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In the present study, wind tunnel experiments were performed to investigate the power and thrust characteristics of a wind turbine model subjected to a variety of surge and sway motions. The turbine model was mounted on a translational stage to simulate the surge and sway motions at prescribed amplitudes and frequencies. The power output and rotor thrust were measured in each sets of the motion frequencies and amplitudes, respectively; and the measurements were also performed in a base-fixed turbine for comparison. Results show that the surge and sway motions with various amplitudes and frequencies all have slight impact on the mean power output and mean rotor thrust of a wind turbine. However, the rotor thrust fluctuations of the surging turbine, not only appeared significantly higher than those of the swaying turbine and base-fixed turbine at the same sets of amplitudes and frequencies, but also increased with the surge frequency and surge amplitude. Moreover, for both the surging and swaying turbine, the rotor thrust fluctuated with the same period as the surge and sway motions, respectively; and the power spectral density of the rotor thrust both exhibited a peak at the motion frequency and its odd multiple frequencies.

On the basis of 24 T magnet, the Institute of Plasma Physics, Chinese Academy of Sciences are developing 35 T all superconducting magnet. The magnet was designed as a combination of 20 T REBCO insert coil and a 15 T 150 mm low temperature superconductor background magnet. The 20 T insert coil consists of two coils nested together. NI winding technology is applied to coil 1 to increase its current density. The 50 μm thick Hastelloy tapes will be used to co-wind with REBCO tapes of the coil 2 to further improve its over-all mechanical strength. Electromagnetic analysis shows that the maximum ratio of operating current to the critical current of coil 1 and coil 2 is 0.63 and 0.37 respectively. A more accurate model for calculating strain considering screening current is under study. The inner joint is applied to the coil 1 to avoid in-plane bending of REBCO tape, and the detailed preparation method is given in the paper. The winding and preliminary testing of all double pancakes of coil 1 have been completed, and the winding of coil 2 is in progress. This magnet is planned to be assembled and tested in 2021.

The delamination and critical current (Ic) degradation caused by thermal stress after epoxy impregnation are threats for the application of REBa2Cu3O7-x(REBCO, RE = Rare earth) superconducting magnets. In this work, two types of REBCO tapes were coated by Stycast 2850FT with controlled coating geometries. Critical currents of coated samples after thermal cycles in liquid nitrogen were measured. Ic degradation was found in coated samples with a free no-coating edge, when the surface coating layer was thicker than 1000 μm. It was also found that additional edge coating can help to suppress the Ic degradation. Samples with degraded Ic after thermal cycles showed an obvious delamination phenomenon. The morphology and location of delamination cracks were carefully observed by using focused Ion beam, scanning electron microscope, and transmission electron microscope. Delamination cracks propagated within the REBCO layer and stopped at reaching the Silver/REBCO interface. Simulations by finite element method suggest that delamination cracks are generated by the stress accumulation within the REBCO layer, which could be reduced by a full epoxy coating on both tape surface and edges.

China Southern Power Grid Corporation started a project to develop a 10-Mvar high-temperature superconducting (HTS) dynamic synchronous condenser (DSC). As the first stage of the project, a 300-kvar HTS DSC prototype was developed to verify some key technologies, such as HTS rotor magnets, helium gas circulating cooling and rotating signal acquisition. In this prototype, conduction-cooled REBCO coils were installed in the rotor and air-core copper windings were used in the stator. Cold helium gas at around 20 K was used as the coolant circulating between the rotor and the external cryogenic unit through a cryogenic rotary coupling. The REBCO coils were conduction cooled by distributed helium-gas tubes embedded in the cover plates of the coils. The rotor magnet was successfully charged up to 200 A under both static and rotating conditions. For the requirement of an HTS DSC, rapid excitation tests from 170 A to 200 A were carried out at both 500 rpm and 1500 rpm.

