straight wires and small 2% C-doped MgB2 coils were investigated. The coils were made using the wind-and-react (W&R) method and annealed at various temperatures from 610 °C to 650 °C for 2–12 h. Critical-current measurements were made for both the coils and straight wires at the temperatures of 4.2 K, 20 K, 25 K, and 30 K. During our research study, we determined the process window that provides the best critical parameters of the coils (annealing at a temperature of 650 °C for 6 h). Moreover, we observed that small coils made with unreacted MgB2 wire and then annealed had morphology and critical parameters similar to those of straight 2% C-doped MgB2 wires. Moreover, small-diameter bending of 20 mm and 10 mm did not lead to transverse cracks, which can cause a large reduction in Jc in the coils. This indicates that the processes of optimization of thermal treatment parameters can be carried out on straight MgB2 wires for MgB2 superconducting coils. [ABSTRACT FROM AUTHOR]

The need for energy is growing as civilization develops. Superconductors have various benefits over regular conductors, including a high current-carrying density and nearly no resistance. Therefore, the objective that scientists continue to seek is the use of superconductors to replace traditional conductors in order to satisfy the demand for energy. As a key part of the superconducting electrical system, the critical current is one of the key properties of high-temperature superconducting (HTS) coils. Scholars have created a variety of numerical simulation models to estimate the performance of HTS coils. However, the extremely nonlinear E–J power law relationship that characterizes the voltage–current relationship in superconducting materials necessitates numerous iterative refinements during the coil design stage, which is time-consuming from the perspective of computing efficiency. Consequently, it is essential to increase computational efficiency. In this study, the critical current of HTS coils was calculated using the J model (integral method with the current density J as the state solution variable), and it was proved to be an efficient research method. The accuracy of the method is verified by comparing with the H model and the experimental measurement results of the critical current of the HTS coil. Moreover, compared with the reference finite element simulation model, this model has a speed advantage of at least four times, which is a good choice, especially, for HTS circular coils with large turns. [ABSTRACT FROM AUTHOR]

An efficient two dimensional T-A formulation based approach is proposed to calculate the electromagnetic characteristics of tape stacks and coils made of second generation high temperature superconductors. In the approach, a thin strip approximation of the superconductor is used in which the superconducting layer is modeled as a 1-dimensional domain. The formulation is mainly based on the calculation of the current vector potential T in the superconductor layer and the calculation of the magnetic vector potential A in the whole space, which are coupled together in the model. Compared with previous T-based models, the proposed model is innovative in terms of magnetic vector potential A solving, which is achieved by using the differential method, instead of the integral method. To validate the T-A formulation model, it is used to simulate racetrack coils made of second generation high temperature superconducting (2G HTS) tape, and the results are compared with the experimentally obtained data on the AC loss. The results show that the T-A formulation is accurate and efficient in calculating 2G HTS coils, including magnetic field distribution, current density distribution, and AC loss. Finally, the proposed model is used for simulating a 2000 turn coil to demonstrate its effectiveness and efficiency in simulating large-scale 2G HTS coils. [ABSTRACT FROM AUTHOR]

Superconductors play a crucial role in the advancement of high-field electromagnets. Unfortunately, their performance can be compromised by thermomagnetic instabilities, wherein the interplay of rapid magnetic and slow heat diffusion can result in catastrophic flux jumps, eventually leading to irreversible damage. This issue has long plagued high-Jc Nb3Sn wires at the core of high-field magnets. In this study, we introduce a large-scale GPU-optimized algorithm aimed at tackling the complex intertwined effects of electromagnetism, heating, and strain acting concomitantly during the quenching process of superconducting coils. We validate our model by conducting comparisons with magnetization measurements obtained from short multifilamentary Nb3Sn wires and further experimental tests conducted on solenoid coils while subject to ramping transport currents. Furthermore, leveraging our developed numerical algorithm, we unveil the dynamic propagation mechanisms underlying thermomagnetic instabilities (including flux jumps and quenches) within the coils. Remarkably, our findings reveal that the velocity field of flux jumps and quenches within the coil is correlated with the cumulated Joule heating over a time interval rather than solely being dependent on instantaneous Joule heating power or maximum temperature. These insights have the potential to optimize the design of next-generation superconducting magnets, thereby directly influencing a wide array of technologically relevant and multidisciplinary applications. Flux jumps can lead to premature quenching and irreversible damage of superconducting magnets. Here, authors developed a GPU-optimized algorithm aimed at tackling the complex intertwined effects of electromagnetism, heating, and strain acting concomitantly on real superconducting coils. [ABSTRACT FROM AUTHOR]

