Persistent Current Mode Operation Research Articles

A flux pump driven non-soldering closed-loop HTS magnet and its electromagnetic-thermal semi-analytical modelling method

We propose a new flux pump driven non-soldering closed-loop (NSCL) high temperature superconducting (HTS) magnet free of any soldering joints throughout the magnet for persistent current mode (PCM) operation. All the superconducting parts of the magnet are wound directly with HTS closed-loop ring split from an 8 mm REBCO tape using our new cutting scheme free of any soldering requirements. The magnet contains two single-pancake coils connected in series forming a closed circuit through two parallel bridge branches. Two thermal switches set on the two bridge branches control the on–off of the two bridges. A copper coil with iron core installed around one of the bridges is employed as the flux pump to drive the HTS magnet. An electromagnetic-thermal semi-analytical modelling method is proposed to analyse the pumping process by which the transport current in the magnet is calculated. The theoretical limit equation of the saturation current is improved as well. The proposed method can predict the current of the NSCL HTS magnet during the pumping process and provide results that are close to experiments. Experiments verify both the feasibility of the proposed flux pump driven NSCL HTS magnet and the modelling method. The results show that the NSCL HTS magnet works well in the PCM, which provides inspiration to the design of PCM operation of high field HTS magnets. The proposed modelling method also helps guide the design of different forms of HTS magnets and the flux pumps driving them.

Evaluation of the Performance of Commercial High Temperature Superconducting Tapes for Dynamo Flux Pump Applications

High Temperature Superconducting (HTS) dynamo flux pumps are a promising alternative to metal current leads for energization and the persistent current mode operation of high current DC superconducting magnet systems for applications in rotating machines, such as Magnetic Resonance Imaging (MRI) or fusion systems. The viability of the flux pump concept has been widely proven by laboratory experiments and research is now in progress for the design and optimization of flux pump devices for practical applications. It has been widely established that the dependence of the critical current density (Jc) on the temperature (T), the magnetic field magnitude (B), and the orientation (θ), has a substantial impact on the overall DC voltage obtained at the terminals, as well as on the current limit and the loss of the flux pump. Since HTS tapes produced by different manufacturers, they show different dependencies of Jc with the amplitude and the orientation of the magnetic field. They also give rise to different outputs when employed in flux pumps. In this paper, we evaluate and compare the performance of several commercial HTS tapes used for flux pumping purposes through numerical simulation. We also investigate the dependence of the flux pump ‘s performance on the operating temperatures. A 2D finite element numerical model is first developed and validated against experimental data at 77 K. Afterward, the same HTS dynamo apparatus used for validation is exploited for the comparison. The Jc(B,θ,T) and n(B,θ,T) relations, which characterize each different tape in the model, are reconstructed via artificial intelligence techniques based on the open-access database of the Robinson Research Institute. It is shown in the paper that certain tapes are more suitable than others for flux pump applications and that the best overall operating temperature is in the vicinity of 77 K.

An efficient approach for superconducting joint of YBCO coated conductors

Superconducting joints are crucial for second generation high-temperature superconducting (2G HTS) closed-loop coils that work in the persistent current mode (PCM) operation. Here, we report an efficient approach for superconducting joints of YBa2Cu3O7-σ (YBCO) coated conductors (CCs). The YBCO layer is etched to and from some microchannels, which serve as oxygen diffusion paths during oxygenation annealing (OA), to accelerate the recovery of critical current (I c) of the joined two YBCO layers. The I c of the superconducting joint is 118 A at 77 K with an optimized joining temperature and a short OA time (10 h), which is about 82% I c of the joined YBCO CCs. The persistent field decay of the closed-loop coil shows a joint resistance (R j) of less than 2.2 × 10−13 Ω at 77 K. Two YBCO films epitaxially grown along the c-axis diffuse into each other at the interface and form a dense joining, enabling superconducting current path. This superconducting joint technique can promote the achievement of the PCM operation in 2G HTS magnet applications, such as magnetic resonance imaging and nuclear magnetic resonance.

Superconducting joining of YBCO coated conductors without a large critical current loss

Achieving superconducting joinings with large current capacities is a challenge for magnetic resonance imaging applications of YBa2Cu3O7-δ (YBCO) coated conductors (CCs). Here we report a novel method to fabricate a superconducting joining between two YBCO CCs using a joining strap consisting of YBCO, Ag, a buffer, and a substrate. The YBCO layer exfoliated from the buffer stack of a YBCO CC can retain more than 90% of its original critical current (Ic). This is particularly due to the Ag layer serving as a fast oxygen diffusion path to promote the recovery of superconductivity of the YBCO layer during oxygenation annealing. The atomic arrangement at the joined YBCO interface is well aligned along the (001) plane, enabling superior superconducting performance. The method reported here provides a practicable solution to achieve persistent current mode operation in second-generation high-temperature superconducting magnet applications.

