Pulsed Coils Research Articles

Design and operation of a sequentially-fired pulse forming network for non-linear loads

While the construction of a linear pulse forming network (PFN) for a constant load impedance is relatively easy, the process is more difficult for a nonlinear or time-varying load. A passive PFN can certainly be synthesized for nonlinear loads, but is usually large and lacks the flexibility to be truly useful in most practical and research applications. This investigation describes the design and construction of a sequentially-fired pulse forming network (SFPFN) that maintains constant voltage and current for a nonlinear load. Operation of the SFPFN consists of charging multiple capacitor banks (or modules) to various levels and sequentially firing these banks into the load at appropriate times. The load and its characteristics determine the module charge voltage. An added benefit with the SFPFN is real-time computer monitoring and control allowing the PFN modules to be charged from a single prime power source via a group of switching relays. The nonlinear load used in this investigation is a helical coil electromagnetic launcher (HCEL). The SFPFN is also tested with a linear load consisting of a pulsed field coil. The nonlinearity of the HCEL is well-known with a factor of 2 change in winding resistance due to joule heating and a large variation in terminal voltage due to changes in the armature back-voltage. Experimental measurements show the SFPFN can deliver a relatively constant current pulse on the order of 5-15 kA into the HCEL load for a pulse length up to 8 ms. The maximum SFPFN operating voltage is 900 V with a total stored energy of 125 kJ. Scaling the SFPFN to larger or smaller pulse amplitudes or lengths is possible.

Cryo-Permanent Magnets—Geometry, Magnetization and Cost Issues

High trapped peak magnetic fields obtained for melt-textured 123-high-temperature superconductor (HTS) bulk cylinders, make these materials highly attractive for cryo-permanent magnets (CPMs). However, to transfer the proven performances of HTS materials into practical magnetic devices with magnetizations of /spl les/5 T over /spl ges/10 cm size, issues related to geometry, magnetization and cost have to be addressed. HTS ring structures appear to be more adequate than just cylindrical bulk parts but the demands for growing, shaping and assembling the HTS materials increase. A novel process is introduced to in-situ magnetize the HTS ring using pulsed copper coils. It involves the injection of an extra current which is induced by the magnetizing pulse field. This allows flux penetration into otherwise strongly shielded rings, and thus reduces the required peak magnetic field. Concepts for "medium-size" assembled structures and results of model calculations for corresponding magnetization processes are presented. Preliminary cost comparisons with winding-based solutions are given as well.

Stability Evaluation of a Conduction-Cooled Prototype LTS Pulse Coil for UPS-SMES

The stability of a prototype conduction-cooled LTS pulse coil for UPS-SMES of 100 kJ was evaluated. This coil has been developed as a first step of a project to develop a 1 MW, 1 UPS-SMES to protect semiconductor chip production equipment and nuclear fusion experimental devices, etc, from momentary voltage drop and power failure. The winding conductor is an NbTi/Cu Rutherford cable, which is extruded with aluminum. This conductor has both low AC losses and high stability under specified orientation of changing transverse magnetic fields. The 100 kJ-coil are wound by the new winding method. In order to improve the heat conduction properties in the coil, Dyneema FRP and Litz wires are used as spacers. Litz wires were connected with the cryocooler as cooling paths. On the pulse operation, the operating current is reduced from 1000 A to 707 A in 1 s. In this paper, the thermal properties of the 100 kJ-coil are calculated by finite element method under pulse operation. In order to estimate the stability, a calibration experiment was carried out. Results indicated that our prototype LTS pulse coil has high stability to enable to allow over 10 times as large heat as AC losses.

Prototype Development of a Conduction-Cooled LTS Pulse Coil for UPS-SMES

We are planning to develop a 1 MW, 1 sec UPS-SMES for a protection from a momentary voltage drop and an instant power failure. As the first step, we have been developing a 100 kJ class prototype UPS-SMES, using a low temperature superconducting coil because of its better cost and performance over the high temperature superconducting coil. However, the difficulty to utilize a pool-boiling LTS pulse coil is the reliability of operation. To solve this problem, a conduction-cooled LTS pulse coil has been designed and fabricated as a key component of the UPS-SMES. The reduction of AC loss and high stability are required for the SC conductor for the conduction-cooled coil because of a limited cooling capacity. The SC conductor of a NbTi/Cu compacted strand cable extruded with an aluminum is designed to have the anisotropic AC loss properties to minimize the coupling loss under the specified orientation of the time varying magnetic field. The coil was wound with a new twist-winding method in which the variation of twist angle of the conductor was controlled with the winding machine designed specifically for this purpose. The fabrication technique and performance of a conduction-cooled prototype LTS pulse coil are described.

