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This article investigates event-triggered and self-triggered control problems for the Markov jump stochastic nonlinear systems subject to denial-of-service (DoS) attacks. When attacks prevent system devices from obtaining valid information over networks, a new switched model with unstable subsystems is constructed to characterize the effect of DoS attacks. On the basis of the switched model, a multiple Lyapunov function method is utilized and a set of sufficient conditions incorporating the event-triggering scheme (ETS) and restriction of DoS attacks are provided to preserve performance. In particular, considering that ETS based on mathematical expectation is difficult to be implemented on a practical platform, a self-triggering scheme (STS) without mathematical expectation is presented. Meanwhile, to avoid the Zeno behavior resulted from general exogenous disturbance, a positive lower bound is fixed in STS in advance. In addition, the exponent parameters are designed in STS to reduce triggering frequency. Based on the STS, the mean-square asymptotical stability and almost sure exponential stability are both discussed when the system is in the absence of exogenous disturbance. Finally, two examples are given to substantiate the effectiveness of the proposed method.

We introduce quantum pin codes: a class of quantum CSS codes. Quantum pin codes are a generalization of quantum color codes and Reed-Muller codes and share a lot of their structure and properties. Pin codes have gauge operators, an unfolding procedure and their stabilizers form so-called $\ell$-orthogonal spaces meaning that the joint overlap between any $\ell$ stabilizer elements is always even. This last feature makes them interesting for devising magic-state distillation protocols, for instance by using puncturing techniques. We study examples of these codes and their properties., 21 pages, 10 figures

Using a fully automated cryogenic wafer prober, we measured superconductor fabrication process control monitors and simple integrated circuits on 200 mm wafers at 4.4 K, including SQIF-based magnetic field sensors, SQUID-based circuits for measuring inductors, Nb/Al-AlOx/Nb Josephson junctions, test structures for measuring critical current of superconducting wires and vias, resistors, etc., to demonstrate the feasibility of using the system for characterizing niobium superconducting devices and integrated circuits on a wafer scale. Data on the wafer-scale distributions of the residual magnetic field, junction tunnel resistance, energy gap, inductance of multiple Nb layers, critical currents of interlayer vias are presented. Comparison with existing models is made. The wafers were fabricated in the SFQ5ee process, the fully planarized process with eight niobium layers and a layer of kinetic inductors, developed for superconductor electronics at MIT Lincoln Laboratory. The cryogenic wafer prober was developed at HPD/ FormFactor, Inc., Comment: 11 pages, 11 figures, 1 table, 27 references

Josephson junctions (JJs) with non-hysteretic I-V characteristic and high critical parameters are required for many applications; in particular, they are needed to create tunable THz range oscillators. We fabricated and studied shunted superconductor-insulator-superconductor (SIS) Nb-AlOx-Nb tunnel junctions with an area of about 1m2, estimated their parameters, and compared them with theoretical models. To assess the performance of shunted junctions in the terahertz range, their characteristics were studied under the influence of a high-frequency signal; the estimated value of the characteristic voltage for the shunted junction at a high frequency is about 1 mV. To improve the circuit parameters, a new type of the shunting based on application of NIS junctions was proposed, which reduces the influence of parasitic inductance, expands the frequency range of the generator, and decreases the size of the structure. The JJs with a new type of shunting have been fabricated and studied. The niobium integrated circuits intended for matching the superconducting oscillator and an SIS junction as a microwave detector were designed, fabricated, and tested. Such structures are required for studying the spectral characteristics and analyzing the operation of the oscillator. The developed circuits have demonstrated efficient operation at frequencies from 300 to 700 GHz; the experimental results are in good agreement with the simulated characteristics. The results obtained will be implemented for further development of the THz range oscillators.

Understanding the properties of grain boundaries in polycrystalline superconductors is essential for optimizing their critical current density. Here, we provide computational simulations of 2D Josephson junctions (JJs) in low magnetic fields using time-dependent Ginzburg-Landau theory, since they can be considered a proxy for a grain boundary between two grains. We present data for junctions with a wide range of superconducting electrodes of different Ginzburg-Landau parameter () values and geometries, as well as normal barriers with different strengths of pair-breaking - characterized by the thickness of the junction and the junction condensation parameter (n) We describe our results using analytic solutions, and hence provide a detailed description of Josephson junctions in low fields up to that required for a single fluxon to penetrate the junction.

Magnetically-levitated superconducting microparticles have been recently proposed as a promising platform for performing quantum experiments with particles in the picogram regime. Here, we demonstrate the superconducting technology to achieve chip-based magnetic levitation of superconducting microparticles. We simulate and fabricate a chip-based magnetic trap capable of levitating superconducting particles with diameters from 0.5$\,\mu$m to 200$\,\mu$m. The trap consists of two stacked silicon chips, each patterned with a planar multi-winding superconducting coil made of niobium. The two coils generate a magnetic field resembling a quadrupole near the trap center, in which we demonstrate trapping of a spherical 50\,$\mu$m diameter SnPb microparticle at temperatures of 4\,K and 40\,mK., Comment: 6 pages, 6 figures, video material available at https://doi.org/10.5281/zenodo.5911190