A Concentrically Arranged Joint-Less HTS Coil System for Persistent Current Mode Operation

We have shown that a persistent current mode operation was possible in various types of HTS coils wound by the joint-less winding technique with 2G REBCO tape. However, in a previous test, a solenoid type of the joint-less HTS coil showed very poor magnetic field quality which was caused by the inevitable gap located at the very center part of the solenoid coil. In this paper, to improve the homogeneity of the magnetic field of the solenoid joint-less HTS coil, we proposed a joint-less HTS coil which had two separable parts of the solenoid coil arranged concentrically, just like a conventional high field magnet. By this arrangement, we could improve the homogeneity of the center magnetic field remarkably. A prototype coil was made and operated at the temperature of 77.3 K by liquid nitrogen cooling and charged up with a persistent current switch that was also a part of the perfect superconducting closed loop in the joint-less HTS coil.

HTS Quadrupole Magnet for the Persistent Current Mode Operation

In this study, a new high-temperature superconducting (HTS) quadrupole magnet with circular coils was designed and built at Fermilab. Several HTS coils were also investigated at the temperature of liquid nitrogen. The main goal of this activity was to investigate the coils and magnet operation in a persistent current mode to reduce accelerator magnets' capital and operational expenses. To accomplish this, HTS short-circuit coils were used. In the paper, HTS coils and magnet design, fabrication, and tests were discussed. The magnetic field in the magnet aperture and the field decay when the magnet operated in the “frozen flux” mode were measured. The test and simulation results were compared, confirming the advantages of the proposed approach.

A Contactless Self-Regulating HTS Flux Pump

A series of high temperature superconducting (HTS) flux pumps have been developed to achieve the persistent current mode (PCM) operation for HTS magnets. However, most of the HTS flux pumps work with a small operating air gap, or with an iron core, the extra heat load brought by the conventional components deployed in the cryogenic environment is inevitable. To address this problem, in this study, in conjunction with the wireless power transfer (WPT) approach and transformer-rectifier type HTS flux pump, we have proposed a contactless self-regulating HTS flux pump. A circuit model and a 2D H-formulation finite-element model have been built for better understanding this system. Furthermore, a basic prototype was proposed to experimentally validate this concept. The measured results indicate that this method has the potential to charge the HTS magnet with all the conventional constituents outside the cryostat, and the operating frequency could be very low to evade inducing large eddy current as well as losses on the cryostat wall.

Field Mapping of the Jointless HTS Solenoid Magnet in a Persistent Current Mode Operation

In order to achieve a persistent current mode operation with high-temperature superconducting (HTS) coils, we have been trying to improve the concept of a jointless HTS coil. The coil has a perfect closed loop without any joints in a coil, and we have proven that a persistent current can be induced in the coil by several charge up methods such as PCS, flux pump, or field cooling with a background magnet. In this paper, based on the previous experiment, which showed partially successful results [10], we designed and fabricated a small solenoid type jointless HTS magnet with a 100-m-long ReBCO wide conductor and tested it in a liquid nitrogen bath. After the persistent current mode operation of 15 h, we tried to measure temporal and spatial homogeneity by a field mapper we developed under the DSV of 10 mm .

Experimental Studies of Current Decay in a Flux Pumped HTS Magnet Subject to Travelling Magnetic Fields

Superconducting magnet, which provides a steady magnetic field if operated in persistent current mode (PCM), can be applied in MRI/NMR, motors, and other power devices. However, due to the joint resistance of high temperature superconducting (HTS) magnet manufactured by coated superconducting tapes, it would be difficult to realize PCM operation. Moreover, the ac loss would affect the decay characteristics of HTS magnet when applied in the ac magnetic field circumstance. In this paper, a shorted racetrack HTS magnet was manufactured and charged by a flux pump. The current decay of the pumped HTS magnet with and without travelling magnetic field was investigated. Results show that subjected to travelling magnetic field, the current decay of the PCM HTS magnet cannot be simply explained by the existence of joint resistance, other factors, which are more important, must be figured out. To understand the underlying mechanism, the dynamic resistance explanation has been adopted and verified by numerical calculation.