In order to cope with the grid fluctuation caused by large‐scale wind power connected to the grid, the wind thermal power generation system has been proposed and extensively studied. The wind thermal power generation system uses a wind turbine to drive a heat generation device to heat the heat storage medium, which is further exchanged to drive a turbine to generate electricity. A superconducting magnetic eddy current heater (SMH) is proposed for the characteristics of wind thermal power generation system, which uses non‐resistive, large current‐carrying superconducting coils for excitation, and has high output efficiency and power density. The working principle of magnetic eddy current heating is analysed, and the structure of SMH with no ferromagnetic material and two heating screens inside and outside is proposed according to the characteristics of SMH. An analytical model of the SMH is established, and the influence of the structure and materials of the SMH on the magnetic field distribution is analysed. Based on this, a 22‐pole SMH was designed and analysed for output power at 10–20 rpm. A three‐dimensional magnetism‐stress combined analysis model of SMH is established, and the strain and stress characteristics of SMH are simulated under the condition of maximum output power, which verifies the feasibility of the mechanical properties of existing superconducting materials for application in SMH. [ABSTRACT FROM AUTHOR]

With the in-depth study of the interaction behavior between a magnet and a superconducting coil, we have developed a new method to determine the inductance of a superconducting coil. With the same apparatus, the resistivity of a normal conductor can also be evaluated. The principle of the new methods is based on the measurement of proper physical quantities during the interaction between a permanent magnet and a superconducting coil or between a permanent magnet and a normal conducting ring. Unlike conventional inductance and resistance measurement, the proposed methods do not require any power supply or voltmeter. We use a simple force measuring apparatus to perform the measurements. The resistivity of the sample is obtained by comparison with a pure copper sample of the same geometry and dimensions. In the case of a superconducting coil, its inductance can be obtained from the measured interacting force and induced current curves as it interacts with a permanent magnet. This article introduces the principle, the procedure, and validity of the proposed methods. The uncertainties of the proposed methods are discussed and compared with the existing methods of measuring inductance and resistivity. These methods may serve as alternatives in measuring the inductance of a superconducting coil or in measuring the resistivity of a normal conductor, especially when the conditions of the conventional methods for such measurements cannot be satisfied. [ABSTRACT FROM AUTHOR]

• In order to better investigate the electromagnetic force distribution characteristics of REBCO superconducting materials after electromagnetic excitation, the electromagnetic characteristics of superconducting tapes are numerically calculated by H method and T-A method on the basis of finite elements (FE), and the effectiveness of T-A method is obtained. • By establishing the real size model and the homogenization equivalent model, the electromagnetic characteristics are analyzed, and the rationality of the equivalent method is obtained by comparing the calculation results of the two models. • Based on the homogenization equivalent model, the influence of different external tensile strains on superconducting tapes is analyzed. Rare earth barium copper oxide (REBCO) superconducting tape is the second generation of high temperature superconducting material with strong current carrying capacity and high mechanical strength, which has been studied most recently. In order to better investigate the electromagnetic force distribution characteristics of REBCO superconducting materials after electromagnetic excitation, firstly, the electromagnetic characteristics of superconducting tapes are numerically calculated by H method and T-A method on the basis of finite elements (FE) in this paper, and the effectiveness of T-A method is obtained. Based on this method, the influence of real size model (RS model) and homogenization equivalent model (HE model) on superconducting tape is further studied. The results show that the radial magnetic field reaches a peak of 1.01 T at the middle position of the upper surface of the pancake, while the axial magnetic field reaches a peak of 16.4 T at the inner side of the pancake, which indicates that the axial magnetic field contributes more to the total magnetic field. At the same time, the rationality of the equivalent method is obtained by comparing the calculation results of the RS and HE models. In addition, the mechanical changes of superconducting tapes are studied by T-A method under the HE model. It is found that the hoop stress and radial stress reach the peak values of 64.7 MPa and 6.84 MPa, respectively, and both are tensile stress, indicating that the hoop stress is more likely to cause damage to the superconducting tapes. Finally, the effect of different external tensile strains on the superconducting tape is discussed. [ABSTRACT FROM AUTHOR]

With the progress of iron-based superconducting wire and tape fabrication technologies, extensive research has been conducted on iron-based superconducting coils. A deep understanding of the energizing characteristics of these coils is crucial for their large-scale application. This study investigates two metal-insulated double pancake coils, each having a different number of turns and co-wound with Ba 0.6 K 0.4 Fe 2 As 2 iron-based superconducting tape and stainless-steel tape. We conducted critical current tests, sudden-discharge experiments, and charge-discharge cycles at 4.2 K to obtain the time constants and characteristic resistances of the coils. Additionally, the impact of stainless-steel as co-wound material on coil performance was analyzed. Theoretical models were employed to calculate the charging processes and validate them against experimental results. This research offers valuable insights for the practical application of iron-based superconducting materials. [ABSTRACT FROM AUTHOR]

