Heat transfer coefficient estimation for high-temperature superconducting coils under light over-current conditions

This paper reports the feasibility of the impregnation of no-insulation (NI) high-temperature superconducting (HTS) coils using solder. NI coils are widely studied and used in many situations. To utilize an HTS coil under high mechanical loads such as field coils for rotating machines, the HTS tapes must be stabilized mechanically. Epoxy impregnation is a common method to protect the HTS field from mechanical disturbances caused by the magnetic field and rotational vibration of the rotor to enhance the mechanical stability. However, traditional epoxy resins are electrical insulating materials which might cause the turn-to-turn contact resistance of NI coils to increase. Therefore, we proposed a new kind of impregnation method using the electrically conductive material, solder. Two coils were fabricated and tested for comparison. One is the NI coil without impregnation and the other is the NI coil impregnated with solder. The charge–discharge and sudden-discharge tests were performed in liquid nitrogen at 77 K. The experimental results were studied with equivalent circuit analyses. The results of this paper could provide useful data for the application of solder impregnated NI HTS coils.

In the case of motors and generators, the benefits of using high-temperature superconducting (HTS) coils could be represented by the reduction of 50% in both losses and sizes compared to the conventional machines of the same rating. However, it is hard to establish the quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. Therefore, the stability of the coils is lower than the quiescent coils applied to NMR, MRI, and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have been developing a novel self-protection method, which removes the insulation turn-turn winding and inserts various metal tapes instead of the insulation. In our previous research, the higher self-protection ability for the thermal disturbances in the HTS coils removed the insulation and inserted Cu tape has been proved. This paper presents the results of an experimental study for the quench current using the insulated, the noninsulated, and the Brass tape inserted HTS coils in liquid nitrogen and background magnetic fields. Also, the measured quench currents of each coil will be discussed.

A quench protection system for a high-temperature superconducting (HTS) coil using Cu tape co-wound with an HTS tape was studied by numerical analysis using a thermal model of a coil winding pack. In normal operation, the voltage across the resistive zone Vs in the HTS coil was monitored by measuring the voltage difference between the HTS coil and the co-wound Cu coil. When Vs exceeded the quench detection voltage, the HTS coil was disconnected from the power supply and the energy stored in the HTS coil was dumped into a dump resistor. At the same time, the terminals of the co-wound coil were connected to another resistor, and some of the current in the HTS coil was quickly transferred to the co-wound Cu tape coil due to the tight magnetic coupling of both coils. This is expected to decrease the hot-spot temperature due to the quick decrease of HTS coil current and to protect the coil from damage caused by overheating at the hot-spot. The analysis investigated the effectiveness of this method.

This paper proposes a millisecond-scale electromagnetic levitator using high-temperature superconducting (HTS) coils. The proposed apparatus consists of an AC power supply, short-circuit switches, an aluminum plate, and two HTS coils. The HTS coils are wound in opposite directions and electrically connected in parallel. Thus, in a symmetric current distribution, the magnetic fluxes generated by the two HTS coils ideally cancel each other. However, in a sudden asymmetric current distribution created by the short-circuit switches, the magnetic fluxes are not cancelled, and the effective magnetic flux induces an eddy current in the aluminum plate placed above the HTS coils. The magnetic flux generated by the eddy current in the aluminum plate and the effective magnetic flux generated by the HTS coils together generate the repulsive force to levitate the aluminum plate. In this paper, numerical calculations and experimental verification of the repulsive force are performed. We show that this apparatus is able to provide a levitation force that we expect will be helpful for devices that need fast switching behavior of the levitation force.

Comparison of heat transfer in liquid and slush nitrogen by numerical simulation of cooling rates for French straws used for sperm cryopreservation

To investigate the electromagnetic characterization of a high temperature superconducting (HTS) coil when subjected to travelling magnetic fields, a no-insulation (NI) racetrack coil was wound with YBCO coated tapes. For comparison, another HTS coil which has the same geometric and structural parameter was wound with insulated tape between the layers. The charge-discharge performance and other typical characteristics were investigated first. The terminal voltage fluctuation of the insulation (INS) coil during the critical current test proved that the NI coil has a better electrical stability. In addition, a flat stator which has traditional copper windings and a controllable three-phase inverter were introduced to provide travelling magnetic field. The HTS coils were injected with DC current and exposed to the external field generated from the stator, respectively. Different terminal voltage and current responses of the coils were measured by experiments. The INS coil burned out when the AC current in the stator rose up to 20 A @30 Hz but the NI coil kept stable during and after all the experiments even with excessive current in the coil. The terminal voltage and current results indicated that the NI coil, which is potentially going to perform as the field magnet of a HTS linear synchronous motor (LSM), has better electromagnetic stability when subjected to the external field from the stator and thus it shows better application prospects in the LSM.

Analysis and experimental study of wireless power transfer with HTS coil and copper coil as the intermediate resonators system

The NASA Glenn Research Center (GRC) has been developing the high efficiency and high-power density superconducting (SC) electric machines in full support of electrified aircraft propulsion (EAP) systems for a future electric aircraft. A SC coil test rig has been designed and built to perform static and AC measurements on BSCCO, (RE)BCO, and YBCO high temperature superconducting (HTS) wire and coils at liquid nitrogen (LN2) temperature. In this paper, DC measurements on five SC coil configurations of various geometry in zero external magnetic field are measured to develop good measurement technique and to determine the critical current (Ic) and the sharpness (n value) of the super-to-normal transition. Also, standard procedures for coil design, fabrication, coil mounting, micro-volt measurement, cryogenic testing, current control, and data acquisition technique were established. Experimentally measured critical currents are compared with theoretical predicted values based on an electric-field criterion (Ec). Data here are essential to quantify the SC electric machine operation limits where the SC begins to exhibit non-zero resistance. All test data will be utilized to assess the feasibility of using HTS coils for the fully superconducting AC electric machine development for an aircraft electric propulsion system.