Experimental Verifications of REBCO Layer Winding Aimed at Persistent Current Operation

Recent development in the superconducting REBCO joints is an encouraging prospect for realizing an HTS magnet in persistent current mode operation. In our ongoing project, we are developing a cryocooled 1 T REBCO magnet system with joints to demonstrate the joint performance in an operating condition expected, for example, for an MRI. Preliminarily, the small test coils have been fabricated to verify the winding method. Additionally, we have designed and manufactured the new leads structures for feeding the conductor from the winding to the joints.

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.

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.

Development of a persistent superconducting joint between Bi-2212/Ag-alloy multifilamentary round wires

Superconducting joints are one of the key components needed to make Ag-alloy clad Bi2Sr2CaCu2O8+x (Bi-2212) superconducting round wire (RW) successful for high-field, high-homogeneity magnet applications, especially for nuclear magnetic resonance magnets in which persistent current mode operation is highly desired. In this study, a procedure for fabricating superconducting joints between Bi-2212 RWs during coil reaction was developed. Melting temperatures of Bi-2212 powder with different amounts of Ag addition were investigated by differential thermal analysis so as to provide information for selecting the proper joint matrix. Test joints of 1.3 mm dia. wires heat treated in 1 bar flowing oxygen using the typical partial melt Bi-2212 heat treatment (HT) had transport critical currents Ic of ∼900 A at 4.2 K and self-field, decreasing to ∼480 A at 14 T evaluated at 0.1 μV cm−1 at 4.2 K. Compared to the Ic of the open-ended short conductor samples with identical 1 bar HT, the Ic values of the superconducting joint are ∼20% smaller than that of conductor samples measured in parallel field but ∼20% larger than conductor samples measured in perpendicular field. Microstructures examined by scanning electron microscopy clearly showed the formation of a superconducting Bi-2212 interface between the two Bi-2212 RWs. Furthermore, a Bi-2212 RW closed-loop solenoid with a superconducting joint heat treated in 1 bar flowing oxygen showed an estimated joint resistance below 5 × 10−12 Ω based on its field decay rate. This value is sufficiently low to demonstrate the potential for persistent operation of large inductance Bi-2212 coils.

Persistent Current Mode Operation of A 2G HTS Coil With A Flux Pump

A flux pump is a kind of power source that energizes superconducting magnets usually embedded in the cryogenic system. Therefore, heat loss through current leads is eliminated, allowing for more efficient cooling design. A winding technique, the so-called “wind-and-flip,” had been proposed by the authors to realize a jointless HTS coil for a perfect resistanceless closed loop, which may enable to induce a persistent current in an HTS coil wound with second-generation HTS tape. In this paper, we investigated charging/discharging characteristics and a possibility of persistent current operation of a jointless HTS coil with a flux pump. Results are discussed with regard to current capacity and temporal stability with a prototype jointless HTS coil.

Experimental Analysis on Initial Current Decay Characteristics of Persistent-Mode HTS Coil by External Alternating Magnetic Field

This paper deals with the current decay characteristics of a high-T c superconducting (HTS) coil operated in persistent current mode (PCM) under alternating magnetic field. In superconducting synchronous machine applications, such as linear synchronous motor (LSM), the superconducting coils are designed to operate in PCM to obtain steady-magnetic field with DC transport current. This superconducting coil operates on a direct current, but it can be exposed to alternating magnetic field due to the armature winding. When the field coils are subjected to an external time-varying magnetic field, it is possible to result in a current decay for those coils in PCM operation due to electrical AC loss. In this research, a current decay measurement system, which consists of field coil and armature coil, was fabricated to verify current decay characteristics of field coil in PCM operation against external alternating magnetic field. The current decay rate for the field coil was measured by using a hall sensor as a function of amplitude and frequency of armature coil.

Persistent Current Mode of a 1-T-Class HTS Pancake Coil for NMR/MRI Applications

We had proposed a winding method, the so-called “wind-and-flip,” which enables a persistent current-mode operation of a high-temperature superconducting (HTS) pancake coil. Two pancake coils had been fabricated using this method and tested in a liquid nitrogen bath with an external magnetic field. The result showed that the time stability was not good enough to use for magnetic resonance imaging or nuclear magnetic resonance when they were magnetized by field cooling at 77 K. In this paper, the test results of a stacked HTS pancake coil for a center magnetic field of 1 T are presented. The coils were cooled to 20 K by conduction cooling using a Gifford-McMahon cryocooler. We measured the time stability of the persistent current-mode operation at 20 K, which shows a strong possibility of a high-field HTS magnet using this method.