The enhanced thermal stability of no-insulation superconducting coils, due to the absent turn-to-turn insulation is demonstrated in several publications. With the self-protecting properties of these coils, the application of second generation high temperature superconductors in real energy applications can be simplified, regarding quench detection and protection. Furthermore, it enables a system design with superconducting coils operating closer to their critical current limitation compared to their insulated counterparts. The usage of superconducting materials, improving the power density in wind turbine generators is an ongoing research topic. As a result, the design and construction of a 10 kW laboratory demonstrator is intended. Therefore, six double pancake racetrack coils are manufactured with two different superconducting tape architectures and compared with respect to the critical current, turn-to-turn resistance and transient performance in liquid nitrogen at 77 K. The superconducting coils are joined together to a six-pole stator system for the installation in the 10 kW generator demonstrator. All 12 pancakes reach the expected critical current and show a similar transient response of the magnetic field in the individual measurements. However, the series-connected stator system has a much longer field delay than the individual pancakes, which cannot be sufficiently explained by the typical equivalent circuit diagram for no-insulation coils. [ABSTRACT FROM AUTHOR]

The thermal conductivity (κ) of polymer nanocomposites with boron nitride (BN) nanoplates is critically dependent on the distribution of BN in the polymer matrix. Here, we heavily functionalized BN nanoplates with hydroxyl groups by ultrasonication in solution with boric acid. Then, the hydroxylated BN nanoplates could be homogeneously distributed in an epoxy matrix, which improved the through-plane κ of 2.09 and 1.96 W/m·K at 25 and -90 °C, respectively. Processable composite resin with hydroxylated BN was utilized as an adhesive for a low-temperature superconducting (LTS) coil to enhance its normal zone propagation velocity (NZPV) and cooling rate performance. Consequently, fast heat dissipation through the adhesive between superconductors contributed to a 65.8% of increase in NZPV and a 13.1% improvement in the cooling rate of the device compared with a commercial adhesive. [Display omitted] • A BN edge was selectively functionalized using boric acid. • Functionalization was done at the sub-kilogram scale. • The higher affinity of b -BN to the epoxy enhanced the κ value of the composite. • The high κ improved NZPV and cooling rate of the superconducting coil. [ABSTRACT FROM AUTHOR]

In order to reduce the levitation energy consumption and increase the levitation air gap, a simulation study of the electrochemistry superconducting magnetic levitation system based on the H equation is proposed. Through finite element simulation, the magnetic field distribution, current distribution, force, and other characteristics of the magnetic suspension system in the superconducting gravimeter are obtained; the relationship between the force of the superconducting ball in the magnetic field and the height of the suspension body and the current of the suspension coil is analyzed; and the penetration rate, the magnetic gradient, penetration depth, and maximum magnetic induction intensity of the superconducting spherical surface of the single-coil electrochemistry superconducting magnetic levitation system are obtained by simulation calculation. Simulation results show that, at 1 s, we start to use 0.2 s, 0.4 s, 0.6 s, and 0.8 s time, respectively, to pass current into the floating coil until it reaches 4.4 A. The magnetic gradient of the electrochemistry superconducting magnetic levitation system using a single coil is too large to meet the requirements of gravity measurement, the penetration depth is much smaller than the thickness of the superconducting sphere, and the maximum magnetic field on the surface of the superconducting sphere is much smaller than the critical magnetic field value of the superconducting material, and no loss will occur. The critical magnetic field value of the superconducting sphere is much smaller than that of the superconducting sphere. The critical magnetic field value of the material will not quench, which verifies that the H equation can simulate the superconducting magnetic levitation system well and has a high simulation accuracy and efficiency. [ABSTRACT FROM AUTHOR]

This article discusses the fabrication of a 25.2 T NMR magnet for an extreme condition user facility in China. The magnet is made using a combination of low-temperature superconducting (LTS) and high-temperature superconducting (HTS) materials. The use of HTS materials allows for higher field strengths and better signal-to-noise ratios in NMR experiments. The article also describes the challenges in winding solenoid coils with HTS materials and proposes a novel random ordering method to minimize inconsistencies. The fabricated magnet demonstrates excellent field stability and homogeneity, making it a useful tool for material science research and potentially expanding its application in medical imaging. [Extracted from the article]

Angle dependence I c (B,θ) of superconducting tape is a crucial parameter to calculate the influence of magnetic field during the design of superconducting applications,. This paper focuses on the asymmetrical characteristics found in REBCO tapes and further applications based on this phenomenon. This paper starts with angle dependence measurements of different HTS tapes, asymmetrical characteristics are found in some of the testing samples. On basis of this property, optimization of superconducting coils in superconducting motor, transformer and insert magnet is discussed by simulation. Simplified experiments which represent the structure of insert magnet were carried out to prove the validity of numerical studies. Conclusions obtained in this paper show that the asymmetrical property of superconducting tape is quite important in design of superconducting applications, and optimized winding technique based on this property can be used to improve the performance of superconducting devices. [ABSTRACT FROM AUTHOR]