The complex and ever-changing harmonics in grid voltage will cause performance deterioration of the frequency-locked loops (FLLs). Although various measurements had been adopted to attenuate the deterioration, they had to make the trade-off between the dynamic and steady-state characteristics generally. Consequently, a novel single-phase FLL that can adaptively adjust the dynamic and steady-state performance was proposed in this paper. The proposed FLL regards the damping ratio of second-order generalized integrator-FLL (SOGI-FLL) as a time-varying parameter rather than a constant, and adjusts the damping ratio adaptively, so the proposed FLL was named SOGI-FLL with damping ratio adaptation (DASOGI-FLL). The adaptive mechanism is to decrease the damping ratio as frequency estimation error decreases and vice versa. To realize the mechanism, a novel variable was constructed to indirectly correlate the damping ratio with frequency estimation error. An objective function containing the constructed variable for damping ratio adaptation was designed, and the adaptive algorithm was derived by using the gradient ascent method. Then the convergence analysis, parameter design, and digital implementation of the algorithm were elaborated. Finally, the results of simulation and experiment reveal that, benefits from the damping ratio adaptation, DASOGI-FLL can achieve rapid convergence, strong filtering capability, and high estimation accuracy simultaneously.

The introduction of high temperature superconducting (HTS) magnet, which is capable of working at liquid nitrogen temperature, and the use of null-flux coil, make the liquid-helium-free electrodynamic suspension (EDS) train available. In essence, the electromagnetic force of a superconducting EDS train is generated by electromagnetic interaction between the onboard superconducting magnets and the ground null-flux coils. In order to obtain an efficient HTS EDS train, the design optimization of the HTS magnets and null-flux coils is indispensable. This study is devoted to this aspect. We first establish an improved load-line method for efficiently estimating the critical current of HTS magnets and an improved semianalytical model to accurately calculate the induced currents and electromagnetic forces in the EDS train. Based on above two improvements, the onboard HTS magnets and corresponding null-flux coils are then designed and optimized to enhance the overall performances of the HTS EDS train. At final, according to the optimized results, the HTS magnets and null-flux coils were manufactured and measured, followed by the performance analyses of the obtained HTS EDS train. The results have shown that the electromagnetic performances of the designed HTS EDS train are excellent, demonstrating the effectiveness of the optimization methodology.

Matrix converter (MC) rotating vector (RV) has the natural advantage for common-mode voltage (CMV) minimization. However, the application of RV in direct torque control (DTC) strategy for motor drive system is limited, mainly due to the extensive torque ripple and current distortion. To improve the steady state torque and current performance of the traditional RV-based DTC, a novel RV- based DTC method is proposed in this paper. According to the analysis of the angle offset between stator flux and the candidate voltage vector, the vector plane is divided into 12 sectors, and virtual vector synthesized by two adjacent rotating vectors is employed to fill in the blank sectors. Based on this, a 12 sector- switching table combined with a novel mapping table of rotating vectors are established to fulfill the proposed RV-DTC method. Experiments are carried out to verify the proposed method. The results show that compared with the traditional RV-DTC, steady state torque and current performances are evidently improved with the proposed RV-DTC at approximate switching frequency, while zero CMV is reserved.

This paper proposes a gradient descent-based transfer delay frequency-locked loop (GDTD FLL) to effectively address the challenges of existing transfer delay-based FLLs in terms of unacceptable frequency and phase angle overshoots during variations in the supply voltage amplitude and distorted voltage conditions. In the proposed GDTD FLL, a linear model of the distorted single-phase supply voltage is developed and the error term is constructed. Consequently, the gradient descent algorithm is employed to drive the error term to zero and provide an estimate of the FLL expression from which frequency estimation is achieved. By designing the learning rate employed in the gradient descent algorithm, a degree of freedom is introduced in the proposed GDTD FLL, giving it the ability to achieve improved performance during variations in the supply voltage amplitude. Furthermore, a delay-based harmonic cancellation approach is developed to reject supply voltage harmonics in the FLL’s phase angle estimation. The effectiveness of the proposed GDTD FLL is verified in comparison with other single-phase FLL schemes through experimental studies where it is shown that the proposed GDTD FLL effectively addresses the challenges of existing delay-based FLLs in terms of peak frequency and phase angle overshoots, especially when the supply voltage undergoes amplitude variation or distortion.

The motion of a domain wall in a magnetic film with an antidot array leads to the formation of a vortex magnetization reversal zone moving along the sample. A description of the magnetization dynamics in the film is obtained by micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation for various film thickness and hole arrangement and size. We show that the magnetization reversal zone either passes through the antidot array or "freezes" on it. Using a special technique for visualizing vortex structures, based on the calculation of topological charges of two types, we establish the relationship between the magnetization dynamics and the types of vortex structures.

A mesoscopic multi-terminal superconducting Nb strip with a spin injection terminal has been developed. We investigate the influence of the spin polarization on the relaxation process of the quasiparticle in the superconducting Nb. The position dependence of the nonlocal voltage provides the quasiparticle relaxation length. The relaxation length for the spin-polarized quasiparticle is found to show 12.5 percent increase from that for the un-polarized quasiparticle. This demonstration opens a new avenue for utilization of the the spin current in a superconducting device.

