Non-insulated Coil Research Articles

A novel and fast electromagnetic and electrothermal software for quench analysis of high field magnets

Abstract High-field superconducting REBCO magnets contain several coils with many turns. For these magnets, electro-thermal quench is an issue that magnet designers need to take into account. Thus, there is a need for a fast and accurate software to numerically model the overall performance of full-scale magnets. High temperature superconductors can be modeled using different techniques for electro-magnetic and thermal (finite element method) analysis. However, it takes a lot of time to model the electro-magnetic and electro-thermal behavior of superconductors simultaneously, especially for non-insulated or metal-insulated coils. In addition, most of the available methods ignore screening currents, which are an important feature of REBCO magnets. We have developed a novel software programmed in C++, which performs coupled electro-magnetic and electro-thermal analysis using variational methods based on minimum electro-magnetic entropy production and finite difference, respectively. The developed software, which takes screening currents into account, is applied to axi-symmetric full scale magnets of more than 32 T field strength under the SuperEMFL project for thermal quench reliability during standard operation. We show that magnets incorporating non-insulated coils are more reliable against quench than metal insulated coils. Also, realistic cooling conditions at the boundaries are essential for such simulations. The model developed can be used for a quick and complete electro-magnetic and electro-thermal analysis of superconducting high field magnets.

Relationship between magnetic field and tokamak size—a system engineering perspective and implications to fusion development

High temperature superconductors (HTSs) offer the promise of operating at higher magnetic field and temperature. Recently, the use of high field magnets (by adopting HTS) has been promoted by several groups around the world, including new start-up entries, both to substantially reduce the size of a fusion power reactor system and as a breakthrough innovation that could dramatically accelerate fusion power deployment. This paper describes the results of an assessment to understand the impact of using high field magnets in the design of DEMO in Europe, considering a comprehensive list of physics and engineering limitations together with the interdependencies with other important parameters. Based on the results, it is concluded that increasing the magnetic field does not lead to a reduction in device size with relevant nuclear performance requirements, because (i) large structures are needed to withstand the enormous electromagnetic forces, (ii) thick blanket and n-shield structures are needed to protect the coils from radiation damage effects, and (iii) new divertor solutions with performances well beyond today’s concepts are needed. Stronger structural materials allow for more compact tokamaks, but do not change the conclusion that scalability is not favourable when increasing the magnetic field, beyond a certain point, the machine size cannot be further reduced. More advanced structural support concepts for high-field coils have been explored and concluded that these solutions are either unfeasible or provide only marginal size reduction, by far not sufficient to account for the potential of operating at very high field provided by HTS. Additionally, the cost of high field coils is significant at today’s price levels and shows to scale roughly with the square of the field. Nevertheless, it is believed that even when not operated at high field and starting within conventional insulated coils, HTS can still offer certain benefits. These include the simplification of the magnet cooling scheme thanks to increased temperature margin (indirect conduction cooling). This in turn can greatly simplify coil construction and minimize high-voltage risks at the terminals.

Fast and accurate electromagnetic modeling of non-insulated and metal-insulated REBCO magnets

REBCO high-temperature superconductors are promising for all-superconducting high-field magnets, including ultra-high field magnets. Non-insulated (NI) and metal-insulated (MI) windings are a good solution for protection against electro-thermal quench. Design and optimization requires numerical modelling of REBCO inserts for high-field magnets. Here, we detail a fast and accurate two-dimensional cross-sectional model for the electromagnetic response of NI and MI coils, which is based on the minimum electro magnetic entropy production. Benchmarking with an A − V formulation method on a double pancake coil shows good agreement. We also analyse a fully superconducting 32 T magnet with a REBCO insert and a low-temperature superconducing outsert. In particular, we analyse the current density, the screening current induced field (SCIF), and the AC loss. We have shown that metal-insulated coils enable transfer of angular current in the radial direction, and hence magnet protection, while keeping the same screening currents and AC loss of insulated coils, even at relatively high ramp rates of 1 A s−1. Surprisingly, soldered coils with low resistance between turns present relatively low AC loss for over-current configuration, which might enable higher generated magnetic fields. The numerical method presented here can be applied to optimize high-field magnets regarding SCIF in MI or NI magnets. It also serves as the basis for future electro-thermal modelling and multi-physics modelling that also includes mechanical properties.