Development of the Current Bypassing Methods into the Transverse Direction in Non-insulation HTS Coils

Flux pumps provide a promising solution for contactless charging of high-temperature superconducting (HTS) coils, eliminating the need for bulk current leads and reducing the heat burden for the cryogenic system. Characterizing the nonlinear effects of an HTS coil charged by a flux pump and understanding the dynamics of the charging process is essential for promoting the practical application of flux pumps. Numerical models provide a fast and cost-effective way of achieving this. In this study, we propose a methodology for coupling HTS coil and flux pump models using an electrical circuit, resulting in reduced computation costs. We validate the effectiveness of our approach against the experimental results of an HTS coil charged by a dynamo-type flux pump. Specifically, we obtain the voltage produced by the HTS dynamo using a 3D model based on the minimum electromagnetic entropy production method and apply this voltage to the load HTS coil using a T−A formulation finite-element method model coupled via an electrical circuit. The simulated charging current shows good agreement with experimental observations, validating our modeling strategy. The results demonstrate that the flux flow state in the HTS coil is the primary factor limiting the charging performance of the HTS dynamo as the charging current approaches the coil’s critical current. Furthermore, based on the simulation, we demonstrate that, when using flux pumps, it is advisable to leave a margin between the operating current and the critical current of the coil. Overall, our approach has the potential to be applied to HTS coils charged by any device.

The authors have proposed a performance evaluation system (PES) for large-scale high temperature superconducting (HTS) wind power generators, which produces strong electromagnetic force and high torque due to the high excitation current and the high magnetic load of the machine, based on the computer simulation results. The PES can physically test the structural stability of HTS coils and armatures against high torque and the effect of the magnetic field before mounting the HTS coil on the generator. This paper deals with the design, fabrication, and performance testing of a full-scale HTS coil to be installed in the PES, a performance analysis system for a 10 MW HTS wind power generator. The HTS coil with metal-insulation consists of four racetrack-type single coils with 310 turns per layer. The manufactured HTS coil was tested at 77 K with and without an iron-core. As a result, the critical currents of the tested HTS coils with and without the iron-core were 90 A and 106 A, respectively. Compared to the simulation results, it was found that the magnitude of the magnetic field was the same. These results will be effectively used to study and fabricate high magnetic field coils for HTS applications, as well as the PES being fabricated.

Axial flux permanent magnet (AFPM) electrical machines, characterized by disc-shaped rotor and stator structures, offer a high power-to-weight ratio. The replacement of traditional copper stator windings with high-temperature superconducting (HTS) windings has led to the development of lighter, more compact, and higher-performing machines, despite the technological challenges posed by HTS windings. Notably, HTS tapes experience AC power loss when subjected to alternating currents or when interacting with time-varying magnetic fields. Research at the Superconductivity Group in ENEA Frascati, in partnership with ICAS Company, focuses on the electromagnetic characteristics of second-generation (2G) HTS materials, particularly concerning AC loss in HTS coils. This paper presents the experimental results of two HTS coils wound using distinct HTS tape structures and tested within the liquid nitrogen (LN2) temperature range. Building on the geometric data from these coils, the electromagnetic behaviour of superconducting stator winding (SSW) topologies made from them was investigated. Finite element method (FEM) models of two AFPM prototypes, associated with two SSW topologies, were created, analyzed, and compared. By combining experimental data from the HTS coils with simulation results, a preliminary analytical design for AFPM superconducting synchronous prototypes is developed, enabling predictions of their performance across a range of operating conditions and rotor speeds.

Provided that a cyclotron system requires DC magnetic field, the no-insulation (NI) high temperature superconductor (HTS) magnet may be a good candidate as it may enable the cyclotron magnet to be more compact and reliable, yet operate under the liquid-helium-free environment. Here we report on a design study on an NI HTS isochronous cyclotron magnet for an acceleration of carbon ion up to an energy level of 385 MeV/u. To obtain a target beam stability, HTS coils were designed with the so-called “hill” and “valley” structure to generate a designated magnetic field profile. After the initial design on yokes and HTS coils was completed, a final design on the cyclotron magnet design was completed and its performance was analyzed using a 3D finite element method. This paper presents the magnet design and performance analyses in details that include: (1) electromagnetic design of the HTS coils and iron yokes for isochronous magnetic field; (2) mechanical stress with the use of stainless steel co-winding to limit the peak magnetic hoop stress; (3) charging and discharging scenarios with the non-linear NI characteristics considered; and (4) first-cut post-quench analysis with an estimated peak temperature and unbalanced force within the NI HTS coils.

In this paper, we propose a new high-temperature superconducting (HTS) coil with shorter HTS tape length. In general, the critical current and n-value of an HTS tape depend on the applied magnetic field magnitudes and angles. Therefore, we derived fitting equations for the critical current and n-value in terms of these parameters obtained from the measurements of a Bi-2223/Ag tape at 77 K. Relatively large electric fields were generated at the coil edges because of the large magnetic fields and angles. This phenomenon prevents improvement of transport-current performance of an HTS coil. To solve this problem, we created a new HTS coil with edge vacancies and central separation. Analysis results revealed that the coil critical current increased significantly. Furthermore, we found an optimum coil shape that results in the improvement of the central magnetic field and stored energy by 33% and 99.5%, respectively, compared with those of the normal coil, irrespective of the reduction in the total HTS tape length owing to edge vacancies.

The normal-zone propagation properties of the non-insulated HTS coil in cryocooled operation