The charging and levitation of particles using UV irradiation in an upward electric field were investigated. Glass beads with a mass median diameter of 61 μm were used for the experiments. The particle layers formed on a glass plate were irradiated by UV light with wavelengths ranging from 240 to 400 nm. The particles in the top layer were positively charged by photoemission and levitated by the electrostatic forces. The fluxes and motions of the levitated particles were analyzed by digital processing of the images obtained with a high-speed camera. The charge of each levitated particle was determined by fitting the results calculated using the equation of motion to the experimentally obtained particle motion. The charge of the particles beginning to be levitated was determined by the force balance and thus was not affected by the UV irradiance. The positive charge of the levitated particles could be increased by continuous photoemission from the particles and decreased by the capture of photoelectrons emitted by other particles. Approximately 40% of the levitated particles descended because of the negative charge clouds formed by the photoemission from the particle layers and the levitated particles. Furthermore, in a second set of experiments, an upward electric field was applied after UV irradiation. In these experiments, the particles in the top layer were levitated after applying the electric field. These particles could be gain more excess charges than the particles for which the levitation was governed by the force balance.

We study the impact of redundant via placement on Fraunhofer interference patterns in superconductor-insulator-superconductor Josephson junctions with thin counter electrodes. By varying the number and location of contact vias to the counter electrode in otherwise equivalent junctions and studying the magnetic field dependence of the critical current, we observe a variety of unconventional features in the interference patterns, including a lifting of critical current minima and differences in node positions and side lobe amplitudes. We attribute these features to local changes in the Josephson phase gradient arising from the non-uniform magnetic thickness resulting from the vias. Our findings highlight the significant role that contact vias on thin counter electrodes play in the response of Josephson junctions to applied magnetic fields.

This letter focuses on the propagation characteristics of the electromagnetic wave in the plasma under high-power microwave (HPM) illumination. A multi-physical method coupling the electromagnetic field and plasma fluid field is proposed as the dynamic Drude model. The differences between the method based on dynamic Drude model and traditional static Drude model are discussed theoretically for the first time. Unlike the static Drude model, the dynamic Drude model fully considers the motion of electrons in plasma driven by electromagnetic field. Numerical examples demonstrate that both the dynamic Drude model-based and static Drude model-based method are consistent with each other under low power microwave illumination, where the plasma can be considered as a static media. While under HPM illumination, electrons gradually gather together to hinder the propagation of electromagnetic waves. Hence, the proposed dynamic Drude model is more logical to analyze the characteristics of electromagnetic wave in plasma than the traditional static Drude model under HPM illumination.

The effects of temperature on the dynamic characteristics of a high-Tc superconductor (HTS) and permanent magnet (PM) system at different field cooled (FC) gaps and different masses are studied theoretically and experimentally. Results indicate that the resonant frequency of the HTS-PM system decreases with increasing FC gap or mass at different temperatures. Although decreasing temperature augments stiffness of the interaction force between the HTS and the PM. The effect of decreasing temperature on stiffness and resonance frequency is more significant at a smaller FC gap. It means that the enhancement of lowering temperature on stiffness and resonant frequency also depends on the magnetic field gradient.

Particle physics and radio-frequency (RF) superconductivity have driven each other on since the 1970s. The unique properties of superconductors (SC) have been the enabling keys for the realization of accelerators with always increased performances thanks to the realization of all-superconducting cavities. The use of increasingly pure superconducting coatings for accelerating cavities, with lower and lower RF losses, determined such high-quality factors [1] that the need to operate at low temperatures (below the superconducting transition temperature $T_{c}$ ) was well paid for. Recently, with respect to the path followed by the high frequency superconductivity [2], a new field opened since SCs are being considered for GHz operation in high dc magnetic fields, and measurements (and optimization) of totally different quantities are needed. The possibility of successfully using SCs in high magnetic fields for these purposes is far from obvious and it depends on the outcome of accurate measurements of usually overlooked quantities.

Superconductors are popular for microwave engineers because they are high performance, high selectivity, and low volume. However, the use of superconductors is limited due to their inherent nonlinearity. Intrinsic nonlinearity in superconductors is due to the dependence of surface current density on the depth of London penetration in them. This leads to adverse effects such as intermodulation distortion (IMD) and the production of a third harmonic in a superconductor. In this article, we intend to improve the effects of nonlinearity by modifying the superconducting geometric structure, which we call the method of geometric reconstruction. To achieve this, we first designed a superconducting transmission line (TL), and then created slots on the superconducting TL to improve the IMD and the third harmonic. Using this method, we managed to reduce the level of the third harmonic from –6 to –31 dBm. To verify the results, after simulating the slotted superconducting TL in ADS software, simulation and solving of London equations using three-dimensional-FEM in COMSOL software has been done again.

At present, superconducting power technology has been applied more and more in the ac field, such as ac superconducting current limiter, ac superconducting cable, and so on. Critical current is the key parameter in the design, manufacture, and operation process of superconducting devices, but few studies were carried out on the ac critical current of superconductors now. In this article, a method for measuring the ac critic al current of the HTS coil based on the thermal stability was proposed, the ac critical current measurement experiment of the superconducting coil is carried out, and the electromagnetic–thermal coupling model is built to analyze the quench principle of the superconducting coil. This article explains the principle of ac critical current from the balance of heating and cooling and gives the rule that the value of ac critical current changes with frequency.