Wrinkling morphology could be utilized in various sensors because of its large specific surface area depending on the amplitude (A) of the wrinkles. The amplitude was proportional to film thickness (h) in the traditional wrinkles fabrication processes, and it is inefficient to increase the wrinkle amplitude by adjusting the film thickness. Hence, it is very necessary to propose a method that can significantly increase the wrinkle amplitude by changing the film thickness. In this study, the wrinkling morphology of mono-layered thin films prepared via chemical solution deposition on rigid substrates was investigated. By using the solution with various cation concentration, thin film samples were fabricated with h ranging from 474 to 1405 nm. A rapid pyrolysis with ramping rate of 5 °C $${\min }^{-1}$$ was applied to obtain the wrinkling morphologies, which were characterized by white-light interferometer and scanning electron microscope. Amplitude values were calculated out of the height images by using two-dimensional fast Fourier transform. The approximate relationship of amplitude with film thickness A ∝ h3 was found. Compared to the traditional multi-layered architecture, wrinkling film was fabricated by depositing a mono-layered thin film on a rigid substrate in this work, which is simple and low-costing. Amplitude A could be more sensitive to h, which might be beneficial for response sensitivity in various fields. The wrinkling mechanism of chemical solution deposited thin films during rapid pyrolysis was discussed based on the assumption of gas accumulation in the precursor film, and a model based on the accumulation of byproduct gas during pyrolysis was proposed to interpret the relationships.

Enhancing the mechanical strength of highly conductive pure metals usually causes significant reduction in their electrical conductivity. For example, introducing phase/matrix interfaces or more grain boundaries, are common and effective methods to strengthen metals. But it simultaneously increases the electron scattering at the interface, thus reducing the electrical conductivity. In this study, we demonstrate that pure aluminum (Al)/carbon nanotubes (CNTs) nanocomposites prepared by friction stir processing have successfully broken through these limitations. The yield strength and tensile strength of Al/CNTs nanocomposites have improved by 104.7% and 51.8% compared to pure Al, while the electrical conductivity remained comparable to that of pure Al. To explore the potential mechanisms, the interface between CNTs and Al was examined and characterized by transmission electron microscopy (TEM) and Raman spectroscopy. Little interfacial reaction compounds were present and no visible physical gaps were observed at CNTs and Al interfaces. We defined it as a clean and tightly bonded interface. Although the quantity of phase interface has increased, the electrical conductivity of the nanocomposite remains approximately unchanged. We attribute the preserved electrical conductivity to the clean and tightly bonded CNTs/Al interface in the nanocomposite.

The second-generation high-temperature superconductor REBCO is one of the most valuable candidates that meet the requirements of future magnetic confinement fusion device. Quench behavior of an improved high-current REBCO composite conductor is simulated by a 1-D finite element method. Indium-tin alloy foil placed in the gap between stabilizer, jacket, and superconducting tape stacks can significantly reduce thermal contact resistance, which leads to an increase on the minimum quench energy of composite conductor. Short perturbations have a better response to this improvement than long perturbations, due to the low normal zone propagation velocity (NZPV) of REBCO conductor. For short perturbation (perturbation length ( $l_{p}$ = 0.1 m), the peak temperature in cable and helium drops 12.76 and 17.99%, respectively, after soldered. Due to the change of heat transfer in a composite conductor caused by helium wave propagation, the development and maximum length of normal zone significantly decrease from 13.06 to 4.78 m under long perturbation ( $l_{p}$ = 10 m) circumstance. High current REBCO composite conductor with proper indium-tin alloy soldered can enhance the stability and safety of composite conductors under severe working conditions, but it also slows down the NZPV and hinders the quench detection of high-temperature superconducting magnet.

With recent advances in second-generation high temperature superconductors (2G HTS) and cable technologies, various numerical models based on finite-element method (FEM) have been proposed to help interpret measured AC loss and assist cable design. The T-A formulation, implemented in COMSOL, shows great potential for reducing the overall computation costs. In this paper, the performance of the T-A formulation for calculating the AC loss of coated superconductors and cables were assessed and compared against the widely accepted H formulation, with benchmark model of a single REBCO tape in 2D/3D and a 14-strand Roebel cable. Evaluation and comparison on key metrics including the computation time, the number of degrees of freedom and the numerical accuracy were presented, which could provide a reference for researchers in applying the T-A formulation for AC loss calculation.

Striation has been proved to be effective in reducing the magnetization losses of Conductor on Round Core (CORC) cables assembled with REBCO coated conductors. In this study, we present numerical models of CORC cable with striated strands considering the inter-filamentary coupling by using 3D $T$ - $A$ formulation. Calculated magnetization losses were compared against experimental data to confirm the validity of our models, including 3 × 2 CORC cables with straited and non-striated strands. Frequency dependence of magnetization losses and the electromagnetic coupling effect were demonstrated and analyzed. The coupling loss fits well with analytical function. And the coupling time constant characteristics was found almost in proportion to the square of pitch length.

