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Inductance is a very commonly used electrical parameter in transient progress analysis, as well as in energy storage evaluations. It was normally considered as constant when the geometrical configuration of a coil is determined. However, experiments on Bi-2223 (Bi 2 Sr 2 Ca 2 Cu 3 O 10+x ) and YBCO (YBa 2 Cu 4 O 7-x ) coils, both show 10% rise inductance while increasing the applied current. The inductance grows approximately a linear relationship from its minimum to maximum. We used both theoretical and experimental analysis to deeply understand the mechanism of the variation in terms of current distribution in high-temperature superconducting (HTS) coils. Taking this inductance change into account would benefit the accuracy in energy storage applications and in transient analysis occasions. Besides, inductance controllable devices could be achieved by manipulating the bias current applied on HTS coils.

Until recently, a new type of open magnetic shielding system was realized by the combination of a series of closed high temperature superconductor coils. We experimentally confirmed that the shielding factor (SF) of the new shielding system can reach more than 40 dB in the frequency spectrum of 0.1–16 000 Hz. Because of its superior ability to resist alternating magnetic field, we named, the system Tesla-shielding (TS), to commemorate the father of alternating current, Nikola Tesla. In this paper, we will report the design and realization of a Helmholtz coil-based TS. The Helmholtz TS has many advantages over the previous planar TS. Especially, the Helmholtz TS has an engineering way of regulating the device for a better SF. We will introduce the design of the Helmholtz TS with the help of numerical simulation. The main electromagnetic properties of the TS, such as SF in the frequency spectrum and its shielding ability in the long duration measurement, are given in this paper.

In this paper, the I c degradation of bent YBCO tapes has been experimentally studied. The I c characteristics in length direction was measured through a non-contact, continuous I c measurement equipment (Mcorder), which is suitable for all kinds of high temperature superconducting (HTS) tapes. The detailed J c mapping of bent tapes was investigated by a Hall microscopy equipment (Mcorder 2-D), which scans the fields distribution on the surface of the films or bulks. A 4.1-m-long YBCO tape was wound on a 4.5-mm-diameter core with YBCO face outside or inside. The critical current of the YBCO tape before and after winding was measured lengthwise. And the 2-D J c map of the samples of degraded sections was measured. The result shows that the smallest safe bending diameter is larger than 14 mm and smaller than 4.5 mm, for YBCO face outside and inside, respectively, which indicates that the YBCO face is recommended to be inside in bending cases. The relation between critical current of YBCO face outside bent tapes and diameters was fitted to a sigmoid-like function, to help further development where YBCO face outside bending is unavoidable.

In this paper, a 10-kW/150-r/min fully superconducting synchronous generator was designed and sent for manufacturing, which installs high temperature superconducting coils in both the stator and the rotor. It has a basic structure of eight poles and nine slots. The concentrated winding was adopted, and there are 16 field coils and 18 armature coils wound by using 5-mmwide YBa 2 Cu 3 O 7-δ (YBCO) tapes. The total tape consumption is around 2.0 km. Iron cores assembled by silicon-steel sheets will be installed in both the stator and the rotor. The peak gap field of 0.87 T can be achieved with a 42-A excitation current in field coils. The armature electric loading can reach 400 A/cm at the rated output. The present machine is designed to be cooled by using liquid nitrogen. The cryogenic structure can be divided into two parts: the conduction-cooled rotating part for field coils, and the immersion-cooled static part for armature coils. Both parts share the same vacuum chamber. A novel thermal insulated torque coupling will be installed to link the cold rotor and the warm shaft. The ac loss of armature coils was studied by H-formulation and it is within a reasonable range.

Chemical solution deposition (CSD) is widely used to fabricate YBCO superconducting and oxide buffer layers for high-temperature coated conductors. Controlling film thickness is important in the CSD technology and can be conducted through several methods. This study proposes a new method of adjusting the temperature of dip-coating. A precursor solution was prepared for fabrication of a YBCO layer. The temperatures of the precursor solution and substrate were fixed at 0 °C, 10 °C, 20 °C, or 30 °C during dip-coating by using a water bath system. Film thickness was measured by scanning electron microscopy after pyrolysis. The average thickness of the pyrolyzed films decreased from 265 to 158 nm when the dip-coating temperature increased from 0 °C to 30 °C, therefore verifying the feasibility of the new method for thickness control. The nonuniformity of thickness distribution along the withdrawal direction was detected and discussed. The influence of temperature on solution viscosity and film thickness was analyzed based on the equation of thickness dependence in dip-coating. The proposed method for thickness control could contribute to increasing the synthesis window in thin-film fabrication through CSD.