With regard to the state-of-the-art technologies in the fields of applied superconductivity and electromagnetic devices, research and development highlights are presented. The recent progress and achievement described with principle and technical details include mainly i) applied superconducting materials; ii) superconducting magnets and their applications such as in ITER and Tokamaks; iii) high T c superconducting (HTS) magnetic levitation and applications; iv) HTS smart grids; v) superconducting and electromagnetic material modelling and characterization; and vi) advanced electromagnetic devices. The applied superconductivity technology and availability are especially focused and verified with the trend of development prospection.

A prototype of the high temperature superconducting (HTS) Maglev system for the electromagnetic launch was built in our laboratory. To meet the higher carrying capacity requirement of this practical application, multi-pole permanent-magnet guideways (PMGs) with different flux concentration methods and various arrangement schemes of HTS bulks have been proposed. The magnetic field distribution of these multi-pole PMG with the iron and permanent magnet (PM) concentration schemes has been estimated by employing both analytical and finite element methods. Meanwhile, a two-dimensional simulation model was established to evaluate the levitation and lateral performance of the Maglev system with several arrangement cases of HTS bulks. It has been validated that the total levitation force of the system can be significantly improved by increasing the pole number of the PMG and applying the optimal arrangement scheme.

The High-temperature Superconducting (HTS) magnet is one of the most promising technologies to accommodate various high field applications. With the high magnetic field, the quench problem of HTS material should be considered with extra care, and the specific quench protection scheme should be projected. In this article, the modeling of the quench protection of an HTS magnet for the MRI system is presented. The modeling strategy was composed of two stages: Stage 1 the HTS magnet was in the normal operation circuit, and Stage 2 the HTS magnet was switched to the heat dissipation circuit. The 1st stage was based on the finite element method (FEM) using the H -formulation together with the thermal-coupled model, which simulated the current distribution of typical HTS tapes and showed the process of temperature increase when the hot source occurred. The 2nd stage was based on the analytical model to investigate the reasonable damping resistance and the hot spot temperature of the HTS magnet.

Superconducting Conductor on Round Core (CORC) cables fabricated by several high temperature superconducting (HTS) tapes are able to carry enormous electrical current density, to be widely used in a variety of superconducting applications such as power transmission, fusion, and magnetic resonance imaging (MRI). However, the AC losses induced in the superconducting tapes under alternating current or magnetic field generate heat dissipation which can reduce the electric power efficiency. Therefore, it is essential to analyze the AC loss behavior of CORC cables as well as consider methods to reduce it. This paper presents an AC loss study of CORC cables with several different superconducting layer thicknesses. The CORC cable simulation model is built in a software suite named COMSOL Multiphysics utilising the finite-element method (FEM) solved by the three-dimensional (3-D) H -formulation. By implementing FEM simulations, the cases of increasing the degree of freedom of superconducting layers have been considered, and the AC losses of a single layer CORC cable model with three tapes mounted around the core have been calculated and analyzed for different layer thicknesses. Simulation results are verified with experimental results measured in previous literature.

The levitation force in the high temperature superconducting (HTS) maglev system has obvious nonlinear characteristics. When the excitation frequency and amplitude meet certain conditions, nonlinear vibration phenomena such as period-doubling bifurcation would appear. Nonlinear vibration will bring difficulties to dynamic analysis and even affect the safety and comfort of driving. While an electromagnetic shunt damper (EMSD) can effectively reduce the vibration amplitude. This paper analyzes the influence of EMSD on the nonlinear vibration of HTS maglev systems from the perspective of experiments and simulations. Firstly, free vibration and forced vibrations in the resonance interval are tested. Secondly, based on numerical simulations, the vibration response under various excitation was analyzed. The results show that the EMSD can effectively suppress its nonlinear vibration. This research has reference value for the detailed understanding of the dynamic characteristics of the HTS maglev and further vehicle design.

Heat generation and serious energy consumption of normal-conductive suspension electromagnets influence the performance and stability of suspension systems. With the development of second generation (2G) high temperature superconducting (HTS) technologies, HTS suspension electromagnets are being considered for energy saving purpose. In this paper, a suspension system with three suspension electromagnets is designed, which is comprised of two normal-conductive suspension electromagnets and a 2G-HTS suspension electromagnet. A three-dimensional (3D) electromagnetic field simulation based on finite element method (FEM) is carried out, and the force test platform of suspension electromagnet is built for performance evaluation of the HTS suspension electromagnet. The experimental results show that the current of HTS suspension electromagnet is steady loaded from 0 A to 70 A under 77 K working conditions, with the suspension force meet the requirements. This work provides a certain theoretical and practical foundation for the future superconducting suspension projects.

