Layer-wound Coils Research Articles

Comparison Between Insulated and NI Layer-Wound ReBCO Coils During Transients

Comparison Between Insulated and NI Layer-Wound ReBCO Coils During Transients

Electro-Thermal Behavior of Layer-Wound BSCCO Coils With and Without Insulation

In this work, the electro-thermal behavior of two layer-wound High Temperature Superconducting (HTS) coils, realized with and without electrical insulation, is compared. Both coils are wound from the same BSCCO tape, and have a very similar geometry, with the same number of turns and layers. A heat input is applied to both coils, by tuning the current supplied to resistive heaters realized through stainless steel tapes wound on the mandrel at the inner surface of both coils. The heaters are in contact with one full inner turn of the winding. The coils are cooled in a liquid nitrogen bath, and the heaters are supplied with a constant current. Then, the windings are charged until the tape critical current is exceeded, and the tests are repeated for different heat loads. The signals acquired through voltage taps, suitably soldered at the same locations in both windings, are compared at the same testing conditions. Finally, the electrical characteristic of the different layers of the coils is related to the temperature of the heater and of the various turns of the coil by means of a 1-D thermal model.

Development of an intra-Layer No-Insulation (LNI) Winding Method for Quench Protection of a Layer-Wound REBCO Coil

The no-insulation (NI) technique is promising as a protection method for high-field high-temperature superconducting (HTS) magnet. However, it is not suitable for layer-wound HTS coils because it has extremely long magnetic field delay due to short circuits inside winding. To solve this problem, a research group, including the current authors, proposed a novel winding method called intra-layer no-insulation (LNI) method. In this paper, we summarize the series of our studies on NI layer-wound coils and LNI coils as well as discuss issues affecting high-field magnet applications.

Shielding currents in multifilament coated conductors wound into pancake coils and layer-wound coils

The effect of multifilament coated conductors on reducing shielding currents was studied by numerical electromagnetic field analyses. We compared the behaviours of the shielding currents in stacked double pancake coils and layer-wound solenoid coils (SLCs) wound with the following conductors: a monofilament coated conductor, a copper-plated two-filament coated conductor, and an insulated two-filament coated conductor. Shielding-current-induced fields (SCIFs) in the layer-wound SLCs wound with copper-plated two-filament coated conductors decay more rapidly than in stacked double pancake coils wound with copper-plated two-filament coated conductors. In addition, we examined the behaviour of the shielding currents in the coils with and without normal metal terminals. The normal metal terminals made the decay time constants of the SCIFs in pancake coils longer, while the decay time constants of SCIFs did not change notably in the layer-wound SLCs with and without normal metal terminals.

Generation of 25 T with an all-superconducting magnet system: field profile and field quality measurements of a layer-wound 4 T REBCO insert coil for a 21 T LTS magnet

High temperature superconductors, especially rare-earth barium copper oxide (REBa2Cu3O7−x or REBCO) tapes are an enabling technology for magnetic fields beyond 23.5 T, the limit of the existing technology based on the low temperature superconductors (LTS) niobium–titanium (NbTi) and niobium–tin (Nb3Sn). Due to their higher prices, REBCO tapes are in general only employed as insert coils in an LTS outsert. Due to the tape geometry, the effect of the screening currents is high in REBCO insert coils. The resulting screening current-induced field (SCIF) can distort the shape and magnitude as well as affect the temporal stability of the field after charging. Their influence has to be well understood and it can be a limiting factor in field quality critical applications such as nuclear magnetic resonance magnets. In this work, we present the test results of a layer-wound 4 T REBCO insert coil operated in the background of a 21 T LTS outsert reaching a combined field of 25 T. Using an array of 13 calibrated Hall sensors, we investigate the spatial and temporal effects of the SCIFs at different background fields. Series expansion is employed to separate the contributions of the insert and of the outsert. We present in this work the characterization of the effect of the SCIF on the shape of the field and on its temporal stability in background fields from 0 T up to 19 T. We compare the experimental results with existing simulation models for the validation of the model at high magnetic fields, which had never been done for layer-wound REBCO coils. Based on the experimental data, we demonstrate the necessity of SCIF-aware simulations for REBCO insert coils, regardless of the insert’s relative operating current. Before summarizing and concluding the work, we give a short overview of proposed methods for SCIF reduction in REBCO coils from the literature.