The superconducting magnet energy storage (SMES) has become an increasingly popular device with the development of renewable energy sources. The power fluctuations they produce in energy systems must be compensated with the help of storage devices. A toroidal SMES magnet with large capacity is a tendency for storage energy because it has great energy density and low stray field. A key component in the creation of these superconducting magnets is the material from which they are made. The present work describes a comparative numerical analysis with finite element method, of energy storage in a toroidal modular superconducting coil using two types of superconducting material with different properties bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO). Regarding the design of the modular torus, it was obtained that for a 1.25 times increase of the critical current for the BSCCO superconducting material compared with YBCO, the dimensions of the BSCCO torus were reduced by 7% considering the same stored energy. Also, following a numerical parametric analysis, it resulted that, in order to maximize the amount of energy stored, the thickness of the torus modules must be as small as possible, without exceeding the critical current. Another numerical analysis showed that the energy stored is maximum when the major radius of the torus is minimum, i.e., for a torus as compact as possible. [ABSTRACT FROM AUTHOR]

Low critical temperature superconducting coils can conduct persistent currents with almost no losses, as the technique to make joints is well developed and the resistive losses are negligible. The replacement of these superconductors by high-temperature superconductors may greatly reduce refrigeration costs. However, even though the technique for making these joints has greatly developed over the last years, there are still inherent difficulties associated to that process. Coils made with partially slit loops of high temperature superconducting tape have no joints, and therefore can conduct current in persistent mode. Since there are no terminals, its current must be induced through magnetization techniques, for instance, by the means of a transformer. The occurrence of persistent current is related to the voltage in the sample, making it interesting to study the voltage behavior for different regions of operation. In this work, a jointless 2G loop was magnetized with pulse magnetization from which the curves of voltage and current as functions of time were obtained. After several tests, it was possible to divide the superconductor behavior into three different stages of operation, depending on the current levels reached: for low current values, the material behaves as a perfect inductor; for current values near the critical current, it behaves as an RL circuit; and for higher values, resistance dependent on the temperature has to be taken into account. A direct relation between persistent current and the arise of a dissipative state was noted and the highest values of persistent current were observed when the pulse was intense enough to produce significant temperature variations in the sample. [ABSTRACT FROM AUTHOR]

Aiming to address the computational difficulties caused by the large width ratio of superconducting tapes, especially in calculating the AC losses of 3D superconducting coils, this paper proposes to study the AC loss laws of a 3D model of racetrack superconducting coil using the T–A equation group model. Firstly, the critical current of the superconducting coil is estimated based on the structure of the coil and the technical parameters of superconducting materials used in high-temperature superconducting (HTS) motors. The transmission AC losses of the superconducting coil under different n values, different transmission currents, different frequencies and different coil structure sizes are studied using T–A equations. Finally, the critical current test system and AC loss test system for HTS tapes and coils are set up. The critical current test and AC loss test for HTS tapes and coils are carried out, and the results are compared with those calculated by finite element method. The test results show the reliability of the T–A mathematical model, which can be used as a pre-valuation in engineering applications. [ABSTRACT FROM AUTHOR]

The second-generation high-temperature superconducting tapes (2G-HTS) such as the REBCO coated conductors are known for their strong mechanical properties and high current carrying capacity with low ac losses compared with first-generation HTS tapes as well as low-temperature superconductors. Therefore, they seem to be good candidates to assemble coils for superconducting rotating electric machines (SuREMs). In this article, to investigate the influence of a soft magnetic composite (SMC) material on the dc and ac HTS coil performances at cryogenic temperatures, GdBCO- and YBCO-based commercial tapes from different manufactures were used to assemble ten HTS coils in double-pancake configuration. These tapes were wound using two different core structures, i.e., fixed and movable cores, having the same circular shape but different diameters as well as materials, i.e., fiberglass and SMC. Critical currents and ac transport current losses of these coils were measured using the electrical method. To evaluate the parallel and perpendicular components of the profile of magnetic flux density for each assembled HTS coil, a finite element model has been developed using Maxwell 2D. Eventually, the performance of different coils for SuREM application was discussed. [ABSTRACT FROM AUTHOR]

Some times ago, in the framework of the Boltzmann kinetic equation in the so-called τ -approximation, it was shown that perpendicular resistivity in quasi-two-dimensional (Q2D) conductors can be impurity independent in a broad range of high parallel magnetic fields [A. G. Lebed and N. N. Bagmet, Phys. Rev. B 55 (1997) R8654]. In this paper, we demonstrate that this result is much more general than the Boltzmann kinetic equation in the τ -approximation and is valid in a pure Q2D metal at low frequencies of the electric field. The above-mentioned statement open possibilities to test the above discussed phenomenon in a number of Q2D organic, high-Tc, and some other Q2D superconductors in their metallic phases and gives a method to extract some important information about their Q2D Fermi surfaces. [ABSTRACT FROM AUTHOR]