High temperature superconductors have zero resistance under critical temperature and the HTS tapes have low thermal conductivity. The China Fusion Engineering Test Reactor(CFETR) project requires current leads with current carrying capacity up to 100 kA, making high temperature superconducting current leads the ideal choice. The main work of this paper is the theoretical analysis and optimization design of the 100 kA HTS current lead. Current leads are designed to minimize heat leakage to the cryogenic system and to reduce the mass flow of cooling gas. In this paper, some important design parameters are introduced firstly, and then one dimensional thermal analysis is carried out for the heat exchanger and high temperature superconductor parts. The LOFA time and overheating time of the current lead were calculated through simulation, and it was verified that the design meets the requirements of the project. Finally, the factors that have great influence on the performance of current leads are analyzed. In view of the four influencing factors, which are joint resistance, GHe inlet temperature, hPw and RRR, we respectively produced an optimized design.

In superconducting system, current lead connecting cryogenic device and normal temperature copper cable is the main leakage heat source of entire system. Its performance directly affects cooling cost of cryogenic system. Multi-current leads have advantages of eliminating Joule heat and reducing heat conduction leakage. As a commonly superconducting material, MgB2(magnesium diboride) has broad application prospects. In this paper, the current lead of 200 A MgB2 cable and normal temperature copper cable are designed and calculated. The heat transfer power of superconducting section and the conventional current lead section are 17.90 W and 18 W, respectively. The numerical simulation method is used to verify and analyze calculation results. The temperature at interface between normal temperature copper wire and current lead is lower than 70 K.

In this paper, the low reluctance superconducting direct drive linear generator (DDLG) for harvesting oceanic wave is presented. At first, a permanent magnet DDLG is designed in the ANSYS/Maxwell environment. The DDLG with both the copper and superconducting winding are characterized with finite element analysis. The DDLG is analyzed with the conventional and recently developed M5 Carlite magnetic cores. High graded neodymium iron boron permanent magnet, N46SH is selected for the DDLG to create strong magnetic field. It is analyzed that the proposed superconducting DDLG with M5 Carlite magnetic core and N46SH permanent magnet produces 294 kW more electrical power compared to that of the DDLG with copper conductor. It is also found that the stator size of the proposed DDLG is much smaller than the conventional one for designing the winding with superconducting tape.

This article investigates an observer-based optimal direct torque control (DTC) strategy for improving the transient and steady-state performance of surface-mounted permanent magnet synchronous motor (SPMSM) drives. Unlike conventional DTC techniques, the proposed method simultaneously controls speed, torque, and flux variables by employing the optimal control theory in the stationary reference frame. This helps achieving a simple structure along with good low-speed operation as the stator flux angle need not be estimated. Further, the feedforward control terms are designed to compensate for system uncertainties. Additionally, an in-depth stability analysis is presented through the Lyapunov theory. Comparative results via a real-time prototype SPMSM test-bed with TI-TMS320F28335 DSP indicate improved performance of the proposed optimal DTC compared to conventional PI DTC, nonlinear optimal DTC, and adaptive DTC, including less computational burden, less torque and flux ripples, more robustness to parameter uncertainty, and improved transient response under very low speed/load step-changes and speed reversal.

The stress and strain analysis is indispensable for the design and development of high field superconducting magnet. In this paper, a stress analysis with elastic and plastic theory for high magnetic field split superconducting magnet was introduced. By the stress analysis theory, the stress of 13 T split superconducting magnet for neutron scattering was simulated and discussed. Based on the analysis results, the structure of coils was modified by three ways including reduce the maximum magnetic field, applied pre-stress and increased thermal stress. After modified, the peak stress is 165 MPa appeared on the third Nb3Sn coils when the magnet is charged to operating current of 114 A. The peak value of radial stress on superconducting coils is 11.4 MPa appeared on the inner Nb3Sn coils reduced from 21.3 MPa. The stress along axis on the coils is less than 80.7 MPa reduced from 90.6 MPa. The hoop stress on the superconducting coils is 154 MPa which effected on the inner side of the third Nb3Sn coils reduced from 171 MPa. The developing risk of 13 T split superconducting magnet for neutron scattering is reduced significantly.

Practical superconducting materials and technologies have been enabled with availability for large scale devices and applications. Various large scale and strong magnetic field applications of superconductors have been developed, and a series of those applications are explored with technical details mainly including i) applied superconducting materials and their characteristics; ii) superconducting magnets and their techniques; iii) large and advanced electromagnetic devices and applications. The applied superconductivity, application technology, and availability are especially focused and verified with the trend of large scale applications’ development prospection.

MgB2 superconducting materials have important application potential in the 20K temperature region, which is a significant advantage compared with other low-temperature superconductors (such as Nb-Ti and Nb3Sn). However, there is a big gap between its superconducting current-carrying capacity and Ni-Ti and Nb3Sn, which is due to the weak magnetic flux pinning force and the poor connection between the MgB2 grains. Chemical doping is a simple and effective method to improve the performance of MgB2. This paper studies the effect of NaCl doping on the performance of MgB2. It is found that NaCl doping has a positive effect on improving the interconnection of MgB2 grains and refining MgB2 grains, and is a potentially effective method to improve the superconducting properties of MgB2. In addition, all of the samples in this study showed surface pinning characteristics, but the smaller the MgB2 grain size, the better the match with the surface pinning characteristics described by Kramer's theory.