Since there is no iron core and the energy is only coupled through the primary and secondary windings, air-core transformers have advantages of no iron loss, no magnetic saturation, and compact in size, which make them attractive to be studied for practical applications. Due to its high current density and low loss, high temperature superconductor (HTS) brings additional advantages for this type of transformer. The HTS wires are highly magnetic field dependent and their characteristics are required for the HTS implementation and the design of the air-core transformers. As a result, magnetic field distribution of the HTS windings is researched, and its influence on the HTS critical currents, ac loss, and the transforming efficiency of the HTS air-core are analyzed.

High-energy high-temperature superconducting superconducting magnetic energy storage (HTS-SMES) is expected to play an important role in stabilizing the future smart micro grid. Large-scale and high-field magnet is required for high-energy SMES. However, the significant hoop stress induced by the Lorentz force has become one of the most serious threats. In this paper, we proposed a solution to reduce the hoop stress by overbanding the solenoidal magnet with high Young's modulus and yielding strength reinforcing structure. A unique graphical method was developed to optimize the geometry of the magnet and reinforcing overbanding structure to achieve high energy capacity. According to our analysis, thin-walled solenoid with large aperture is recommended as an economical geometry for HTS-SMES.

Most HTS (high-temperature superconductor) coils will quench due to defects in the winding. Some of these defects come from tape manufacturing, but more commonly are generated in the process of making a coil, especially for the YBCO racetrack coils that use epoxy resin to solidify the winding. In order to find out defects, chemical solutions are used to dissolve epoxy resin of three damaged YBCO racetrack coils. YBCO tapes reclaimed from these coils are scanned by MCorder, a continuous critical current measurement device that can tell amplitude of defects and distribution of defects in a high speed and high resolution. The scan result shows severe defects everywhere in the winding. However, during the solidification process, outer turns are more vulnerable to the defects than the inner turns. We believe that the delamination of YBCO tape caused by contraction of the epoxy resin contributes to the defects.

A 15-kW/150-rpm fully high-temperature superconducting (HTS) synchronous generator, which installs HTS windings on both the rotor and the stator, is under development. The thermal insulated torque coupling (TITC) is one essential part in the generator, which can transmit torque from the outside warm shaft to the inside cryogenic rotor shaft and reduce the heat conduction as well. In this work, a modular TITC structure has been proposed, which mainly consists of several G10 rods and two metallic flanges. An analytical model for evaluating the stress level and the heat leakage power was built. Design principles for TITCs at different load levels were discussed. Based on the aforementioned knowledge, a sample TITC for our 15-kW/150-rpm generator was designed, produced, and tested under the static load. It was found that the TITC cracked at the load of 2500 Nm, which corresponds to a safety factor of 2.6. Key structural parameters for TITCs working under higher torque levels (0.5-10 MNm) were also calculated.

The magnetic circuit (MC) method has been developed to measure the critical current (I c ) of long high temperature superconductor (HTS) tapes continuously since 2010. So far, with the apparatus based on the MC method, namely MCorder, industrial HTS tapes produced by main manufactures worldwide have been measured using the MC method, including Bi2223/Ag multifilament tapes and coated conductors with/without magnetic substrates. The high speed (360 m/h), high accuracy measurement (resolution 1 A) and its adaption to all kinds of HTS tapes make large quantity measurement and statistical evaluation of HTS tape possible. In this paper, criteria for I c assessment of long HTS tapes is discussed, based on which analysis of samples from different manufacturers worldwide is provided. The measurement result of a Bi2223/Ag tape by MC method is compared with section-by-section four-probe method, and the two methods show good agreement. Fourier analysis is given for analyzing I c periodicity of a tape with periodical artificial defects.