Laser drilling: enhancing superconducting joint of GdBa2Cu3O7 − δ coated conductors

While GdBa2Cu3O7 − δ (GdBCO) coated conductors (CCs) have been proposed for superconducting applications, they have not been used in devices with persistent current mode (PCM) operation because of a lack of joining techniques. A superconducting joint of CCs, formed via melting diffusion and oxygenation annealing, offers no electrical resistance between the CCs, thus establishing a superconducting closed loop for PCM operation. Because superconductivity degrades with oxygen out-diffusion during melting diffusion, oxygenation annealing allows oxygen diffusion into the GdBCO lattices. As effective oxygenation annealing requires oxygen pathways in the joint, low solubility and diffusivity of oxygen in the buffer and CC substrate hinder full superconductivity recovery. Here we show a laser-drilling technique to produce microholes as conduits on the surfaces of GdBCO CCs’ to promote oxygen in-diffusion, which resulted in reduced superconductivity recovery time. Superconductivity was fully recovered after laser drilling, melting diffusion at 850 °C for 1 min, and oxygenation annealing at 500 °C for 350 h.

Super-Stable Superconducting MRI Magnet Operating for 25 Years

This paper describes a superconducting magnet performance continuing persistent current operation for 25 years. The central magnetic field is 0.5 T for whole-body magnetic resonance imaging (MRI). The observed stability of the magnetic field was extremely stable compared with the specification. Because of the ultrafine electrical resistances in persistent current mode operation, it was difficult to observe an accurate magnetic field stability at the factory test. By long-term operation, we are able to observe the magnetic field stability and the joint resistance precisely. The magnet was started to energize on January 1988 and has been operated consecutively without re-excitation. At that time, a superconducting whole-body MRI system was appeared in the world among a normal conducting magnet and a permanent magnet system. We made superconducting joints to connect superconducting conductors between superconducting coils. We carried out the measurement of the central magnetic field by the nuclear magnetic resonance method. Because the device is used for the medical examination at the hospital, the magnetic field data acquisition is in a limited situation. The stability of the last magnetic field becomes 0.0006 ppm/h and gets the good performance that is more than 100 times than the specification. The total resistance of the superconducting joints is 4 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> Ω and supports good superconducting connection.

A superconducting joint for GdBa2Cu3O7−δ-coated conductors

Second-generation (2G) GdBCO-coated conductors (CCs) are promising for superconducting magnet applications because of their high critical current (Ic) density, low dependency of the Ic on the external magnetic field, good mechanical properties and reasonable cost, which offer opportunities to develop ultra-high-field magnets. However, they have not been used in high-temperature superconducting (HTS) applications with persistent current mode (PCM) operation such as nuclear magnetic resonance/magnetic resonance imaging magnets owing to unavailability of fabrication techniques for proper joining and contacts. Here we report a resistance-free joint, termed a ‘superconducting joint’, for 2G GdBCO CCs that forms a direct connection to establish a superconducting closed loop for PCM operation. The Ic value of the joined CCs is identical to that of the parent conductors in a liquid nitrogen bath (77 K). Moreover, the initially induced magnetic field of a model GdBCO coil containing a superconducting joint is maintained without decreasing, indicating the complete absence of electrical resistance. Thus, this fabrication method is a unique practical solution for lengthening the 2G HTS CCs and, more importantly, achieving PCM operation in 2G HTS magnet applications, including ultra-high-field nuclear magnetic resonance/magnetic resonance imaging magnets generating more than 1 GHz.

Development of $\hbox{Nb}_{3}\hbox{Sn}$ Superconducting Coil Manufacture Technology

The nuclear magnetic resonance superconducting magnet technology is being developed in our laboratory. Because the upper critical magnetic field of NbTi alloy is not high enough, an Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor has to be used in the manufacture of magnets for 500 MHz and above. In this paper, an experimental Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting coil was manufactured to study the fabrication technology. The bobbin with Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ceramic coating is made of stainless steel. A vertical tube type vacuum heat treatment furnace was established for Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn formation. Heat treatment parameters for the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coil were done at 670 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C in vacuum. Vacuum pressure impregnation was done to strengthen the assembly. The Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coil, the switch, and the current leads were joined together using superconducting solder matrix replacement method. In the test the maximum current of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coil was 263 A, corresponding to a central magnetic field of 3.47 T. Persistent current mode operation was also successfully performed.