In recent years, along with the unceasing development of High-temperature Superconductor (HTS) technology, the performance of HTS tape has been improved significantly, including the higher critical current, lower joint resistance, and longer tape length etc. To use HTS instead of Low-temperature Superconductor (LTS) in Magnetic Resonance Imaging (MRI) applications, the flux pumping method with travelling wave has been investigated. In this paper, our new experimental results were regarding to the frequency, amplitude, and wavelength characteristics of charging an HTS coil with an HTS square bridge, using an electromagnetic flux pump designed at the University of Cambridge.

An HTS flux pump is an effective device to compensate the field decay in HTS magnets running in persistent mode. This paper contains updates to the model for the travelling wave HTS electromagnetic flux pump, in which the magnetization system is modelled as an L-R circuit. In this paper we consider the various sources of resistance. There are three components, joint resistance, resistance of superconductor and dynamic resistance. Joint resistance is treated as a constant. The superconducting resistance is derived from the E-J model and finally the dynamic resistance is a function of applied field strength and frequency. In addition the model includes the electromotive force which is a function of the applied field, frequency and surface area of the tape. This is a calculated value based on the φ model we built. In order to test the model, a series of experiments were performed in which an HTS coil was actuated using our flux pump. The flux pump test rig was made from 4-pair rectangular copper solenoids with laminated iron cores, which were fixed on the supporting iron frame. This rig was used to magnetize a double layer HTS racetrack coil at 77 K. The induced current in superconducting coil can reach more than 90% of critical current. Our model was found to be a good fit to the experimental data.

Etching the grating on the magnetic film surface is able to increase the film effective longitudinal permeability ( $\mu _{\textit {eL}}$ ) significantly, which has been demonstrated by a magnetic sensor consisting of a solenoid and grooved grating patterned film. The feasibility that the magnetic polarization field generated by the groove end faces compensates for the shape demagnetization field and intensifies $\mu _{\textit {eL}}$ is verified theoretically. After the optimal structural parameters are obtained, the planar and patterned films are manufactured with the microfabrication process. The effects of groove width, sensor dimensions, and excitation frequency on the sensor performance are systematically investigated. For the prototype with the patterned magnetic film of $10~\mu \text{m}$ groove width, the impedance ratio and impedance sensitivity are 4049% and 347 %/Oe at 1 MHz. Compared with the prototype with the same thickness planar smooth magnetic film core, they are improved by 436% and 659%, respectively. It is revealed that the grooved grating patterned films offer the possibility of developing film-based microsensors, making their performance comparable to that of wire-based sensors.

Levitation control is a core technique of magnetic levitation (Maglev) trains, whose performance has a significant impact on the driving safety and ride comfort during the operation of Maglev trains. With the rapid development of high temperature superconducting (HTS) materials, a novel hybrid levitation system for Maglev trains, composed of HTS and normal conducting electromagnets, is becoming a more promising alternative to the conventional levitation system, due to a lager levitation air gap and lower energy consumption. The controller of the hybrid levitation system is hard to design due to strong nonlinearity, open-loop instability and fast-response requirements. In this study, we propose a robust model predictive control (MPC) strategy for the novel hybrid levitation system in which state constraints are taken into account, including the air gap, and the velocity and acceleration of the hybrid magnet. In the MPC strategy, the prediction model employs a stabilized model through nonlinear and linear state feedback. With regard to the online computational load, the primal-dual interior-point (PDI) algorithm is adopted to solve the optimization problem with high efficiency to deal with constraints in a real-time manner. Finally, simulation results are demonstrated to verify the effectiveness of the proposed MPC-PDI control strategy for the hybrid levitation system, and the significances considering state constraints are revealed. This study provides an engineering realization solution to the novel hybrid levitation system of a Maglev train.

On-chip filters play an invaluable role in most detectors and instruments, blocking unwanted, harmonic, or mirrored signals. In this article, two coplanar waveguide bandpass filters working at 350 GHz are designed based on quarter-wavelength stepped impedance resonators, which can be easily integrated in on-chip systems. A second-order filter with a classical Chebyshev response and a fourth-order filter with quasi-elliptical characteristic is developed together to define the roll-off performance. Both filters are patterned based on superconducting niobium film by the optical lithography and lift-off process, which could reduce the significant metallic loss in high terahertz band. All the chips integrated with transitions are evaluated through a quasi-optical system, which is built based on the sensitive superconductor–insulator–superconductor tunnel junction detector at cryogenic temperature. The calibrated transmission responses, which are matched with simulations, have indicated that such superconducting coplanar waveguide filters can be enabled in future terahertz astronomical spectrometer.

Superconducting (SC) direct-drive wind generators are one of the best candidates for the offshore wind energy applications. The gearbox that exists in the traditional wind generators can be removed. Using superconductors in the armature and excitation windings will largely increase the machine torque density. However, the synchronous reactance in a traditional superconducting synchronous machine is usually low due to its large air gap. Consequently, operating faults will largely threat the machine itself. Also, the superconductor needs to work in the low temperature, usually around 30 K. The magnetic field around it and the current flows in it are also limited. Thus, to improve the machine reliability, the superconducting system needs to be taken care. In this paper, a double stator superconducting flux modulation machine (DSSFMM) is studied and its fault performances are analyzed. From the results, both fault current and fault torque in this kind of machine are low. It is also indicated that the AC loss of the SC coils during fault condition is low.