This work presents the design and validation of a vibrating coil magnetometer for the characterization of the field dependence of the critical current density of centimeter-sized bulk superconductors as an alternative to the destructive methods typically used. The magnetometer is also shown to be capable of measuring the magnetic moment in an applied field of up to 5 T for diverse magnetic materials, such as soft and hard ferromagnets and high-temperature superconducting pellets. The vibrating coil magnetometer was first optimized using finite element simulations and calibrated using a commercial vibrating sample magnetometer. The vibrating coil magnetometer was benchmarked with hysteresis measurements of a Nd2Fe14B disk made with a commercial hysteresisgraph, showing good agreement between the different setups. The magnetic hysteresis of a YBa2Cu3O7−x superconducting pellet was measured at 77 K, showing a penetration field of 1 T and an irreversibility field of 4 T. The field dependent critical current density of the superconductor was then inferred from the magnetic hysteresis measurements and extrapolated at low fields. Finally, the resulting critical current density was used to successfully reproduce the measured magnetization curve of the pellet at 2 T with finite element simulations. [ABSTRACT FROM AUTHOR]

The detection of a short circuit inside the superconducting coils of the ATLAS detector after their installation in the casing is mandatory for the mechanical and electrical safety during a fast discharge of the magnet The detection of a short circuit by means of a capacitor discharge becomes very difficult once the coils are integrated inside the casing, which behaves as a short-circuited turn strongly coupled with the coils. Under these circumstances a preliminary and careful analysis of the electrical parameters of the electrically coupled system and of the discharge behavior is required in order to clearly detect a short circuit. The paper presents and discusses the results of the analysis and the measurements carried out on the 16 double pancakes of the ATLAS Barrel Toroid after their integration inside the casing. [ABSTRACT FROM AUTHOR]

We suggested an original method to investigate the Fermi surfaces (FSs) in the quasi-two-dimensional conductors some time ago [A.G. Lebed and N.N. Bagmet, Phys. Rev. B 55, R8654 (1997)]. It was based on a consideration of a perpendicular conductivity in quasi-two-dimensional metals in parallel magnetic fields in the framework of the Boltzmann kinetic equation, where it was shown that the conductivity was independent on impurities. In this paper, we demonstrate that the above mentioned result is much more general than the kinetic equation and can be obtained even in a fully quantum mechanical case. We suggest to investigate this possible phenomenon in the quasi-two-dimensional organic, high- , and some others superconductors in a metallic phase to judge if the Fermi liquid picture is valid for them or not. If the Fermi liquid picture is valid, then study of the perpendicular resistivity in the rotated parallel magnetic field allows to extract important information about the two-dimensional Fermi surfaces. [ABSTRACT FROM AUTHOR]

• Simultaneous simulation of screening current and bypass current of a 32 T magnet. • Effect of charging sequence on loss, stress, and magnetic field in NI magnet. • Asynchronous charging alters NI insert coil's azimuthal current direction. • Synchronized charging reduces loss, evens field ascent. • Insert coil charging first reduces hoop stress but increases power and loss. The hybrid superconducting magnet with low-temperature superconductor (LTS) for background coil and high-temperature superconductor (HTS) for insert coil is a dependable approach towards achieving high magnetic fields. However, challenges related to screening currents considerably limit the realization of higher field strengths. Due to the historical dependence of screening currents in superconductors on the magnetization path, the shunt currents in non-insulated (NI) superconducting coils can alter the effective magnetization path. Consequently, the charging sequence of LTS coils and NI HTS coils can influence the behavior of screening currents. We establish an electromagnetic and mechanical model for a 32 T NI hybrid superconducting magnet and conduct a comprehensive comparative analysis of screening current induced stress (SCIS), screening current induced field (SCIF), and loss under three different charging sequences. The results indicate that when the NI insert coils are charged first, the hoop stress is slightly lower compared to the other two charging sequences, but the instantaneous power and total losses are the highest. When both the insert and background coils are charged simultaneously, the hoop stress is the highest, yet the instantaneous power and total losses are the lowest, accompanied by a more uniform magnetic field ascent. These findings provide valuable insights for the design and selection of charging sequences of ultra-high-field NI hybrid superconducting magnets. [ABSTRACT FROM AUTHOR]

The A15-type superconductors (\Nb3\Sn, \Nb3 \Al) have been used to fabricate the 10-15-T high-field superconducting magnets. Due to the strain sensitivity of this type of conductors, minimization of the strain in conductors is an important issue to realize the highest critical current density during operation. It is also known that the superconductors in epoxy-impregnated coils exhibit much less critical current degradation than wire samples when subjected to transverse compressive stress, due to the stress buffer effect of the epoxy and insulation materials among strands. This paper presents analytical and numerical investigation of the stress and strain distributions in A15-type superconducting coils, which are usually wound with copper-stabilized Rutherford cable and are epoxy impregnated. Analytical formulas have been derived to calculate the strain in superconductors from the average strain in coils. In addition, a FEM simulation model has been built to get a clear view of the strain distribution between superconductors and the surrounding insulation materials. [ABSTRACT FROM PUBLISHER]