Performance of demountable solder joints for no-insulation superconducting coils produced by vacuum pressure impregnation

No-insulation high temperature superconductor (HTS) stack coils show both increased thermal and electrical stability, and present a simplified geometry for the integration of demountable joints. Demountability is a desirable feature for many superconducting applications including fusion magnets, which motivate this research. In this work, a novel vacuum pressure impregnation (VPI) solder process was developed to couple superconducting paths via a low resistance, mechanically simply, demountable joint for a non-insulated coil design. The three low temperature solders considered were (mp = 118 C), (mp = 156.6 C), and (mp = 29.8 C), all with a lower melting point than the (mp = 183 C) used to solder the HTS tapes into the coil, thus allowing for the advantages of solder in the joint, yet facilitating demountability without disturbing the primary solder in the non-insulated coil stack. A multidisciplinary campaign was undertaken to design, build, test and identify the major challenges with these small-scale demountable solder joints. A multi probe voltage tap system was used to infer the effective resistances to exit a superconducting stack, cross the solder layer, and enter the second superconducting stack, at 77 K. The experimental resistivities show good agreement with a newly developed finite element model that breaks down domains to the level of individual layers of an HTS tape. When taking into account the thermal degradation that can occur to the HTS stacks during the VPI processes, the normalized joint resistances are found to be 528, 668, and 671 for the , and solders, respectively. The benchmarked finite element model is used to predict normalized joint resistivities for fusion-relevant temperatures and magnetic fields using a 100 µm solder layer thickness, finding 38, 125, and 103 for the respective solders; these results are competitive with the lowest resistance cable joints presented in the literature.

Enhancement of high-temperature superconducting coil stability using doped smart insulation materials for superconducting magnet applications

This study reports a high-temperature superconducting (HTS) coil containing molybdenum doped vanadium trioxide (V2O3:Mo) as turn-to-turn insulation material to ameliorate the electrical properties of both insulated and non-insulated (NI) coils. The electrical characteristics and thermal stability of the V2O3:Mo insulated coil against NI and V2O3 insulated coils were investigated through charge-discharge, over-current, and quench tests. The findings revealed that the V2O3:Mo and V2O3 insulated coils demonstrated a faster charge/discharge rate than the NI coil due to the high resistance of the V2O3: Mo or V2O3 insulator which obstructed the current to flow away from the azimuthal current path. In the over-current test at 160A, the V2O3:Mo insulated and NI coils exhibited higher electrical stability than the V2O3 insulated coil because the excessive currents above their critical values bypassed between the turn-to-turn contact. Moreover, the V2O3:Mo insulated coil demonstrated a 2.1times improvement in the minimum quench energy compared to the V2O3 insulated coil at 70 % of the coil's current-carrying capacity, which is an essential performance parameter for the operation of superconducting magnets.

Aging effect on copper stabilizers in second-generation high-temperature superconducting tapes

Copper stabilizers play an essential role in second-generation high-temperature superconducting (2G-HTS) tapes. Corrosion or aging of the copper stabilizer is unavoidable and affects the tape performance. In this study, aging tests were performed on fresh copper-plated 2G-HTS tapes under steady-state damp heat conditions, in accordance with the IEC 60068–2-78:2012 regulations. To accelerate the aging, the conditions were set as 40 °C, a relative humidity of 93%, and test durations of up to 48 h. The surface contact resistivity (ρc) in liquid nitrogen was measured for fresh and aged specimens under normal pressure of up to 160 MPa. The results indicated that ρc increased with the test duration and tended to become saturated. After 48 h aging tests, the ρc of the specimens was three times higher that of the fresh 2G-HTS tapes. Moreover, the microstructure of the copper stabilizer was characterized. Height fluctuations were observed on all the specimen surfaces, which might be the reason for the ρc scattering. X-ray photoelectron spectroscopy elemental analysis revealed higher amounts of carbon and oxygen on the surfaces of the specimens after aging, which significantly reduced the purity of the copper stabilizer. It was also found that Cu2O was the main product on the tape surface during the reaction with oxygen and, water vapor and so on. This study provides a reference for the design of non-insulated coils and the preservation of 2G-HTS tapes.