Terahertz (THz) electromagnetic (EM) wave emitters have been expected for applications because these waves are suitable for non-destructive inspection, telecommunication technologies, and other uses. Strong and coherent THz EM waves are known to radiate from large-sized intrinsic Josephson junction (IJJ) stacks in which the self-heating effect is considerably larger and the temperatures in the mesa are non-uniformly distributed. In this study, we numerically investigate and discuss the temperature and current distributions in large-sized IJJ stacks because these parameters are difficult to analyze through experimental investigation. The temperature and current distributions can be obtained by self-consistently solving the non-linear diffusion equation in an equivalent circuit of the mesa by considering the temperature dependence of several parameters. As shown in the numerical results, self-heating caused the local temperature of the center in the mesa to exceed the critical temperature ( $T_{\rm c}$ ). To improve the non-uniform temperature and current distributions, we propose an improvement in which an external heat generation system is placed on the edges of the mesa. By applying this measure, the temperature distribution is improved because the edges of the IJJ mesa are heated externally. Furthermore, the local temperatures throughout the mesa were held below $T_{\rm c}$ , and the current $-$ voltage characteristics were improved using the proposed improvement.

This paper proposes and analyzes a novel radial partitioned stator HTS-excitation flux-switching (RPS-HTSFS) machine. The key is that the single ring-shaped HTS-excitation coil is independently placed in the coaxial claw-pole outer stator while the armature windings are wound around the U-shaped iron core on the inner flux-switching armature stator without permanent magnets. This solution has been explored to realize the physical isolation of armature winding and excitation winding, relieving the influence of armature field on the HTS-excitation coil. Moreover, only a single ring-shaped HTS-excitation coil is employed, therefore, reducing the HTS wires and the bending strain of the HTS-excitation coil. Also, the special refrigeration system can realize stationary seals of the double-layer ring-shaped cooling Dewar pattern, improving the cooling performance. By using the three-dimensional finite element analysis (3D-FEA), the machine performances are investigated and the validity of the proposed machine is verified.

Application method of static-field magnetization to magnetically levitated mover system with high critical temperature ( T c) superconductors is discussed. The system is composed of a levitated mover, magnetic rails, a linear induction motor (LIM) and some power supplies. In the paper, static-field magnetization is applied to superconductors in the levitated system from bottom superconductor surfaces. Then, magnetizations in superconductors are tried to control using static-field on superconductor surfaces. That is, magnetic flux density distributions are successfully forced to change to initial flux density distributions.

We present our studies on the evolution of the normal and superconducting properties with thickness of thin Nb films with a low level of non-magnetic disorder ( $k_Fl\approx 150$ for the thickest film in the set). The analysis of the superconducting behavior points to the presence of magnetic moments, hidden in the native oxide on the surface of Nb films. Using the Abrikosov-Gorkov theory, we obtain the density of surface magnetic moments of $10^{13}$ cm $^{-2}$ , which is in agreement with the previously reported data for Nb films.

We report on the magnetic levitation of a millimeter-sized neodymium permanent magnet within the interior of a superconducting radio frequency (SRF) cavity. To the best of our knowledge, this is the first experimental work on levitating a magnet within an SRF cavity. The cavity is a coaxial quarter-wave microwave resonator made from 6061 aluminum, having a resonance frequency of 10 GHz and a loaded Q of 1400. The cylindrical magnet (N50) has a height of 1 mm, a diameter of 0.75 mm, a mass of 4 mg, and a remanence of 1.44 T. This produces a peak magnetic field 140 times greater than the critical field of aluminum. The magnet is placed either on the top of the coaxial portion of the cavity or on the cavity floor before cooling it below the superconducting transition temperature of aluminum. The coaxial mode's resonance frequency shifts as a function of the levitation height of the magnet and gives an idea of the magnet's position and mechanical motion. We observe a transition at a temperature of 650 mK where the Meissner effect levitates the magnet as the material beneath the magnet becomes superconducting. The magnet is levitated to a height of 2.5 mm above the surface of the cavity stub, which is a sufficient separation for the field strength of the magnet at the surface of the stub to be less than the critical field strength of the superconducting aluminum. We measure a 120 MHz upshift in the cavity resonance as the magnet is levitated from the top of the stub and 15 MHz downshift as it levitates from the floor of the cavity. Our measurements are consistent over several heating and cooling cycles. Our work provides a path towards a novel optomechanical system.

We report the fabrication and measurement of an ab -plane Josephson junction created in an 80-nm thick thin-film-like exfoliated Bi $_{2}$ Sr $_{2}$ CaCu $_{2}$ O $_{8+x}$ single crystal flake. The junctions are formed using ion irradiation from a focused helium ion beam which locally converts the material from a superconductor to an insulator. Current-voltage characteristics show the device to be a weak link with I $_\text{C}$ R $_\text{N}$ product of 40 $\mu$ A. The critical current exhibits a well-behaved Fraunhofer pattern with magnetic field applied perpendicular to the film confirming the DC Josephson effect. Simulations reveal a large amount of ion straggle deep into the film which is likely influencing the electrical properties. Combination of these devices with the intrinsic junctions could provide a method for multi-dimensional Josephson devices.