In this paper, the use of magnetic materials to divert flux in high-temperature superconductor superconducting coils and reduce transport ac loss is investigated. This particular technique is preferred over other techniques, such as striation, Roebel transposition, and twisted wires because it does not require modification to the conductor itself, which can be detrimental to the properties of the superconductor. The technique can also be implemented for existing coils. The analysis is carried out using a coil model based on the \bf H formulation and implemented in comsol multiphysics. Both weakly and strongly magnetic materials are investigated, and it is shown that the use of such materials can divert flux and achieve a reduction in transport ac loss, which, in some cases, is quite significant. This analysis acts to provide a foundation for further optimization and experimental work in the future. [ABSTRACT FROM PUBLISHER]

Non-insulation high-temperature superconducting coils provide a much lower risk of burnout in fault/abnormal conditions, such as hot-spot quench and overcurrent. This study employs an equivalent circuit grid model, coupled with magnetic field calculation and the E–J power law of superconductors, to deeply and systematically investigate the overcurrent charging process in a double-pancake non-insulation coil. An evident saturation of the magnetic field in the axial direction of the coil was observed and verified by experiments. Experimentally, the entire process, including the behavior of the magnetic field, was consistent with the numerical results. Based on the verified model, two main points were addressed: (1) Transient current distribution inside the coil during overcurrent charging was studied. Potential quenching risks were found to be at the innermost and outermost turn near the electrodes, as well as the pancake-to-pancake connection part. (2) Magnetic field saturation, which is a unique phenomenon in non-insulation superconducting coils during overcurrent charging, was studied in detail and first quantitatively defined by a new concept "converged load factor". Its relationship with turn-to-turn resistivity was revealed. [ABSTRACT FROM AUTHOR]

The charging/discharging delays in superconducting coils wound without insulation (NI coils) are a major drawback of the technique. While removing the insulation improves safety margins, the increase in the characteristic time constant τc can make a coil unfit for a particular purpose. It is widely accepted for instance that NI coils will not be used in ac applications where τc ~ 1/f. To decrease τc of the NI coils, the same length of superconductor can be wound/connected in parallel rather than in series — decreasing the inductance L, and hence the time constant τc, while maintaining the number of amp-turns IopN. Here we investigate the effect of parallel connecting coils in a magnet using a 2D axially symmetric model which captures all the necessary electromagnetic properties of the HTS NI coils. These properties include: critical current anisotropy Jc(B,θ), turn-to-turn conductivity, as well as winding parallelism. Our modeling results show that the parallel connected magnet experiences magnet-wide shielding current effects. Whilst these shielding currents affect field homogeneity — the model enables this effect to be quantified. Furthermore, shielding currents are not an issue when running NI coils in saturated mode. The modeling work presented here provides a simple initial example of how magnet designers may approach designing, optimizing, and operating high current, HTS NI coils. [ABSTRACT FROM AUTHOR]

Abstract: Wendelstein 7-X (W7-X) is an advanced stellarator which is presently under development and construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The W7-X is a magnetic confinement device where the main magnetic configuration is produced by a set of 70 superconducting coils, arranged in five identical modules. A module can be further divided in two half-modules, each made up of five non-planar coils (Types 1–5) and two planar coils (Types A and B). To monitor the integrity of the coil system and its neighbouring components during the operational and accidental transients of the W7-X device, a strain gauges (SG) system was included in the machine design, distributed in those positions which will have, realistically and according to finite elements (FE) calculations, the most high stress levels. Specifically, each superconducting coil of the W7-X system is monitored via a set of four SG sensors which are firmly attached in specific locations of the coil steel casings. Each sensor consists of a bridge of two individual foil gauges, connected so to compensate the effects of the broad excursion from room to cryogenic temperature, and the four sensors are arranged in a double redundant array for increased reliability. To estimate the response of the SG system in terms of accuracy and precision, so actually in terms of degree of conformity to FE anticipated results and degree of repeatability of similar measurements, and also to explain some unpredictable results obtained at cryogenic temperature, a comprehensive data analysis was carried on, based on the SG data collected during the cold acceptance tests of the W7-X superconducting magnets. The cold testing phase was, indeed, a valuable tool to verify the response of the SG system in the framework of the overall acceptance process of the coils. Thanks to the consistent data pool, a meaningful statistical analysis was possible, and the behaviour of the SG system was observed from a new perspective. This paper describes the procedures adopted in the aforementioned data analysis, and provides a critical overview of the main results. The main issues faced and solved in the accomplishment of the analysis were mainly related to the handling of huge raw data files, and to the conversion of such data into meaningful information. [Copyright &y& Elsevier]

To achieve the first plasma of the Korea superconducting tokamak advanced research (KSTAR), the KSTAR superconducting coils were tested in advance. As they should operate in excessively low temperature of 4.5 K and high magnetic field environment of 7.5 T, it is crucial to monitor the cryogenic and the structural behaviors of KSTAR device during the commissioning period including a cool-down. The temperatures of the KSTAR toroidal field (TF) coil and the poloidal field (PF) coils were measured during the entire operating period. The mechanical stresses on the TF and PF structures were continuously monitored to check if they go beyond the limiting value calculated through the simulation. The alignment of the KSTAR device was checked by using displacement sensors. The TF coils were successfully supplied with 15 kA DC current for 8 hours, and the maximum 5 kA/s current variation of the PF coils were tested. For the main experiment, the interlock test of the quench detection system for the KSTAR coils was carried out at reduced currents of 1 kA. From these results the quench protection circuit, and the current-flow of the KSTAR superconducting coils proved to be well performed for the first plasma operation. [ABSTRACT FROM AUTHOR]