Optimising ultrasonic wideband Rayleigh wave generation by pulsed electromagnetic coils

Transient ultrasonic Rayleigh wave generation by pulsed electromagnetic coils operating via the Lorentz force mechanism is investigated. Optimal coil design is calculated by considering the entire ultrasonic generation system including the pulse generator, coil shape and size, and the electromagnetic coupling between coil and metal sample. The study is carried out through theoretical modelling and comparison with experimental measurements taken using an 80 MHz bandwidth Michelson laser interferometer as a detector. The time dependent Lorentz force generated by the electromagnetic acoustic transducer (EMAT) is modelled using finite element calculations, and the resultant ultrasonic field is calculated using an efficient numerical algorithm. Excellent correlation is observed between the simulated waveforms and experimental measurements. This demonstrates how the approach reported here will greatly assist in optimising the design of generating EMAT for specific applications.

系統安定化用SMESのコスト低減技術開発

The authors have developed a solenoid model coil to stabilize the power system in superconducting magnetic energy storage (SMES) system with the aim of drastically reducing system cost. The single solenoid model coil is designed with the rated current, maximum magnetic field and coil charge rates equivalent to those of a practical 100 MW/15 kWh-class SMES using multi-pole solenoid coils. In addition, the coil is characterized by the use of a stabilized Al-NbTi CIC conductor that has a graded structure, layered winding and a high magnetic field with a compact design. To verify coil performance, several tests such as a the measurement of rated current and AC loss, high-speed pulse operation, coil quenching, and insulation characteristics were conducted using a model coil at Kyushu Electric's Imajuku testing center. Test results proved that the SMES model coil performs as well as the initially designed coil, while doing so at a drastically reduced cost.

An FDTD Model for Calculation of Gradient-Induced Eddy Currents in MRI System

In most magnetic resonance imaging (MRI) systems, pulsed magnetic gradient fields induce eddy currents in the conducting structures of the superconducting magnet. The eddy currents induced in structures within the cryostat are particularly problematic as they are characterized by long time constants by virtue of the low resistivity of the conductors. This paper presents a three-dimensional (3-D) finite-difference time-domain (FDTD) scheme in cylindrical coordinates for eddy-current calculation in conductors. This model is intended to be part of a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The singularity apparent in the governing equations is removed by using a series expansion method and the conductor-air boundary condition is handled using a variant of the surface impedance concept. The numerical difficulty due to the "asymmetry" of Maxwell equations for low-frequency eddy-current problems is circumvented by taking advantage of the known penetration behavior of the eddy-current fields. A perfectly matched layer absorbing boundary condition in 3-D cylindrical coordinates is also incorporated. The numerical method has been verified against analytical solutions for simple cases. Finally, the algorithm is illustrated by modeling a pulsed field gradient coil system within an MRI magnet system. The results demonstrate that the proposed FDTD scheme can be used to calculate large-scale eddy-current problems in materials with high conductivity at low frequencies.

Development of ‘Poly-Layer’ Assembly Technology for Pulsed Magnets

Historically the analysis of pulsed magnets has indicated that a coil assembly comprised by a layer-by-layer graded conductor coil is nearly optimum. Additionally the layer-to-layer winding transitions, associated with monolithic coil construction, are problematic and often the source of catastrophic failure in pulsed coils. FEA analysis of transition structures has confirmed the lessons of operational experience. Additionally the layer-to-layer winding transitions present an unacceptable risk a short circuit fault to the reinforcement structure. These attributes of monolithic coil construction have limited the fields where pulsed magnets can be operated to approximately 60 T. Several operational issues arise above this field: coil reliability degrades, and the time required to cool the magnet between shots is increased, since higher field monolithic coils require large amounts of distributed internal reinforcement to limit conductor strain. We are developing a new 'poly-layer' coil construction to address these engineering requirements. In this construction, each layer is wound separately on a forming mandrel, and then installed onto an assembly mandrel. Spool pieces are installed at each end to support the lead exits as they transition out of the windings. Each layer is joined in series with the next in joint structures that is constrained in the radial and circumferential directions, but free to move with the coil in the axial direction. Furthermore, the coil reinforcement is used to support the lead in the transition region between the windings and the joint. The new design represents a change in our manufacturing template that is intended to allow development of higher field coils. This paper report design details and the results from prototype testing.