This paper describes recent developments for increasing the output voltage of the Josephson Arbitrary Waveform Synthesizer (JAWS) at PTB. For this purpose, we developed modified broadband Wilkinson power dividers, which enable the operation of two or four Josephson junction series arrays per RF channel of the pulse pattern generator and thus the generation of increased output voltages. In detail, we designed and investigated two different Wilkinson power dividers: the one-stage three-section Wilkinson power divider with extended bandwidth and the two-stage single-section Wilkinson divider with more outputs to combine four parallel series arrays. They were both integrated with triple-stacked Josephson junction series arrays. Additionally, a new modulator pulse amplifier was introduced into the JAWS set-up to provide sufficient power for all the junction arrays. Our measurement results show that spectrally pure sinusoidal waveforms were successfully generated with both types of power dividers. With the one-stage three-section Wilkinson power divider, we obtained RMS voltages of nearly 53 mV at a clock-frequency of 15 GHz combined with a test array of 3000 Josephson junctions. As for the two-stage single-section Wilkinson power divider combined with a test array of 6000 Josephson junctions, we synthesized RMS output voltages of about 105 mV. These test results show a promising prospect for quantum voltage standard applications.

A high temperature superconducting (HTS) Bitter magnet consisting of REBCO annular plates with slits is proposed. The REBCO annular plates are soldered to each other at the edge of the slits. The resistance of the soldering joint, which can help control current distribution, is experimentally proven to be small enough to avoid producing excessive loss. Furthermore, a fast-computational method based on a convolution neural network (CNN) is used to calculate the magnetic field and stress distribution of the proposed magnet. With the help of this algorithm, a specific structure of a 25 T HTS Bitter magnet is designed and distribution of the magnetic field and stress are numerically computed. Finally, the feasibility of the structure of the magnet and the efficiency of the algorithm are experimentally verified on a model magnet.

The current distribution of high temperature superconducting (HTS) Bitter magnets soldered by REBCO annular plates can be controlled by adjusting the area of the soldering joint and the number of grooves on the plates. In this paper, the principle of controlling the current distribution of HTS Bitter magnets is analyzed. The magnetic field distribution and current saturation of HTS Bitter magnets under three different current distributions are analyzed. Finally, an approach of measuring the current distribution of the HTS Bitter magnet based on integral equation is proposed through which the current distribution of the model magnet is measured. The results show that the current control method of HTS Bitter magnet is effective. Besides, it is proved that the central magnetic field of the HTS Bitter magnet can be increased while decreasing the maximum current saturation of the magnet by a reasonable current distribution controlled by the suggested method.

We have been studying no-insulation (NI) high temperature superconducting (HTS) coils that do not use turn-to-turn electrical insulation. The no-insulation winding technique can provide high stability of the HTS coils against the quenching, but it cannot prevent mechanical deterioration of the HTS coil due to electromagnetic force and multiple cooling. To solve these problems, we propose the installation of metal protection ring on the outermost turn of NI HTS coil to improve thermal, mechanical, and electrical stability. In this study, the current bypass characteristics in the transverse direction within an NI test coil wound with 37 turns of REBCO wire with/without copper protection ring were experimentally investigated for various amounts of heating by a heater and various heater positions. Eight repeated cooling experiments with liquid nitrogen proved that the proposed copper protection ring was effective against shape deformation of the NI test coil and deterioration of contact resistance between turns due to thermal stress.

This paper proposes a novel high-temperature superconducting stator-excited axial-flux magnetic gear (HTS-SEAFMG) for the high-torque transmitting system. The proposed magnetic gear combines the merits of both HTS-excitation and stator-excitation. By using the HTS-excitation, a high air-gap flux density can be easily obtained, thus achieving the high torque density. The HTS-excitation windings are placed on the stator with a static seal of cooling Dewar which greatly reduces the manufacturing difficulty and the cost of refrigeration system compared with the dynamic seal. Thanks to the stator-excitation pattern, both the high-speed rotor and the low-speed rotor comprise only iron, thus contributing to the high mechanical reliability. The axially sandwich structure can also simplify its assembly in the high torque transmission applications. The dimensional parameter optimization and the electromagnetic performance evaluation are conducted by using the finite element method, respectively. The results show that the torque density of the proposed HTS-SEAFMG can reach 123 kNm/m3. Theoretical analysis and results are both given to verify the feasibility of the proposed HTS magnetic gear design.

Superconducting coils may suffer from electromagnetic vibration due to changing electromagnetic fields under AC conditions. In the long term, this vibration may cause performance degradation of superconducting tapes. In order to study the influence of electromagnetic vibration between superconducting tapes on their performance, the study can be started from the simplest form of electromagnetic vibration, that is, the electromagnetic vibration between two superconducting tapes. The degree of electromagnetic vibration under different AC currents were analyzed by simulation, and the variation of critical current ( I c) and the n-value of Y-Ba-Cu-O (YBCO) tapes after undergoing long-term electromagnetic vibration through experiments were measured. The results show that two YBCO tapes does not show obvious performance degradation under tens of hours the electromagnetic vibration caused by the AC current amplitude of 1.2 to 1.4 times Ic .