Modelling the Quench Behavior of an NI HTS Applied-Field Module for a Magnetoplasmadynamic Thruster Undergoing a 1kW Discharge

Recent advances in commercial miniaturised cryocoolers and high-temperature superconductors (HTS) have revived the discussion of using HTS electromagnets to enhance the thrust and efficiency of electric thrusters for space applications. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">An HTS applied-field magnetoplasmadynamic (AF-MPD) thruster is currently being developed. While the thruster is operating, there will be a large time-variable heat load on the cryogenic environment. The operation of a low-power cryocooler and energised HTS coils (operating at 70 K) adjacent to streams of hot plasma and large electrical discharges (on the order of 1 kW) represents a significant thermal management problem. The electromagnetic and thermal behaviour of non-insulated (NI) coils under these conditions, and how resilient they are to quenching during thruster operation, is not well understood. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, a model is formulated to study the transient electromagnetic and thermal behaviour of an HTS-AF-MPD thruster with NI coils. The thruster and a conduction cooled cryogenic design are coupled via surface-to-surface radiation heat transfer. The model predicts current flow within the HTS, copper stabiliser and between turns, with the contact resistivity being a key input variable. Critical current is determined locally using temperature, magnetic field, and field angle in combination with a measured data set for a specific conductor. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">This model reveals conditions where the cryocooler can passively compensate for large instantaneous heat loads on the coils, demonstrating quench resistance.

Characteristics of Non-Insulated High-Temperature Superconductor Coils in Time-Varying Magnetic Fields

Non-insulated coils made of second-generation high-temperature superconductors (HTSs) have been developed to prevent the burning of superconducting wires. Although these coils perform reasonably well in a constant magnetic field, the induced current in the coil is a serious concern in a time-varying magnetic field. Hence, we aimed to determine the characteristics of non-insulated HTS coils in a time-varying magnetic field. We manufactured two HTS coils: 1) non-insulated coil and 2) metal-insulated coil with a higher turn-to-turn resistance. Experiments were performed under a time-varying field generated using the stator of a 1 MW rotating machine. The HTS coil in the rotating machine was severely affected by an asynchronous or transient magnetic field. In addition, we designed a special structure to simulate severe operating conditions. As a result, the magnetic field of the HTS coil decreased in the time-varying field. The metal-insulated coil with a higher turn-to-turn resistance exhibited a lower field reduction than the non-insulated coil. We also performed circuit analyses on the experimental results. Our findings suggest the feasibility of applying HTS coils without insulation in rotating machines.

Modeling HTS non-insulated coils: A comparison between finite-element and distributed network models

High-temperature superconducting (HTS) non-insulated (NI) coils have the unique capability to bypass current through conductive turn-to-turn contacts, mitigating the possibility of a catastrophic failure in the event of a quench. However, this turn-to-turn conductivity leads to a significant increase in the coil decay/charging time constant. To understand this phenomenon, several modeling techniques have been proposed, including the lumped and distributed network (DN) circuit models, and more recently the finite-element (FE) models. In this paper, the decay results obtained from modeling HTS NI pancake coils using both a DN model and a 2D FE model approach are evaluated and compared. Steady-state fields, and transient charging and decay behaviors are calculated with each model and the results compared. Key differences are highlighted, including the computation speed and the capturing of various physical phenomena. Both models exhibit non-exponential decay during initial coil discharge due to current redistribution between the inner and outer turns. In addition, the FE model exhibits other effects arising from current redistribution in both the radial and axial directions, including remanent magnetization, and variation of the “apparent total inductance” during charging. Simulations of sudden discharge have also been analyzed using the common “lumped circuit” formula. This shows that extracted values for the apparent surface contact resistance between coil windings can differ by more than a factor of 5 from the initial input value. Our results confirms the optimal choice of architecture for future NI coil models and emphasize that caution should be exercised when interpreting experimental results using the lumped circuit approach.