Ion deposited nanoscale permalloy and Co films generated from laser excitation

A modified pulsed laser deposition system has been used to deposit permalloy and Co films from ions generated due to the impact of 248 nm excimer laser pulses. A synchronously pulsed magnetic field coil with a wide entrance throat and a tapered bore in conjunction with a set of permanent magnets has been used to capture and concentrate the ion and electron beam flux under vacuum conditions onto the substrates. Co and other films such as diamond like carbon (DLC) are strongly adherent when deposited onto even room temperature substrates. Permalloy films deposited onto Si (111) substrates with thicknesses from 10 to 50 nm exhibit anisotropic magnetoresistance of approximately 2% for magnetic fields applied in-plane parallel versus perpendicular to the current direction. Over 1 order of magnitude intensity modulation has been observed in x-ray reflectivity measurements for Co films, 7–15 nm thickness, deposited onto R-plane sapphire substrates and onto Si (111) substrates. In contrast to this Co films simultaneously deposited onto A-plane sapphire substrates hardly exhibited x-ray reflectivity fringes indicating a large surface roughness. Similarly DLC films deposited onto (111) Si substrates exhibited order of magnitude intensity modulations indicating a very small surface roughness. Scanning electron microscope measurements indicated smooth films were obtained with no discernible particulates.

Development of UPS-SMES as a Protection From Momentary Voltage Drop

We have been developing the UPS-SMES as a protection from momentary voltage drop and power failure. The superconducting system is suitable as electric power storage for large energy extraction in a short time. The most important feature of superconducting coil system for the UPS-SMES is easy handling and maintenance-free operation. We have selected low temperature superconducting (LTS) coils instead of high temperature superconducting (HTS) coils from the viewpoint of cost and performance. However, it is difficult for the conventional LTS coils to fulfill maintenance-free operation since the cooling methods are either pool boiling with liquid helium or forced flow of supercritical helium. Thus, a conduction cooled LTS pulse coil has been designed as a key component of the UPS-SMES. The development program of 1 MW, 1 sec UPS-SMES is explained.

Winding Techniques for Conduction Cooled LTS Pulse Coils for 100 kJ Class UPS-SMES as a Protection From Momentary Voltage Drops

In order to develop the 100 kJ class UPS-SMES as a protection from momentary voltage drops, design of the conduction cooled LTS pulse coil was carried out and special winding machine has been developed. Such coil is required to simultaneously attain low AC loss and high stability and the distributions of temperature in the coil are sensitively controlled. For this purpose, an aluminum stabilized conductor with circular cross-section composed of a Cu stabilized NbTi Rutherford cable was used as the winding conductor, and in the winding process the twist angle of the conductor around its axis was controlled to adjust the direction of edge-on orientation to the Rutherford cable to direction of local transverse magnetic fields applied to the conductor in winding area of the coil. The developed winding machine is used for this winding method. As a result, conduction cooled LTS pulse coil can be expected to operate stably in adequate temperature margin.

Pulse Magnet Development Program at NHMFL

In 1989, the NHMFL pulsed magnetic field user facility was set up at the Los Alamos National Laboratory. Two programs were initially conceived: 1) The construction of a 60 T long pulse magnet energized by the 600 MJ, 540 MW generator, and 2) a high field magnet program for the 1.6 MJ/32 mF capacitor bank with an operating range of 10 kV. These magnets are designed and built at the NHMFL. Our pulse coil development has its roots in the work accomplished by the group in Leuven which served as a starting point for our technical development programs. After having established the technology, we have focused our efforts on increased reliability. Our present user magnet design is based upon the use of distributed MP35N and Zylon reinforcement. These 60 T short pulse (5.7 ms pulse rise time) and 50 T mid-pulse (40 ms pulse rise time) magnets have become the "workhorse" magnets of the pulsed magnetic field user facility since the failure of the first 60 T long pulse magnet. Coil longevities of 1500 shots, of which half are at full field, are becoming the standard. Presently, the short pulse group is engaged in several programs to increase the science opportunities at our user facility. The development activities encompass fast-cool magnet systems, allowing a shot at least every 20 minutes, the production of a new generation of user coils for higher fields, and a 100 T insert. We present a technical overview of our development programs and new coil designs, including materials development and characterization, identifying those areas critical to progress toward even more reliable and stronger magnetic fields.