China Southern Power Grid Corporation has started a project to study the feasibility of large-scale high-temperature superconducting (HTS) dynamic synchronous condensers (DSC) in Ultra High Voltage Direct Current (UHVDC) transmission grid. To investigate some key technologies, a small-scale prototype of HTS DSC of 300 kvar/2 poles/3000 RPM is under development. The electromagnetic design, especially the critical current of REBCO rotor coils was discussed. Besides, the configuration and properties of the air-core stator, the damper and the rotor were also described. Water-cooled windings in the air-core stator have a rated current density of 9.55 A/mm 2 . Helium cooling pipes were embedded within REBCO rotor coils, and HTS windings will be cooled down to

A 300 kvar high temperature superconducting synchronous condenser is under construction. In this work, a test superconducting magnet consisting of two double-pancake coils was designed, fabricated and tested. The coils had an effective length of 300 mm. They were wound by 5 mm wide REBCO tapes and vacuum impregnated by paraffin. The test magnet was cooled by a distributed-tubes-inside-pancakes conduction cooling structure. In this structure, 20 K helium gas was forced to flow in the tubes which were embedded in stainless-steel cover plates of coils. The magnet was installed in a vacuum chamber for cooling and transport tests. The results showed that the magnet was effectively cooled to about 22 K and operated stably at 281 A. The magnet steady-state temperatures at different operation current were recorded, approximately 2 K higher than the helium gas. Moreover, the thermal stability of the magnet was evaluated by adjusting inlet gas temperature from around 20 K to 25 K, and no magnet quench was observed. It was concluded that the cooling structure of the test magnet could fulfill the requirements of the 300 kvar HTS synchronous condenser.

Chemical solution deposition is a promising technology to fabricate YBCO superconducting thin films due to its low-cost advantage. However, the decreasing of critical current density ( J c ) usually occurred in the fabrication of multi-layered YBCO films. In this study, Ag and Au heterogeneous interlayers were fabricated by using sputtering deposition in order to improve the bi-layered YBCO films, and the influence of crystallization temperature from 740 ∘C to 800 ∘C was investigated. It was found that J c could be enhanced by both interlayers relative to the counterparts without interlayers, and the effect of Ag interlayer was superior to that of Au interlayer. When the crystallization temperature was 800 ∘C, the optimal sample with Ag interlayer had a J c 3.65 MA/cm2, which was about twice of the J c of its counterpart. The surface morphologies and cross-sections were observed by using scanning electron microscope, and the content variations of c-axis oriented grains were measured by using Raman spectroscopy, which could help to investigate the influences of interlayers on the microstructure of YBCO films. The possible reasons for the improvement effects of Ag and Au interlayers were discussed based on the experimental results.

Fluxgate magnetometer is an important device for weak magnetic-field measurements. By adding a large dc bias excitation current, the performances of fluxgate can be highly improved. In this article, based on the assumption of the single-domain state within the probe core wire, the quasi-static Stoner–Wohlfarth model (SW model) was used to construct a mathematic model for the output signal of the orthogonal fluxgate probe operated in fundamental mode (FM-OFG). The dynamic hysteresis loop characteristics were also considered here, and the expected first-harmonic output signal corresponding to the theoretical analysis was obtained from our model. A prototype FM-OFG probe was fabricated by using a 20 mm-long Co–Fe–Nb–Si–B amorphous microwire as core wire and a 1500-turn solenoid as pickup coils. Output performances were analyzed at different external fields, ac excitation currents, and dc bias currents. A good agreement between the modeling results and the experimental ones was found. The noise level was reduced down to 9.01 pT/rt (Hz).@.1 Hz. The simulation method and experiment results provide theoretical guidance for the subsequent design and improvement of the FM-OFG.

A wind-tunnel experimental study was performed to investigate the impact of the surge and sway motions of a wind turbine model on the power output, rotor thrust and wake characteristics. A wind turbine model was mounted on a translation platform to simulate the surge and sway motions under given amplitude and frequency. The power output and rotor thrust of the turbine model subjected to surge and sway motions were measured by using a DC variable electronic load and a six-component force sensor, respectively. For comparison, these measurements were also performed in a bottom-fixed wind turbine. The results show that the mean power output and mean rotor thrust of the turbine model under surge and sway motions are almost the same as those of the bottom-fixed turbine. However, the thrust fluctuation amplitude of the turbine model under surge motion is significantly higher than those of the turbine model under sway motion and the bottom-fixed turbine. In addition, the wake characteristics of the turbine model were also investigated by using a particle image velocimetry system. The results show that the surge and sway motions have slight effect on the near and intermediate wake of the turbine model in the horizontal plane at the rotor hub height.