Varistor Insulation for HTS Magnets

A variable resistance thin dielectric insulation coating for REBCO tape HTS coils has been developed. This new type of insulation system switches between high and low resistance, after an increase in inter-turn voltage. Non-Insulated (NI), fully soldered, HTS coils have proven to be very reliable; NI coils are achieving high magnetic fields above 25 Tesla and are almost impossible to quench. Over-current operation simply redirects the excess current out of the superconducting tape, to flow radially through the coil then back to the power supply. The internal coil resistance can then run the current down when the power supply is switched off. The disadvantage with NI coils is, as the coil volume and inductance increase, the charging / discharging time can take many hours, even days. This is not compatible with magnet systems that need accurate and fast current to magnetic field control, such as accelerators or other systems. With the Varistor Insulation (VI) we aim to achieve both robust performances as seen in NI coils and fast ramping with controlled current to field transfer functions. In this paper we present the electrical characterization of the insulation at room temperature and cryogenic temperatures, along with simulated magnet operation during ramping, normal operation and failure modes. We discuss other features of the VI insulation such as, application methods to provide thin layers, and alternative formulations to tune its properties. Its ability to act as a distributed quench heater when the voltage threshold is exceeded is also discussed.

Design of a Cloverleaf-Racetrack Dipole Demonstrator Magnet With Dual ReBCO Conductor

For future 20+ T accelerator type magnets, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO superconductors are ideal for their high current density in high magnetic field. At CERN, demonstrator dipole magnets using <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO conductor are being developed in order to study their feasibility. In this work, a design of a cloverleaf-racetrack magnet is presented, which consist of two poles. A key problem in such magnets is how to realize the coil-ends without causing degradation due to coil winding, cool down and operation. The cloverleaf geometry accommodates the particle beam pipe without any hard-way <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO tape bending. To design such geometry of the coil-end, a new method using Bézier splines is presented. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO coils are wound with dual tape conductor where the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO layers are facing each other. For quench protection, the coils are non-insulated, allowing the coil current at quench to redistribute transversally through the winding pack. A comparison between dry-wound and soldered non-insulated coils is made. Progress on the electromagnetic and mechanical design of the cloverleaf magnet is reported in this paper.

Effective Time Constants at 4.2 to 70 K inReBCO Pancake Coils With Different Inter-Turn Resistances

For future <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO tape based accelerator magnets it is proposed to use no- or partial inter-turn insulation to deal with quench detection and protection. In a non-insulated coil the turns are separated by a finite electrical resistance, providing a bypass for the current at hot-spots, improving thermal stability and quench detection time. However, such coils show different dynamic electromagnetic behavior compared to insulated coils under normal charging and transient quench conditions. To study such coils in detail two pancake coils, one dry-wound and one with solder in between turns, are prepared and tested in a variable temperature cryostat between 4.2 and 70 K. Properties of the coils that are studied are charge and discharge time behavior, turn-to-turn resistance, response to current stepping, and operational stability. In this paper, the first results are presented and compared to a simplified network model in order to gain further understanding into the underlying physics.

A Pulsed Current Charging Mechanism for High Temperature Superconducting Magnets

Persistent Current Switches (PCS) which switch between superconducting and resistive states enrich the operation scenarios of superconducting magnets. A commonly used thermally controlled PCS fits DC current feeding but too slow for AC or pulsed current feeding for the magnets. Thanks to the high thermal stability of high temperature superconductors, pulsed current feeding becomes feasible with a practical over current PCS control strategy, which is verified through experiments on a non-insulated coil made of ReBCO tapes in this work, leading to a novel charging procedure for superconducting magnets. The structure is simple and straightforward, and has a prospect in the field of excitation and field supplement for a superconductingmagnet.