Magnetic resonance imaging for the assessment of myocardial viability.

The identification of myocardial viability in the setting of left ventricular (LV) dysfunction is crucial for the prediction of functional recovery following revascularization. Although echocardiography, positron emission tomography (PET), and nuclear imaging have validated roles, recent advances in cardiac magnetic resonance (CMR) technology and availability have led to increased experience in CMR for identification of myocardial viability. CMR has unique advantages in the ability of magnetic resonance spectroscopy (MRS) to measure subcellular components of myocardium, and in the image resolution of magnetic resonance proton imaging. As a result of excellent image resolution and advances in pulse sequences and coil technology, magnetic resonance imaging (MRI) can be used to identify the transmural extent of myocardial infarction (MI) in vivo for the first time. This review of the role of CMR in myocardial viability imaging describes the acute and chronic settings of ventricular dysfunction and concepts regarding the underlying pathophysiology. Recent advances in MRS and MRI are discussed, including the potential for dobutamine MRI to identify viable myocardium and a detailed review of the technique of delayed gadolinium (Gd) contrast hyperenhancement for visualization of viable and nonviable myocardium.

50 T pulsed field coils using multi-composite wire

The multi-composite (MC) wire is a new concept in the pursuit of obtaining an ideal conductor for high-performance pulsed magnets. Prototype MC wires were made with an ultimate tensile strength of 1.2 and 1.7 GPa and a conductivity of 40% IACS at 300 K. With the 1.2 GPa wire, a series of three coils with 26 mm bore were wound that performed well above expectation. Using the 1.7 GPa wire, a coil was designed and manufactured with similar bore (18 mm) and pulse duration as our standard 50+ T user coils with internal reinforcement. This coil generated 53 T before failure, even without internal stress optimization; there is still much room for improvement.

Development of reliable 70 T pulsed magnets

A capacitor-driven pulsed magnet coil has been designed to generate fields in the 70–75 Trange, with a life expectancy of at least 100 pulses, thus qualifying as a ‘75 T class usermagnet’. The bore is 10 mm and the rise time used in our experiments is 4 ms. The coilconsists of two coaxial sections: the inner section, where stresses are highest, is made withCuNb microcomposite wire and optimized Zylon reinforcement; the outer section is madewith soft copper and glass fibre composite. In the inner section, the stress in each layer isself-contained, while the stresses induced in the outer section are transmittedto a thick shell made from steel and carbon fibre composite. The cross sectionof the copper wires is adjusted to redistribute the heating evenly between theinner and the outer section. Another innovative design feature is a system foraxial compression that can be easily retightened during coil training. Two nearlyidentical coils were manufactured and tested to 72 T; this is a limit imposed due tooverheating when using our 10 kV, 0.5 MJ capacitor bank (at an energy of 380 kJ).At 75 T, the calculated von Mises stress in the Zylon composite is 2.6 GPa, wellbelow the UTS of more than 3 GPa, and the CuNb wire is still in an elastic state.

Inductive heating on a NbTi CICC magnet: energy calibration and stability analysis

An experiment on a NbTi coiled cable-in-conduit conductor has been carried out, aimed at measuring its stability under the disturbance conditions determined by the discharge of a capacitor bank on pulsed coils, which produces rapidly varying fields, uniformly distributed over the conductor length. The energy amount actually deposited on the superconducting strands during the external field variations has been determined, on the basis of an experimental analysis of the AC losses and the results of a semi-empirical model of AC losses in saturation regime. The heating power has been taken as input for the simulation code Gandalf: a good agreement has been found between the simulated and the measured evolution of temperature along the conductor length during the inductive disturbance. Transient stability has been measured, as function of electromagnetic and thermo-hydraulic variables. A satisfactory agreement has been obtained in the stability diagrams between experimental data and the results of numerical simulations.

