High Temperature Superconducting Double Pancake Coil Research Articles

Through-Wall Excitation of a Conduction Cooling HTS Magnets by a Linear-Motor Type Flux Pump

The flux pumps can realize non-contact excitation of high-temperature superconducting (HTS) magnet and eliminate heat leakage caused by current leads. However, earlier proposed flux pump needs to be installed inside the cryostat, which leads to additional heat load to the cryogenic system. Here, we report the successful demonstration of a through-wall linear-motor type flux pump, with its heating parts such as the copper windings completely outside the cryostat, while its generated heat is dissipated in the air and insulated from the cryostat. With an insulation HTS double pancake coil installed in the cryostat and cooled by a G-M cryogenic cooler at 32.6 K, we have demonstrated the injection of direct currents of 42 A into the closed-loop, without the current leads. Thus, the feasibility and reliability of using the linear-motor type flux pump as the power source to excite the HTS through the cryostat is verified, making it possible for future applications such as in MRI and superconducting motors, etc.

Fast Discharging Behavior of High Temperature Superconducting Coils With Copper Rings and Plates

Fast energy extraction using dump resistances is an effective method for quench protection of high temperature superconducting (HTS) magnets. Copper frames are often applied in HTS magnets for conduction cooling and mechanical support. Previous studies have shown that copper plates can accelerate the energy release of HTS coils during discharging operations with dump resistance, which is favorable for fast quench protection. Copper rings distributed at the inner and outer diameters of HTS coils can provide a strong mechanical support for the possible electromagnetic strain in high magnetic fields. This paper studies the effect of copper rings and plates on fast discharging process of an insulated HTS double pancake coil by finite element method. An optimized structure composed of copper rings and plates is proposed, and its effect on fast discharge is analyzed. The results show that copper plates show better performance than copper rings in accelerating energy release and reducing hot spot temperature of the coil. During discharging process, the optimized structure can absorb considerable energy from the HTS coil and significantly reduce the hot spot temperature. The optimized structure is a promising alternative for fast quench protection of HTS magnets.

A New Approach to Measure Magnetic Field of High-Temperature Superconducting Coil Based on Magneto-Optical Faraday Effect

Magnetic field monitoring devices are essential for studying and operating high-temperature superconducting (HTS) magnets. Apart from conventional Hall sensor, passive optical fiber sensor is also an option for magnetic field monitoring. In this article, we proposed a novel access based on yttrium iron garnet magneto-optic (YM) sensor, which is one kind of optical fiber sensors, to measure magnetic fields generated by an HTS double pancake coil. Moreover, we used a magnetic field bypass tube made of silicon steel material to expand the measurement range of YM sensor. The test results of YM sensor in liquid nitrogen demonstrated that the YM sensor was benchmarked against Hall probe up to 24.7 mT at 77 K. Using a silicon steel tube to shield magnetic field could extend the field measurement range to 37.6 mT. YM sensor could be one of the promising methods for monitoring HTS magnet magnetic field.

A Novel Method to Eliminate the Screening Current–Induced Magnetic Field in a Non-insulated REBCO Double Pancake Coil

The screening current–induced magnetic field (SCIF) brings bad effects on the field quality of REBCO high-temperature superconducting (HTS) magnet, which is mainly due to its specific large aspect ratio structure. In this paper, numerical simulation and experiments were conducted to investigate SCIF in a non-insulated HTS DP coil firstly. Then, a heating flux method was proposed to help eliminate the effect of screening current in a GdBCO HTS double pancake (DP) coil. A heating plate was mounted under the DP coil and used to generate a heating flux, which could increase the temperature and decrease critical current of the HTS coil in a short time. The decreasing of critical current could reduce the margin left for the screening current and eliminate the effect of SCIF further more. Heating flux experiments were performed to verify its effects on eliminating SCIF in the DP coil at the end. The results show that the SCIF could be reduced obviously by the method of heating flux. The research provides an important reference to eliminate SCIF in the HTS magnet.