Concept design and optimization for a high-temperature superconducting (HTS) rotor of a 100-kW generator is studied in this paper. The rotor has eight HTS magnets constructed by air-cored racetrack coils. The HTS magnets with a unique circular cross-sectional shape can produce a gap field over 2.0 T. The present design is based on a new graphical method developed to estimate the maximum operating current of an HTS magnet that takes both I c -B characteristics and angular dependence into consideration. The optimized stepped shape design saved more than 8% of the HTS material compared with the rectangular one without sacrificing the gap field.

A YBCO racetrack coil was developed for HTS machine applications. The coil was constructed as step-type to fit in the limited space of the machine rotor. Two double-pancake (DB) windings were fabricated for the whole coil that has used 388 m of YBCO wires purchased from American Superconductor Corporation (AMSC). A finite-element method (FEM) software was used to model the coil and the critical currents for each DB winding were estimated. It was found out during the winding process that by using AMSC wires the coils can be wound in a rather simple way. The coils show no degradation even after a few accidents occurred during the winding process.

Field penetration and ac loss of high-temperature superconductor (HTS) windings wound by parallel-connected (PC) tapes are simulated and calculated. The simulation is given to a winding with six tapes, which are connected to produce either n = 1, 2, 3, or 6 PC current turns. The PC turns are basically organized as two main types according to whether the turn end is soldered or not. The soldering state is controlled by shunt resistances between HTS tapes. Therefore, six independent PC structures are used for simulation. In addition to the HTS tapes, normal conductors, representing the possible reinforcement layers or stabilizer layers, are also built in the finite-element analysis (FEA) model. The simulation is carried out using the FEA software ANSYS, which is combined with the self-developed resistivity-adaption algorithm, a code that can cope with arbitrary FEA software to simulate field penetration into HTS. More importantly, we used the circuit-field analysis, so that we can adjust n values and the shunt resistance independently while we are simulating the field penetration process. AC loss contributed from different parts of the winding is separately studied as QHTS, which is hysteresis loss generated in HTS; Qnor, which is eddy current loss generated in normal conductors; and Qcc, which is coupling loss generated in shunt resistances. The current profiles and QHTS in each tape of the winding are very sensitive to the specific n value. However, the total QHTS for different PC structures does not significantly change. Even under overestimation, Qnor can only be comparable with QHTS when the frequency exceeds 1000 Hz. Qcc is prominent at low transport currents but is naturally restrained at high transport currents.

Hermetically sealed samples are required for HIP experiments of Bi-2223/Ag tapes. Otherwise the high pressure gas might penetrate into the inner sample and balance the outside pressure. In this work, a new high temperature pressure charging (HTPC) procedure was proposed for the HIP process of Bi-2223/Ag tapes. Some liquid phase was firstly formed during the normal pressure heat treatment and the gas pressure was then applied directly at high temperature. Dense filaments could be achieved after this new HIP process. It was found the gas inflation temperature (Tadd) and dwelling time before inflation (tadd) have some influence on the final microstructure morphology and critical current of HIP samples. A preliminary improvement of Ic of 13% was achieved after optimization, compared with that of normal pressure heat treated samples.

A new method based on the principle of magnetic circuits is proposed and realized for continuous Ic examination of HTS tapes. The greatest advantages of the new method are that it first eliminates all the noise caused by mechanical fluctuations, and thus makes high speed and high stability measurement possible, and second has a natural ability to measure HTS tape with a magnetic substrate. The principle of the method is introduced with the help of Finite Element Analysis. An apparatus for examination of kilometer long tapes has been constructed, by which continuous Ic examination for a YBa2Cu3O7-x tape with and without a magnetic substrate and a Bi2Si2Ca2Cu3Ox multi-filamentary tape is reported.

A conduction-cooled 1.5 T high temperature superconducting (HTS) magnet was designed. The magnet will be used to provide a steady and confining magnetic field, which requires the uniformity within ±5% of the central field, over a ? 25 mm × 120 mm cylindrical region. Totally 0.8 km Bi-2223 HTS tapes will be used to wind 22 double-pancakes. The inner and outer diameters are 60 mm and 88 mm respectively; the notched shape was employed to improve the field uniformity. An improved method other than the traditional was employed to determine the size of the magnet. The magnet is expected to be cooled down to 30 K by a GM cryocooler through the conduction cooling method. The operating current is 100 A, with which the magnet generates 1.5 T at the center of the bore. In order to facilitate the winding process and enhance the heat conduction, a new method was specially developed to wind the double-pancake, which was named as the "cassette-pancake".