Numerical analysis of thermal stability and mechanical response in a no-insulation high-temperature superconducting layer-wound coil

To achieve magnetic field spatial homogeneity and reduce resistive joints, no-insulation (NI) high-temperature superconducting layer-wound coils are highly desirable for nuclear magnetic resonance and magnetic resonance imaging. A multiphysics quench model is built to study the thermal stability and mechanical behaviors in an NI layer-wound coil. The numerical results show that when an NI layer-wound coil is subjected to heat disturbance, the transport current will be redistributed owing to the low turn-to-turn contact resistance, and thus the coil can achieve post-quench recovery. Although the layer-wound coil can rapidly return to its initial temperature after a quench, the current diffusion process within the coil leads to a long characteristic field delay time, which means that the circumferential current needs more time to recover. The increase of the peak temperature for coils with high turn-to-turn contact resistivity is not significant, which indicates that the temperature rise is mainly caused by the heat disturbance. Additionally, the recovery time of the central field is dramatically shortened. Thus, it is feasible to reduce the field delay time without sacrificing high thermal stability by designing a layer-wound coil with high turn-to-turn contact resistivity. Moreover, owing to the fast temperature rise during a heat disturbance, the coil undergoes remarkable deformation. The hoop and axial stresses exhibit larger changes than the radial and shear stresses. It can be also found that the mechanical behaviors are mainly affected by the heat disturbance rather than the electromagnetic force in the self-field.

Mechanism of notable difference in the field delay times of no-insulation layer-wound and pancake-wound REBCO coils

The characteristic magnetic field delay time for a no-insulation (NI) REBCO layer-wound coil is three orders of magnitude longer than that for a NI REBCO double-pancake coil. In a NI layer-wound coil, the circumferential current firstly flows along the periphery of the coil winding, and then it diffuses from the top and bottom turns into the middle turns of the winding, resulting in a long characteristic magnetic field delay time due to the current diffusion process. In contrast, the characteristic magnetic field delay time for a NI double-pancake coil is dominated by the circumferential current decay in individual turns. On the basis of a derived scaling law, the characteristic magnetic field delay time for a NI REBCO layer-wound coil for a 400 MHz LTS/REBCO nuclear magnetic resonance (NMR) magnet is 37 h, while that for a NI REBCO double-pancake coil is only <1 min. Thus, it is demonstrated that a double-pancake-winding is greatly preferred to a layer-winding for NMR applications from the view point of the characteristic magnetic field delay time.

Degradation of a REBCO Coil Due to Cleavage and Peeling Originating From an Electromagnetic Force

This paper presents the results of a high field generation test for layer-wound HTS coils in a background magnetic field. The test revealed new types of degradation for a REBCO coil caused by an electromagnetic force. A series-connected REBCO coil and a Bi-2223 coil were charged at 4.2 K in a background magnetic field of 17.2 T to generate a central magnetic field of 28.2 T (nuclear-magnetic-resonace frequency of 1.2 GHz). However, a premature normal voltage appeared on the REBCO coil, and the charging was stopped at 25 T. Unwinding the REBCO coil after the experiment found: 1) delamination, due to a cleavage and peeling, of the conductor in a soldered joint inside the winding; and 2) axial movement and edgewise bend deformation of the REBCO conductor at the outermost layer. The keyword of the present degradation is stress concentration, which is also the origin of the more widely known thermal stress-induced degradation.

Degradation analysis of REBCO coils

RE-Ba-Cu-O (REBCO) layer-wound coils were operated to investigate their properties under electromagnetic forces in an external magnetic field of up to 17.2 T. While REBCO coils were successfully operated under electromagnetic forces over 200 MPa, some showed degradation after quenching. To develop high-temperature superconducting (HTS) magnets, the reasons for the degradation of REBCO coils should be investigated. In this study, the degraded REBCO coils were carefully rewound. The critical current (Ic) of the rewound REBCO conductor was measured to check the damaged parts in the degraded REBCO coils, and the possible causes for the degradation were discussed.

REBCO Layer-Wound Coil Tests Under Electromagnetic Forces in an External Magnetic Field of up to 17.2 T

A nuclear magnetic resonance (NMR) system using a high-temperature superconducting (HTS) magnet and a probe with an HTS radio frequency coil is currently under development. The HTS NMR magnet is expected to reduce the volume occupied by the magnet and to encourage users to install higher field NMR systems. RE-Ba-Cu-O-coated (REBCO-coated) conductors offer the advantages of a higher critical current density than low-temperature superconductors in magnetic fields above 10 T as well as tolerance to high tensile stress. Both of these factors are expected to lead to a reduction in the volume of the magnet. Four REBCO layer-wound test coils were fabricated in order to investigate their properties under electromagnetic forces in an external magnetic field of up to 17. 2 T. The REBCO coils with a practical inner diameter were successfully operated under electromagnetic forces of over 200 MPa in high magnetic fields.