A moving of the superconducting wire due to an electromagnetic force induces heat, and that is one factor of instabilities in superconducting coils. There is a wire motion under a DC operation; on the other hand, there is a mechanical loss under an AC operation. To prevent those movements, it is effective to enhance the winding tension at coil-operating temperature. For that purpose, we propose the use of a Dyneema fiber reinforced plastic (DFRP), which has a negative thermal expansion coefficient, as the bobbin of the superconducting coils. To investigate a contribution to improvement of stability due to the DFRP, we performed DC and AC experiments. In the DC experiment, rates of the attainment to the critical current were evaluated by quench training measurements, and in the AC experiment, losses were measured. In both experiments, the thermal expansion characteristics of the DFRP prevented the wire's movement, therefore it was demonstrated that the use of the DFRP bobbin was effective about the improvement of stability. [ABSTRACT FROM AUTHOR]

When a Dyneema fiber reinforced plastic (DFRP) is used as a superconducting coil's bobbin, the decrease of the winding tension during cooling down is prevented. Therefore, it is expected that a mechanical loss generated in AC superconducting coils is reduced. The DFRP has the high Young's modulus to the fiber direction, however the modulus to the perpendicular direction is lower than the modulus of the glass fiber reinforced plastic (GFRP), which is generally used as a structural material in super- conducting coils. Therefore, a Dyneema and glass fiber reinforced plastic (DGFRP), which was compounded the Dyneema and the glass fibers, was fabricated for the purpose of the intensification of the DFRP, and total losses of the superconducting coils having some kinds of DGFRP bobbins were measured under AC operation. In loss measurements, the coil whose loss was the smallest was the coil having the strongest winding tension at cryogenic temperature, and the coil whose loss was the largest was the coil having the weakest tension. Moreover, comparing with the results of the loss measurement in the coils fabricated with the DFRP bobbin, the losses of the coils having the DGFRP bobbins were slightly large due to the mixture of the Dyneema and the glass fibers. This paper reports the effect of the reduction of the mechanical losses in the coils having the DGFRP bobbins and the comparison with the coils whose bobbins are the DFRP and the GFRR [ABSTRACT FROM AUTHOR]

Frictional losses in windings are one of the loss types in AC superconducting coils. When an AC current is supplied to the coil, a superconducting wire in the winding start to vibrate. And frictional heat generates at contact segments between the wire and a coil bobbin. And hence, the losses are not electromagnetic losses such as coupling losses but mechanical losses in superconducting coils. We prepared four types of bobbin materials. Two bobbins expanded during a cooling process from room temperature to liquid helium temperature. The other two bobbins contracted during the cooling down. Winding tensions for the four kinds of coils were 0.5, 3.5, and 5.0 N. And then, the AC losses of the twelve coils were measured. When the coils whose bobbins have thermal expansion were used, the AC losses increased with the gain of the winding tensions, in spite that the experimental conditions such as coil currents and background magnetic field were same. On the contrary, in case of the contraction bobbins coils were used, the losses decreased with the tensions increased. To analyze the experimental cases of the winding tensions of the sample coils at the liquid helium temperature were calculated, and the relation between the tensions at the cryogenic temperature and the frictional losses was discussed. [ABSTRACT FROM AUTHOR]

Describes a numerical method which analyzes adiabatic quench of superconducting coils. Information on Wilson's method for superconducting coils analysis; Computational formulas; Applications of the method.

In order to achieve a high intensity and compact multifunctional air-core cyclotron, the behavior of non-circular coils in a high magnetic field at 4.2 K were investigated. Two types of isosceles triangle shaped double-pancake coils: a coil with reinforcement by Yoroi-coil (Y-based oxide superconductor and reinforcing outer integrated coil) structuring and a coil without reinforcement, were manufactured and their behaviors were observed when current was applied to each coil in a high magnetic field. The non-reinforcement coil showed in 10 T external magnetic field that the superconducting properties of the coil were partially lost and a voltage generated in the coil winding was observed when the applied current exceeded 160 A. After disassembling this coil, it was found that the superconducting tape in the coil winding was plastically deformed and permanently bent at the corners. The reinforced coil due to Yoroi-coil structuring was powered up to the coil current of 300 A in a magnetic field of 14 T. A voltage of 0.3 mV was observed when the coil current was 277 A, and the excitation was terminated when the voltage reached 0.6 mV at 300 A. The coil winding, after the current flowing up to 300 A in 14 T field, indicated that the deformation of it was significantly limited. It was confirmed that the Yoroi-coil type structural reinforcement, even for triangular coils, has sufficient mechanical strength under the conditions of our air-core cyclotron. [ABSTRACT FROM AUTHOR]