Hot isostatic pressure (HIP) process has been carried out on multifilament Bi-2223/Ag tapes. The influence of the crack-healing process (CHP) on the microstructure and critical current (I c) of HIP samples has been studied. I c values of HIP samples show a strong depend on the CHP sintering temperature and dwelling time, with the optimum CHP condition at about 828degC for 3 hours. It was suggested that CHP is related to the presence of liquid phase, leading to a self-sealed effect in Bi-2223/Ag tapes. By controlling the CHP temperature, a preliminary I c improvement of about 14% has been achieved on HIP samples, compared to those sintered at normal pressure.

Closed superconducting coils made from Bi2223/Ag multi-filamentary tapes were successfully fabricated. By measuring the magnetic flux density at the center of the coil, we were able to observe the persistent current mode of superconducting closed coil in liquid nitrogen. The persistent current measured was 1.39 A and remained unchanging for more than 500 hours after several hours decay. It was estimated from the decay curve that the upper limit resistance of the closed-loop coil was in the order 10-13 Omega of . Current-voltage characteristic was derived form the decay curve and different models were applied in fitting the experiment data..

Residual magnetic field in a high temperature superconductor (HTS) magnet is a factor which should be avoided when the field of the magnet is being operated under a high uniformity and high accurate control. In this paper, the residual field in a Bi2223/Ag pancake magnet at 77 K is systematically studied. The amplitude of the residual field as a function of maximum charging current and the relaxation of the residual field are measured. For a magnet with an inner diameter of 60 mm, an outer diameter of 80 mm and a height of 30 mm, the maximum residual field is about 14.5 G after ramping the charging current up to 15 A, followed by dropping the current to zero. A simple model is proposed, from which the residual field can be estimated as along as the configurations of the magnet and the critical current (Ic) of the original short sample are given.

The current lead is an important application of high temperature superconductors. In this work, superconducting current leads are assembled by using Bi-2223 tapes with Jc = 32000 A/cm2 at 77 K under self-field. The influence of self field on the critical current (Ic) is studied by changing the distance (d) between neighboring tapes. With our in-situ d changing device, Ic values of current leads with 2 and 3 tapes are measured as a function of d (0.5-15 mm). It is found that the total Ic decreases greatly if d < 1.0 mm due to the influence of self-field. An analysis method concerning the field dependent property and self-field distribution of the current lead is suggested and compared with the experimental data. Finally 6-tape current leads are prepared. Copper layers are added as protection circuits, which are put between neighboring tapes. The total Ic is about 0.681 of the sum of Ic of each individual tape when d = 0.26 mm. This ratio increases to 0.835 if 1 mm thick copper layers are put between two adjacent tapes (d = 1.26 mm). It agrees with our calculation quite well. This knowledge helps us to design tape arrangements in current leads and other HTS devices using large current.

Alternating current (ac) losses in high-temperature superconductor tapes and devices with transport current are solved by using the resistivity-adaption algorithm (RAA). The most advanced feature of the RAA is that it enables the simulation of any model derived from the flux motion theory on finite-element analysis (FEA) packages that have an eddy current solver. The principle of the RAA, as well as its realization on the ANSYS FEA package, is introduced. The simulation begins with the calculation of the ac loss of an ellipse and of strips with aspect ratios ranging from 50 to 2000. The accuracy and efficiency of the calculation are verified through comparisons with the Norris theoretical curves. The possible errors and the method to overcome such errors are discussed. The most significant improvement in the proposed RAA from that discussed in a previous study is that the RAA was proven to be valid for calculating the field-dependent critical state model by using the descendant process from +Im to -Im. We then extend this method to calculate the transport ac loss of a stack of ellipses with Jc(B) characteristic from a typical Bi2223/Ag tape and the transport ac loss of a stack of strips with Jc(B) characteristic from a typical YBCO-coated conductor.