Design, construction, and testing of no-insulation small subscale solenoids for compact tokamaks

Fusion energy systems studies (FESS) for next-step devices based on the most promising magnetic configurations indicate that high magnetic fields and high current density for magnet coil systems may reduce device size and lower the cost. High current density and radiation resistant fusion magnets are particularly beneficial for low cost, low aspect ratio compact reactor designs such as Fusion Nuclear Science Facility (FNSF), Fusion Pilot Plant (FPP) of low-aspect ratio spherical tokamak (ST) or compact stellarators. Unlike typical high field magnets for nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), or high energy physics research applications, neutron irradiation damage to organic insulations in the coil winding pack is a critical issue for next generation fusion reactors where orders of magnitude higher neutron fluence than those in present experimental reactors such as ITER are expected. Moreover, a high coil winding pack current density is needed for high field magnets in low cost, compact radial build next step fusion reactors. The slow current charging time, however, is an issue in fully non-insulated coils. We present the design, construction and testing of subscale Nb3Sn solenoids, up to half the diameter of the central solenoid (CS) for National Spherical Torus Experiment (NSTX), with and without inter-layer insulation for enhancing radiation resistance while improving the winding pack current density and coil performance. The major radius for the NSTX/NSTX-Upgrade (reference for compact ST) is 0.854/0.934 meter, and 4.8 meters for FNSF. The magnetic field for NSTX/NSTX-U at the plasma center is 0.6/1 Tesla and 9 Tesla for FNSF. The inner diameter of the central solenoid coil is 0.2/0.4 meter for NSTX/NSTX-U and 1.2 meters for FNSF. The current charging and discharging behavior of the prototype solenoids was investigated to quantify performance advantages of a simplified coil fabrication without error-prone vacuum pressure impregnation (VPI) for the removal of epoxy organic insulation. Scalability of the no insulation concept for fusion can be addressed via engineered coating for novel insulation on contact resistance or intra-layer no insulations. Radiation resistance and coil winding efficiency were significantly improved in the no insulation coils for next step compact fusion reactors.

Measurement of parameters of the superconducting non-insulated coils

The electrical characteristics of superconducting coils with non-insulated windings are studied. The procedures for measuring the parameters of uninsulated superconducting windings are described. In particular, the inductance is measured by voltage with a linear current input at a given rate. Attention is focused on the impossibility of correctly determining the inductance in a winding with an uninsulated superconductor in a normal state. It is noted that in a superconducting state at currents below the critical value, the inductance of the winding is comparable to the inductance with an insulated wire. The results of measurements of inductance, radial resistance, static current-voltage and magnetic characteristics of two tape coils with non-insulated superconducting windings, one of which had a soldered connection, are presented. Conditions for measuring the parameters of non-insulated superconducting windings are formulated when they are compared with insulated windings.

PCB-Based Current Sensor Design for Sensing Switch Current of a Nonmodular GaN Power Semiconductor

GaN-based power semiconductors exhibit small on-resistance and high dv/dt of the switch characteristics, thereby enabling the construction of high-efficiency, high-density semiconductor systems with fast switching and low power loss characteristics and miniaturization of passive devices. However, owing to the characteristics of GaN devices that result in them being significantly faster than other devices, the accuracy of the switching transient response significantly affects the noise or inductance in the device. Therefore, securing sufficient sensor bandwidth is considerably important for accurate current measurement in GaN-based devices. Conversely, the current sensor in the form of a non-insulated coil must secure sufficient bandwidth and overcome the tradeoff relationship with measurement sensitivity; moreover, the sensor configuration must be applicable to various power semiconductor types. This study proposes a current sensor model that applies the principle of the printed circuit board Rogowski coil to a surface mount device-type GaN-based half-bridge structure. This structure is applicable to a nonmodular power converter and is designed to secure sufficient bandwidth with a minimum area while simultaneously exhibiting high sensitivity. For the coil design, mutual inductances with existing coil structures were compared and analyzed, and the frequency response and magnetic analysis were evaluated. Experimental verification was performed and the transient response characteristics in various DC voltage ranges are discussed.