Higher magnetic fields are always favoured in the magnetoplasmadynamic thruster (MPDT) due to its superior control of the plasma profile and acceleration process. This paper introduces the world's first integrated study on the 150 kW level AF-MPDT equipped with a superconductive coil. A completely new way of using superconducting magnet technology to confine plasma with high energy and extremely high temperatures is proposed. Using the PIC method of microscopic particle simulation, the plasma magnetic nozzle effect and performance of the MPDT under different magnetic-field conditions were studied. The integrated experiment used demonstrated that, in conjunction with the superconducting coil, greater homogeneity and a stronger magnetic field not only caused more even cathode ablation and improved its lifespan but also improved the performance of the MPDT (maximum thrust was 4 N at 150 kW, 0.56 T). Maximum thrust efficiency reached 76.6% and the specific impulse reached 5714 s.

REBCO-based high temperature superconductor (HTS) cable conductors, due to its large transport current, are crucial for the requirement of large-scale applications, such as accelerator magnets, fusion magnets, and superconducting magnetic energy storage system (SMES) systems. One key characteristic of these conductors is their electromagnetic response under alternating external magnetic field. Test samples of a 10-strand REBCO-based Roebel cable were studied both, experimentally and numerically. Samples were magnetized without transport current at 77 K by ramping up and down the external magnetic field. The remnant fields (screening current field) after the removal of external fields were mapped using a scanning Hall probe with fixed distance (1–5.5 mm) above the sample surface. Corresponding three-dimensional finite element method (FEM) models were developed using T – A formulation with thin strip approximation. The experiment results and simulation solutions showed good agreements, which confirmed the validity of the numerical model, as well as provided us with a better understanding of the magnetization process and remnant field characteristics of Roebel cable.

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.

Scanning Hall probe (SHP) system is a powerful instrument to investigate the planar distribution of areal critical current density (j c ) and defects of high temperature superconducting (HTS) thin film samples. The distance between the effective measuring position and probe bottom (h 1 ) is usually unknown for the commercialized Hall probes. However, h 1 value has a significant influence on the measurement results, according to the calculation of the magnetic flux density (B 0 ) above the planar center of a rectangular HTS thin film sample. In this paper, a method was proposed to obtain h 1 by using a series of copper strips and analyzing the relationship of B 0 against the parameters, such as sample width and current. Then, SHP system was employed to characterize the j c distribution of an HTS thin film fabricated via chemical solution deposition. The relationship of B 0 and h 1 could also be utilized to obtain the average j c value (j c B0 ). A series of commercialized coated conductor samples were measured with both SHP and the traditional four-probe method. The feasibility of preliminary evaluation for rectangular HTS thin film samples by using j c B0 was discussed based on its correlation with the results of four-probe method.

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.

The scaling characteristic of surface roughness could reveal the fractal property of a surface, which is common for the thin films fabricated via physical vapor deposition. In our previous study, a roughness scaling extraction (RSE) method was proposed to accurately calculate the fractal dimension (D), which is an effective parameter to represent the irregularity and fragmental property of a fractal surface. RSE method was based on a single morphological image of the concerned surface, and was much more accurate than the traditional methods with a low mean relative error of 0.64%. In this study, RSE method was firstly optimized by using the planarization modification on the morphological images of artificial surfaces with ideal dimensions (Di) ranging from 2.1 to 2.9, which were generated through Weierstrass-Mandelbrot (W-M) function. The calculation accuracy of RSE method could be enhanced by using the second order planarization modification, with a lowest mean relative error of only 0.42% between Di and the calculated dimensions (Dc). Secondly, artificial surfaces with high resolution and Di from 2.2 to 2.8 were utilized to investigate the influence of atomic force microscopy (AFM) probe-tip geometry on the calculation accuracy. A typical geometry with a probe-tip radius (r) of 10 nm was employed to carry out the simulation of morphological filtering for the surface images, and the filtered images with various scales (L) from 0.5 μm to 7.5 μm were generated. RSE method was found to be robust only when the ratio of L/r was above 400, while a significant deviation occurred at lower ratio values, which would be instructive for the fractal analysis on the AFM images of various surface morphologies.