AC Loss Measurement in HTS Coil Windings Coupled With Iron Core

Utilizing high temperature superconducting (HTS) magnets coupled with iron core on the rapid cycling synchrotrons is considered as an effective way to reduce their construction and running costs. However, the heat generation by ac loss in HTS wires is a critical obstacle for the application, and prediction and reduction of ac loss in HTS magnets coupled with iron core are essential. In this work, we built one HTS double pancake coil (DPC), a stack of two DPCs, and an iron core comprising laminated steel to couple with the coil windings. The HTS-coated conductors are 200 A-class 4-mm–wide Shanghai Conductors. We report the ac loss measurement results on the coil windings with/without the iron core at three different frequencies. The ac loss in the HTS windings was significantly increased due to the existence of the iron core. Increasing the number of stack of the DPCs may help reduce the difference in ac loss values in the coil windings with/without iron cores.

Charging 2G HTS Double Pancake Coils With a Wireless Superconducting DC P ower Supply for Persistent Current Operation

We have designed and built a novel high-temperature superconducting (HTS) flux pump device to charge a second-generation (2G) HTS double pancake coil (DPC). This device generates dc-biased ac travelling wave on sections of 2G HTS coated conductor (superconducting stator), which pumps flux into the superconducting closed loop formed by the DPC and superconducting stator. In this paper, we discuss the importance of the dc-biased field, ac travelling wave, and the travelling direction in the flux pump process, and how to control the magnetization of an HTS coil with the above factors. We have found that a pure ac travelling wave can only transport net flux into the superconducting closed loop in the presence of dc-bias field. The magnetization direction of the DPC can be reversed by reversing the dc-bias direction of the dc background field or travelling direction of the ac travelling wave. This paper shows the versatility and flexibility of this novel flux pump device in controlling the magnetization of the HTS coils, which will work as a wireless dc power source to help persistent current mode operation of a small-scale MRI magnet built by HTS DPCs later in this project.

Influence of Iron Core on Critical Current and Inductance of an HTS Double Pancake Coil

In a high temperature superconducting (HTS) DC induction heater, an HTS coil and an iron core are essential to generate DC magnetic field and form the magnetic circuit. To understand effect of the iron core on the critical current and the inductance of the HTS coil, we prepared a double pancake coil (DPC) and an iron core in this study, and measured the critical current and inductance of the DPC. The experimental results were compared with the numerically calculated ones as well. The measured critical currents of the DPC without and with the iron core are 89 A and 84 A, respectively. Thus, the iron core does affect the critical current. Moreover, the measured inductances are 6.94 mH and 45 mH. The measured data of the critical currents and the inductances are in good agreement with the calculated results

Study on the Persistent Current Switch in HTS Coils Wound with 2G Wire for Compact NMR Magnets

High temperature superconducting (HTS) magnets wound with REBCO wires are used in many applications, and the superconducting magnet which is operated in persistent current mode has many advantages. Therefore, we have been developing compact NMR relaxometry devices using HTS coils operated at liquid nitrogen temperature. The required strength and homogeneity of magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm3 respectively. The proposed HTS magnet for NMR relaxometry device consists with stacked HTS double pancake coils wound with REBCO wires and operated by persistent current mode (PCM) using superconducting joint between REBCO wires. In PCM operation, the ability of persistent current switch (PCS) is very important, so, in this study, the thermal properties of YBCO wire against the various thermal inputs by heater were investigated experimentally to design the PCS for PCM HTS magnets. The thermal behaviors of the YBCO wires were measured as functions of amount of heat input using two types of epoxy resin. The current bypassing properties on the YBCO loop coil with developed PCS were studied experimentally.

Shape Optimization of the Stacked HTS Double Pancake Coils for Compact NMR Relaxometry Operated in Persistent Current Mode

The low temperature superconducting (LTS) coils are used in NMR devices, and liquid helium is used for their cooling. It is possible to reduce the operating costs by manufacturing the superconducting magnet that is consisted of the high temperature superconducting (HTS) coil using liquid nitrogen cooling. We have been developing the NMR relaxometry devices using the HTS double pancake coils and operated in the persistent current mode at liquid nitrogen temperature. The required strength and homogeneity of the magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm 3 , respectively. To achieve the target field strength of 1.5 T, it is necessary to increase the numbers of pancake coils and winding turns of each coil, and the large operating current is required as much as possible. In this paper, I c - B properties of HTS wire and superconducting joint are measured to determine the operating current (load line) for the proposed NMR relaxometry devices. Moreover, 3-D finite-element-model-based analysis was carried out to design the stacked HTS magnets for compact NMR relaxometry with downsizing and low cost.