Abstract: The large helical device (LHD) has been operated since 1998 and the 13th experimental campaign was conducted in 2009. Before final assembling, cool-down and excitation tests for the Inner Vertical (IV) field coil, which is one of the LHD poloidal field coils, were carried out in 1995. This coil, which consists of a cable-in-conduit conductor, (CICC) is cooled by the forced-flow of supercritical helium. During the tests of the IV coil, hydraulic characteristics, such as flow distribution among cooling channels and friction factors, were measured. In this paper, the consistency of the behavior of the IV coil will be presented and comparison with other fusion devices using superconducting coils will also be made at not only cryogenic temperatures but also at room temperature. [Copyright &y& Elsevier]

Wendelstein 7-X (W7-X) is a superconducting stellarator under construction at the Max-Planck-Institute for Plasma Physics in Greifswald, Germany. The magnet system consists of 70 superconducting coils providing magnetic confinement-fields. Before assembly every coil had to pass an acceptance test at the French partner-institute CEA, Saclay, to examine its performance on leak tightness, high voltage strength, ampacity, and hydraulic flow at cryogenic conditions. The testing procedure was mandatory, as access for maintenance or repair after installation will not be viable. Testing followed a well specified routine including tests in warm conditions, during cool down, in cold conditions, during warm up and at last in warm conditions. Measurement was obtained firstly by the facility's instrumentation and secondly by the coil-casing's instrumentation. The received data was transmitted to different ASCII-files for each gauge. To record testing in detail, a meta-document was edited, which follows in its structure the routine mentioned above. Accordingly, the main distinctive functions of the coil during testing are recorded in so called test reports, while deviations from given thresholds and parameters are described in non conformity reports. During testing, organizational and technical challenges had to be mastered and the test program was regularly updated to meet technical improvements, manufacturing requirements and revised assembly schedules. This paper will present the acceptance test program, its development and the gained experiences. [ABSTRACT FROM AUTHOR]

A new quench monitoring system by using the Poynting Vector method was proposed. This new system can indirectly detect normal transitions which are suddenly induced during an operation of superconducting coils. In this system, the superconducting coil is surrounded by the thin metallic cylinder composed of several pieces, where the cylinder is electrically insulated from the coil; potential leads to measure local electric fields are directly attached to this cylinder surface and pickup coils to measure local magnetic fields are put on the surface. The Poynting vector distribution around the coil can be obtained by using many sets of both the potential lead and the pickup coil. Since sudden changes of energy flow profiles around the superconducting coil are induced in the event of local normal transitions of the coil winding, such an event of coils can be detected by the change of the Poynting vector. This system has no risk of the destructive voltage breakdown which may happen in case of the existing balance voltage method, because there is no need of direct contacts on the coil windings. In addition, this method is expected to be higher reliability and sensitivity than the acoustic emission method. A demonstration test was carried out of monitoring the quench for the Bi-2223 coil partially immersed in liquid nitrogen, and a sign of normal transitions before the quench was successfully detected by this method. [ABSTRACT FROM AUTHOR]

To protect the KSTAR (Korea Superconducting Tokamak Advanced Research) superconducting coils against a quench, the quench detection system based on voltage measurement was fabricated. It used a detect/dump scheme which detects the presence of non-recovering normal zones and activates a dump circuit that transfers most of the stored energy into a dump resistor. Even though it is desirable to discharge the coil energy as fast as possible after quench detection, a time delay between the quench detection and the complete dump circuit actuation will be necessary in order that the quench voltage is to be distinguished from various noises. The quench threshold voltage and the delay time should be set before operation for quench voltage detection so that the maximum hot-spot temperature could be limited to 150 K. This paper describes the hardware techniques to prevent the malfunction of the quench detection due to voltage noises arising from the KSTAR operating scenarios. During the first operation of the KSTAR machine, the inductive voltages and other voltage noises were measured and effectively compensated below voltage thresholds. A quench did not occur and the quench detection system was well operated without any false activation. [ABSTRACT FROM AUTHOR]

We investigated the effect of thermal cycles between room and liquid-nitrogen temperatures on the critical current (Ic) and AC loss of two superconducting coils: one with a bobbin that expands during cooling from room temperature to cryogenic temperature and one with a bobbin that contracts during the cooling. After 100 cycles, neither bobbin suffered degradation. The Ic of the contraction-bobbin coil did not decrease, and that of the expansion-bobbin coil decreased due to the repeated thermal strain. The expansion coil's AC loss was significantly smaller than the contraction coil's loss at the first cycle; however, it increased with the thermal cycles and eventually surpassed the contraction coil's loss because the Ic of the expansion coil decreased due to the thermal fatigue. These results indicate that a moderate expansion in the size of the bobbin effectively decreases AC loss and that excessive expansion reduces the Ic and increases the AC loss. [ABSTRACT FROM AUTHOR]