Chemical solution deposition is a promising technology to fabricate YBCO superconducting thin films due to its low-cost advantage. As found in our previous study, the addition of lead acetate in precursor solution could enhance the critical current density of YBCO films, since the deteriorating a/b-axis orientated grains could be suppressed. In this study, the precursor solutions with lead acetate weight percentages of 0%, 1%, 2% and 3% were prepared. The molar concentration of all metal ions was 2.0 mol/L for all solutions. Crystallization temperatures of 720°C, 740°C, 760°C, 780°C, 800°C and 820°C were used in the heat treatment. The thickness of the finally obtained YBCO films was about 400 nm. It was found that 1% of lead acetate addition was favorable to achieve high critical current at 77 K in self-field, and the critical current generally increased along with the crystallization temperature. The cross-sectional microstructure of YBCO thin films fabricated by using lead acetate addition was denser than that of the counterpart samples. In situ measurement of the film's resistance during the heat treatment was carried out to analyze the growth and nucleation processes of YBCO thin film deposition. The mechanism of lead acetate addition for the a/b-axis oriented grain suppression reported in our previous study was investigated.

Fractal dimension (D) is an effective parameter to represent the irregularity and fragmental property of a self-affine surface, which is common in physical vapor deposited thin films. D could be evaluated through the scaling performance of surface roughness by using atomic force microscopy (AFM) measurement, but lots of AFM images with different scales (L) are needed. In this study, a roughness scaling extraction (RSE) method was proposed to evaluate D values of a single AFM image, in which the roughness at smaller L was estimated by image segmentation with flatten modification. Firstly, a series of artificial fractal surfaces with ideal dimension ( D i ) values ranging from 2.1 to 2.9 were generated through Weierstrass-Mandelbrot (W-M) function, in order to compare RSE method with traditional methods such as box counting method and power spectral density method. The calculated dimension ( D c ) by RSE method was much closer to D i than the other methods, with a mean relative error of only 0.64%. Secondly, RSE method was utilized to deal with real surfaces, which were AFM images of amorphous alumina thin films with L of 1–70 μ m. D c obtained by RSE method based on a single AFM image was also close to the result in our previous study by multi-image analysis at L above 10 μ m, while the larger D c at smaller L was consisted with the actual surface feature. The validity of RSE method and the physics nature of real surfaces were discussed, which might be helpful to obtain more understandings of fractal geometry.

Superconducting fault current limiters (SFCLs) are attracting increasing attention due to their potential for use in modern smart grids or micro grids. Thanks to the unique non-linear properties of high-temperature-superconducting (HTS) tapes, an SFCL is invisible to the grid with faster response compared to traditional fault current limiters. The quench recovery characteristic of an HTS tape is fundamental for the design of an SFCL. In this work, the quench recovery time of an HTS tape was measured for fault currents of different magnitudes and durations. A global heat transfer model was developed to describe the quench recovery characteristic and compared with experiments to validate its effectiveness. Based on the model, the influence of tape properties on the quench recovery time was discussed, and a safe margin for the impact energy was proposed.

Pitch regulation plays a significant role in improving power performance and achieving output control in wind turbines. The present study focuses on a novel, pitch-regulated vertical axis wind turbine (VAWT) with inclined pitch axes. The effect of two pitch parameters (the fold angle and the incline angle) on the instantaneous aerodynamic forces and overall performance of a straight-bladed VAWT under a tip-speed ratio of 4 is investigated using an actuator line model, achieved in ANSYS Fluent software and validated by previous experimental results. The results demonstrate that the fold angle has an apparent influence on the angles of attack and forces of the blades, as well as the power output of the wind turbine. It is helpful to further study the dynamic pitch regulation and adaptable passive pitch regulation of VAWTs. Incline angles away from 90&deg, lead to the asymmetric distribution of aerodynamic forces along the blade span, which results in an expected reduction of loads on the main shaft and the tower of VAWTs.