Development of a Bi-2223 HTS Magnet for 3T MRI System for Human Brains

We are developing a cryogen-free high temperature superconducting (HTS) magnet system for a compact 3T MRI system for human brains. In the conceptual design, the magnet system consists of 5 main coils that are layer windings of Bi-2223 tapes. The magnet system will have 500 mm room temperature bore and be operated at 20 K using G-M cryocoolers. The target field is 3 T ±5 ppm for 250 mm (dia,) and 200 mm (length) volume. All the coils will be connected in series and operated in driven mode. Controlled overpressure (CT-OP) processed Bi-2223/Ag tapes which are reinforced with Cu-alloy laminations are to be used for these coils. We investigated Ic-B-T performance in detail for short samples of the tape. We fabricated and tested five small layer-wound coils using the tape (38.4 ~ 46.2 m piece for each coil). Each small coil could be energized up to its expected current that was calculated using the short sample performance and the coil parameters. The maximum electromagnetic force (hoop stress) reached 137 MPa, and caused no degradation in the coil performance. These results show that our layer-winding techniques and the conductor performance (Ic-B-T and homogeneity along length) can be applicable and suitable for our 3T MRI magnet.

High Field Magnets With HTS Conductors

Development of high-field magnets using high temperature superconductors (HTS) is a core activity at the NHMFL. Magnet technology based on both YBCO-coated tape conductors and Bi-2212 round wires is being pursued. Two specific projects are underway. The first is a user magnet with a 17 T YBCO coil set which, inside an LTS outsert, will generate a combined field of 32 T. The second is a 7 T Bi2212 demonstration coil set to be operated in a large bore resistive magnet to generate a combined magnetic field of 25 T. Owing to the substantial technological differences of the two conductor types, each project faces different conductor and magnet technology challenges. Two small coils have been tested in a 38-mm cold bore cryostat inserted in a 31 T resistive magnet: a Bi2212 round-wire layer-wound insert coil that generated 1.1 T for a total of 32.1 T and a YBCO double-pancake insert that generated 2.8 T for a total central field of 33.8 T. Four larger layer-wound coils have been manufactured and tested in a 20 T, 186-mm cold bore resistive magnet: a sizeable Bi-2212 coil and three thin large-diameter YBCO coils. The test results are discussed. The current densities and stress levels that these coils tolerate underpin our conviction that >30 T all-superconducting magnets are viable.

Conceptual Design of a Superconducting 30 T Solenoid Using Wire-in-Conduit Conductors

A conceptual design is presented for a distributed reinforcement conductor that enables the use of presently available, high current density, round cross-sectional wire for high field superconducting magnets. One of the disadvantages of providing separate structural reinforcement on the outer diameter of layer wound coils is the inherent thermal and mechanical stresses developed at the interface between the superconductor and reinforcement. This paper proposes a new concept, a single wire in conduit (WIC), for a distributed reinforcement conductor configuration which not only eliminates the problem of interface stresses, but introduces coolant directly to the superconducting wire where it will be more effective in providing a stable operating environment. The materials required for the proposed configuration are discussed and a conceptual design for a 30 T superconducting solenoid using the WIC conductor is described. The areas of WIC conductor technology requiring development and characterization are discussed.

Acoustic emission from superconducting magnets: the difference between layer wound coils and pancake wound coils

The energy counting rate of acoustic emission (AE) is measured as a function of energizing current for several simple-shaped unimpregnated superconducting magnets. A simple theory based on the frictional motion of the conductor predicts that the AE rate increases monotonically with increasing current. This is confirmed by experimentation using test coils. The practical magnets, however, exhibit a different behaviour. For layer wound coils, the AE rate has a peak in the high current region when the current increases. For pancake wound coils, the AE rate has a peak in the low current region. On the basis of these results, the mechanism of the characteristic AE behaviour of each type of winding is discussed.

超伝導マグネットからのアコースティック・エミッション 層状密巻コイルとパンケーキ巻コイルの違い

The energy counting rate of acoustic emission (AE) has been measured as a function of energizing current for several dry wound superconducting solenoid in order to pursue the frictional motion of the conductor during the energization. A simple theory based on the frictional motion predicts that the AE rate increases monotonically with increasing current. This has been confirmed by the experiment for some test coils. Some practical magnets, however, exhibit different behaviors, depending on the type of winding: layer winding and pancake winding. For the layer wound coils, the AE rate has a peak in the high current region when the current is increased. For the pancake wound coils, the AE rate has a peak in the low current region. These behaviors are also discussed on the basis of the conductor motion.

New winding techniques for superconducting dipole magnets

Superconducting, saddle-coil dipole magnets are used for bending and magnetohydrodynamic generator magnets. The uniform transverse fields may be produced by coils of tubular form with a sin θ current distribution or by uniform current density coils with cross sections of overlapping circles or ellipses. It is shown that overlapping circles and ellipses use less conductor. A simple new approximation to the overlapping circle or ellipse form is shown, in which pancake windings are used rather than the layer-type windings used previously. Pancake windings have an advantage in that the large transverse mechanical forces are transmitted through the pancakes without passing through the cooling channels as in layer-wound dipole coils. Pancake coils are simple to wind and permit an additional important innovation in that bell-shaped ends can prevent the field at the windings exceeding the uniform central field anywhere, thus allowing conductors to be optimized for actual working field rather than a higher peak field. Reduced end effects and high homogeneity are obtainable with these techniques. Models and tests are described.