Investigation of Thermal Quench Characteristic in the HTS Bi-2223 Racetrack Coil

The thermal stability and quench propagations are one of the most fundamental issues for the High Temperature Superconducting (HTS) coil. Superconductivity is confined within the phase surface bounded by magnetic field, temperature, and current. Of these three parameters, the temperature is neither completely controllable, nor predictable, because the energy stored in the magnet, can easily be converted into heat up setting the thermal equilibrium within the winding. Therefore, in HTS coil operation, the temperature stability or more generally the thermal behavior of the conductor is the most important. In this paper, a HTS double racetrack pancake coil was prepared to examine the thermal stability, quench propagation and minimum quench energy (MQE). HTS coil was wound using the AMSC Bi-2223. The experimental results are discussed in this paper.

A Versatile Laboratory Electromagnet With HTS Coils

We report the development of a laboratory electromagnet utilizing high temperature superconducting (HTS) coils which is designed to generate fields in excess of 3 Tesla, but with power consumption and size significantly below that possible with copper-wound coils for an equivalent useful field volume. The components are complete and the system is currently being assembled. The electromagnet is based around a compact iron yoke with four HTS double pancake coils, employing a total of 1.6 km of first generation HTS (Bi-2223/Ag composite) wire, to generate the field across an air-gap of 50 mm. The electromagnetic design was tailored to take into account the anisotropic in-field performance of the HTS conductor, requiring a minimization of the radial field component at the coils while retaining sufficiently open access for installation of the components of a vibrating sample magnetometer with which this magnet will be used. The HTS coils are conduction cooled with a single-stage cryocooler that delivers approximately 27 W of cooling power at the design operating temperature of 35 K; HTS current leads are employed to reduce the thermal load at the coils. In the present configuration the coils are housed in a single cryostat, but the design is easily adaptable to provide for a number of specific application requirements. Contrary to the generally held view that HTS magnets are still too expensive, this project demonstrates that utilization of HTS conductor is now an attractive option in a number of application areas.

Electrical insulation characteristics in the simulated electrode system of HTS double pancake coil

For the experiment four types of spacer were distinguished by an arrangement that could improve the dielectric strength by making the path of flashover longer. One of the spacers could be applied to an insulator between windings of a high temperature superconducting (HTS) transformer. The flashover characteristic of each type was investigated and the flashover phenomena were observed to understand breakdown mechanism in liquid nitrogen (LN/sub 2/). In the first, the flashover characteristic in LN/sub 2/ was compared with that in air using the simulated electrode made from five turns of HTS tape. The dielectric strength of spacers was improved with an arrangement in air but not in LN/sub 2/. Through the observation of flashover phenomena, a micro gap between the spacer and the coil electrode generating bubbles was regarded as the main cause. In the second, the effect of the micro gap on flashover was investigated with a plane-plane electrode system. The micro gap decreased the flashover voltage by 70 percent. Finally, the dielectric strength of the spacer in LN/sub 2/ also could be improved by filling of a small amount of epoxy in the gap between the coil and the spacer.

Advances in the development of high-temperature superconducting Bi-2223/Ag pancake coils for electric power applications

Abstract High-temperature superconducting (HTS) double-pancake coils have been fabricated using (Bi,Pb)2Sr2Ca2Cu3O10+ x (Bi-2223) multifilamentary (MF) Ag-clad tapes. The thermal-mechanical processing of Bi-based composites has shown an improvement in the transport properties of composite tapes by optimizing powder stoichiometry, deformation rate and reduction, sintering temperature and duration, and adjusted rate of heat treatment. Ag-clad Bi-2223 MF tapes up to 55 m have been fabricated using the powder-in-tube technique. An overall Ic of ∼7.5 A with a fluctuation along the length of less than 12% was achieved. An improvement in the electrical transport properties of ∼12 mT Bi-2223/Ag wind-reacted HTS double-pancake coils prepared from long partially processed tapes was achieved by means of post-sintering.