An experimental study was performed in a uniform incoming flow wind tunnel to investigate the effect of surge and sway motions on the wake characteristics of a wind turbine model. The platform surge and sway motions can be simulated at prescribed amplitudes and frequencies through a translation stage mounted on the base of the turbine model. The flow fields of near, intermediate, and far wakes of the turbine model were characterized by a particle image velocimetry system. Results show that the mean velocity profiles during the whole surging and swaying process of the downwind wakes were almost unchanged with that of a base-fixed turbine; however, the maximum mean velocity differences appeared near the blade tips for both motions compared with the base-fixed cases. Little difference was found between the mean velocity profiles of the turbine model while moving in different directions, for both the surge and sway motions. Later, the characteristics of instantaneous velocity profiles were investigated, and the lateral offset of the instantaneous velocity profiles for the swaying turbine at the two ends of the sway motion was observed. The offset distance was very close to sway amplitudes within the near wake, and it gradually decreased until it disappeared as the downstream distance increases. This work may provide an instrumental guide in the further research on the wake of floating offshore wind turbines.

Surface roughness is widely used in the research of topography, and the scaling characteristics of roughness have been noticed in many fields. To rapidly obtain the relationship between root-mean-squared roughness (Rq) and measurement scale (L) could be helpful to achieve more understandings of the surface property, particularly the Rq-L curve could be fitted to calculate the fractal dimension (D). In this study, the robustness of Rq against low number of picture elements was investigated. Artificial surfaces and the surfaces of two actual samples (a silver thin film and a milled workpiece) were selected. When the number of picture elements was lowered, Rq was found to be stable within a large portion of the concerned scope. Such a robustness property could validate the feasibility of Rq-L curve obtained by segmenting a single morphological picture with roughness scaling extraction (RSE) method, which was proposed in our previous study. Since the traditional roughness (TR) method to obtain Rq-L curves was based on multiple pictures, which used a fixed number of picture elements at various L, RSE method could be significantly more rapid than TR method. Moreover, a direct comparison was carried out between RSE method and TR method in calculating the Rq-L curve and D, and the credibility and accuracy of RSE method with flatten order 1 and 2 was verified.

The wake of upstream wind turbine is known to affect the operation of downstream turbines and the overall efficiency of the wind farm. Wind tunnel experiments provide relevant information for understanding and modeling the wake and its dependency on the turbine operating conditions. There are always two main driving modes to operate turbines in a wake experiment: (1) the turbine rotor is driven and controlled by a motor, defined active driving mode; (2) the rotor is driven by the incoming wind and subject to a drag torque, defined passive driving mode. The effect of the varying driving mode on the turbine wake is explored in this study. The mean wake velocities, turbulence intensities, skewness and kurtosis of the velocity time-series estimated from hot-wire anemometry data, were obtained at various downstream locations, in a uniform incoming flow wind tunnel and in an atmospheric boundary layer wind tunnel. The results show that there is not a significant difference in the mean wake velocity between these two driving modes. An acceptable agreement is observed in the comparison of wake turbulence intensity and higher-order statistics in the two wind tunnels.

In this study, we demonstrated simultaneously enhanced mechanical properties and electrical conductivity of pure aluminum (Al) by incorporating graphene as the reinforcement. The graphene/Al nanocomposite was fabricated by friction stir processing (FSP) combined with hot extrusion. It was found that graphene was homogeneously dispersed into Al matrix and directly bonded graphene/Al interfaces were formed, which were the structure basis for the simultaneously improved electrical and mechanical properties. This study provides new strategy for fabricating high-strength and highly conductive graphene/Al nanocomposites.

In this study, a series of Hastelloy tapes were electro-polished, and the dividing method was used to carry out a detailed investigation on the influence of polishing current (I) on root mean square (Rq) at various image scales (L). The electro-polishing is found to be effective mainly at L smaller than 10 μm, where the Rq–I relationship could be fitted by an exponential decay function with a residual roughness value. An approximate model of electro-polishing process was established to interpret the exponential decay function. This study provides a quantified insight into the electro-polishing process, which could help to obtain more understanding of its mechanism.