Fluorescent thermal imaging of a quench in insulated and non-insulated REBCO-wound pancake coils following a heater pulse at 77 K

High temperature superconductors (HTS)-wound coils are being developed for use in motors, generators as well as magnet applications. Determining the stability and safe operating margins of such coils still poses challenges. While the recently introduced no-insulation winding method provides a remedy for many problems, it comes with its own limitations. For comparison, we have wound two pancake coils from HTS coated conductors with the insulated and non-insulated winding techniques. Both coils were coated with a fluorescent, temperature-sensitive coating, which allowed monitoring the surface temperatures during operation. The coils were cooled to 77 K via a combination of conduction and gas cooling, and their electrical and thermal behaviour was observed in operation. Here we present the normal transition of both coils caused by an artificially introduced instability due to a surface-mounted, resistive heater element. In the insulated coil, the localized disturbance caused a local transition of the superconductor to the normal conducting state, triggering a thermal runaway. Merely the turns in contact with the artificial disturbance heated up, while the rest of the coil remained in the superconducting state. In the non-insulated coil—although a much longer heater pulse was required—the normal transition started from the weakest point of the coil (around the bobbin) and the whole coil was heating thereafter, with the centre heating more.

Electrical and thermal analyses of a second generation high temperature superconducting magnet with vanadium III oxide and Kapton polyimide film insulation materials under an over-pulse current

In this study, we developed a novel simulation code to analyze a second-generation high-temperature superconductor (HTS) magnet that was insulated with a polyimide film and vanadium III oxide (V2O3). In particular, V2O3 is one of the metal–insulator transition materials that has resistivity that varies with temperature. For its commercialization for large-scale HTS magnet applications, the HTS magnet that uses V2O3 is expected to achieve both of the desired characteristics, specifically, (a) high electrical and thermal stabilities for the non-insulated coils, and (b) prompt controllability of the insulated coils. First, an equivalent electrical-circuit model is proposed to develop a simulation code for the analysis of electrical and thermal characteristics of polyimide and V2O3. Then, all simulated results were compared with experimental results of a small-scale HTS single pancake coil and discussed to verify the reliability of the developed simulation code.

Flux pumping for non-insulated and metal-insulated HTS coils

High-temperature superconducting (HTS) coils wound from coated conductors without turn-to-turn insulation (non-insulated (NI) coils) have been proven with excellent electrical and thermal performances. However, the slow charging of NI coils has been a long-lasting problem. In this work, we explore using a transformer-rectifier HTS flux pump to charge an NI coil and a metal-insulated coil. The charging performance comparison is made between different coils. Comprehensive study is done to thoroughly understand the electrical-magnetic transience in charging these coils. We will show that the low-voltage high-current flux pump is especially suitable for charging NI coils with very low characteristic resistance.

An Experimental Investigation of the Transient Response of HTS Non-insulation Coil

A single pancake coil without turn-to-turn insulation was tested in this paper to investigate the transient responses under different situations. We performed charging and discharging test, AC current test, and regional quench emulation test on the non-insulated (NI) coil. The experimental test results show a significant time delay for charging and discharging characteristics of NI coil and can be validated by a simple proposed equivalent electrical circuit. Under the AC operating current, the NI coil can bypass nearly all the AC current from the coil spiral path to the radial path such that it is not possible for NI coil to store or be affected by the AC magnet field. Additionally, while carrying AC current, the AC loss dissipation of NI coil is inversely proportional to the frequency of the AC operating current. When a regional quench occurs, the NI coil can bypass the current in the regional quench zone to avoid further temperature accumulated and protect the NI coil itself.