High-performance high-temperature superconductor superconducting magnetic energy storage (HTS-SMES) is a promising technology to play an important role in stabilizing the future power system. However, due to the high prices of HTS materials, a hybrid energy storage system consisting of micro HTS-SMES and Li-ion battery is proposed as a transitional solution. In this paper, a 30-K/4-kJ HTS-SMES magnet is designed and manufactured. The solenoidal magnet spends 500-m 4.55-mm-wide rare-earth-barium-copper-oxide (REBCO) tape and its geometry is optimized to achieve a balance between energy capacity and charging/discharging rate. A flexible and extensible cryogenic system with dual Gifford-McMahon (G-M) cryocoolers is designed for maintaining the operating temperature of the magnet. Solid nitrogen is utilized to enhance the thermal stability during dynamic operation. According to our design, the magnet can deliver/absorb an instant high power with a minimum risk of thermal runaway. A preliminary test of the cryogenic system successfully demonstrated the capability of nitrogen solidification.

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.

A double pancake coil wound by HTS Roebel cable was designed, fabricated, and tested at 77 K. This coil winding had eight turns with the inner diameter of 340 mm and a total conductor length of 10 m. With ten ReBCO strands, the transposition length of the cable was 300 mm. To avoid delamination of ReBCO strands, this coil was impregnated with paraffin wax by a vacuum impregnation method. For protecting ReBCO strands, protective copper plates (PCP) were inserted and soldered between cable surface and external current terminal. The voltage probes were not directly soldered on strands, but on the PCP instead. The critical current and central flux density of the coil were measured at 77 K. With the criterion of 1 μV/cm, the critical current was 1025 A at 77 K. Finally, the additional resistance and the heat loss at 77 and 20 K caused by adding the PCP were estimated.

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.

In the fabrication of a high-temperature superconducting coated conductor, the surface roughness and texture of buffer layers can significantly affect the epitaxially grown superconductor layer. A biaxially textured MgO buffer layer fabricated by ion beam assisted deposition (IBAD) is widely used in the coated conductor manufacture due to its low thickness requirement. In our previous study, a new method called energetic particle self-assisted deposition (EPSAD), which employed only a sputtering deposition apparatus without an ion source, was proposed for fabricating biaxially textured MgO films on non-textured substrates. In this study, our aim was to investigate the deposition mechanism of EPSAD-MgO thin films. The behavior of the surface roughness (evaluated by Rq) was studied using atomic force microscopy (AFM) measurements with three scan scales, while the in-plane and out-of-plane textures were measured using X-ray diffraction (XRD). It was found that the variations of surface roughness and textures along with the increase in the thickness of EPSAD-MgO samples were very similar to those of IBAD-MgO reported in the literature, revealing the similarity of their deposition mechanisms. Moreover, fractal geometry was utilized to conduct the scaling analysis of EPSAD-MgO film's surface. Different scaling behaviors were found in two scale ranges, and the indications of the fractal properties in different scale ranges were discussed.

High magnetic fields play a critical role in the development of modern science and technology, breeding many significant scientific discoveries and boosting the generation of new technologies. In the last few years, China has undertaken a great deal of work on the application of ultra-high-field (UHF) superconducting magnet technology, such as for the Synergetic Extreme Condition User Facility in Beijing, the UHF nuclear magnetic resonance/magnetic resonance imaging, nuclear fusion energy, particle accelerator, and so on. This paper reports the research status of UHF superconducting magnets in China from different perspectives, including design options, technical features, experimental progress, opportunities, and challenges.

Rare-earth-based barium copper oxide (REBCO) coated conductors are promising candidates for the development of ultra-high-field (UHF) magnets, due to its high in-field performance, engineering current density, tensile strength and commercial availability. However, technological challenges pertaining to the large screening currents still remain. The major issues caused by the screening currents in REBCO conductors in UHF applications involve two aspects: the screening current induced magnetic field (SCF), and the screening current induced stress (SCS). In the past decades, extensive research has been devoted to the SCF, offering a variety of possible remedies. With latest advances in the construction and testing of UHF magnets, new observations of the SCF involving REBCO coils were reported. The SCS was identified in recent years and has raised growing concerns. The excessive and highly concentrated Lorentz force, rooted in the high magnetic field and the screening currents, poses threats to the mechanical strength of REBCO coated conductors. The aim of this paper is to review recent research efforts in understanding and tackling the screening current related technological issues. For the SCF, we focus on the latest observations in high-field experiments and its various mitigation methods. For the SCS, we present recent studies including experimental characterizations, numerical modelling and possible countermeasures. It is still an open question to precisely predict the SCS in large-scale HTS magnets. How to minimize the influence of SCF and SCS is one of the key technical challenges for the design of future UHF magnets.