Dependence on winding tensions for stability of a superconducting coil

The purpose of this study is to manufacture a high performance superconducting pulse coil by using high strength polyethylene fiber (DF; Dyneema ® fiber) reinforced plastic (DFRP) or Dyneema–glass hybrid composite fiber reinforced plastic (DGFRP) as material of a coil bobbin, which has negative thermal expansion, low frictional coefficient and high thermal conductivity. Description in this paper is as follows. First, thermal strains of several kinds of FRP pipes made by filament-winding (FW) method are measured, and the measured results well agreed with calculated ones by our proposed calculation method about thermal strains of a FW-pipe form. This shows that thermal expansion can be controlled by the proposed design technique of a DGFRP FW-pipe. Moreover, frictional coefficients of FRP plates using as coil structural material are measured and frictional heats are calculated for respective material when contact forces are changed. From these results, we find that the lower winding tension of a coil generates the smaller frictional heat when the frictional coefficient of the coil structural material is low. Furthermore, we systematically measure quench characteristics of many specimens of small superconducting coils using DFRP or DGFRP bobbins with different thermal expansions. From the results of quench tests, we find that the higher winding tension’s coils tend to decrease quench current when the coil’s bobbin expanded to a direction of circumference during the cooling down to cryogenic temperature, and suitable values of winding tension in a coil are located in region of 2–4 kg/mm 2. Finally, we design and manufacture 100 kJ class superconducting pulse coil by using a DGFRP bobbin, which wound in winding tension of 4 kg/mm 2. In addition, we prepare another 100 kJ class coil with the higher winding tensions of 8 kg/mm 2, and the quench characteristics of the coils are compared. The quench currents in the coils exceed the 95% rating on the load line for critical current value of a conductor, and a coil with winding tension of 4 kg/mm 2 is more stable.

The Effects of Pulse Configuration on Magnetic Stimulation

A study is presented in which the authors have examined the effects of pulse configuration, stimulation intensity, and coil current direction during magnetic stimulation. Using figure-8 and circular coils, the median nerve was stimulated at the cubital fossa and at the wrist of 10 healthy volunteers, and the response amplitude and site of stimulation were determined. The key findings of this study are in agreement with other researchers' findings and confirm that biphasic stimulating pulses produce significantly higher M-wave amplitudes than monophasic stimulating pulses for the same stimulus intensity. Mean response amplitudes for biphasic stimuli applied by both coils at the elbow and wrist are consistently higher for the normal current direction. Mean response amplitudes for monophasic pulses are almost always higher for reversed currents. The site for effective stimulation (the position of the virtual cathode) cannot be defined within a fixed distance from the center of the coil (3 to 4 cm), as has been suggested by other researchers, but was found to vary depending on the coil current amplitude and direction as well as the degree of inhomogeneity of the tissues surrounding the nerve. There is a statistically significant relationship between virtual cathode shift and stimulus intensity for biphasic and monophasic pulses. Reversing the coil current direction has no statistically significant effect on the virtual cathode position. Virtual cathode shifts can be measured for biphasic and monophasic stimulations using a figure-8 coil at the wrist and the elbow. However, such a shift is difficult to determine with a circular coil.

Magnetic field angular dependence of AC losses in Bi-2223 superconducting wires

We investigated the magnetic field angular dependence of the AC loss in stacked Bi-2223 silver-sheathed superconducting multifilamentary tapes. The form of sample stacked tapes corresponds to parallel conductors, which were adopted to a 22 kV–1000 kVA liquid-nitrogen-cooled superconducting transformer and a 4 kJ conduction-cooled pulse coil. We measured the AC losses in stacked Bi-2223 tapes by using a saddle-shaped pickup coil and rotating the sample stack around its central axis. On the basis of the observed results, we discussed the influence of stacking and the field angular dependence of the AC losses in stacked superconducting tape. In addition we confirmed that the AC losses in stacked superconducting tapes can be estimated for any field angle as the simple sum of the respective AC losses due to the parallel magnetic field component and the perpendicular one.

Fabrication and test of a 4 kjJ Bi-2223 pulse coil for smes

We designed and fabricated a 4 kJ conduction-cooled high-Tc superconducting (HTS) pulse coil. The coil is wound with an interlayer-transposed 6-strand parallel conductor which is composed of Bi-2223 silver alloy-sheathed multi-filamentary wires. We had developed a complete 3.6 MJ/1 MW low-Tc superconducting (LTS) SMES system for testing on a power line at Imajuku substation. Aiming at the feasible operation of SMES applying a HTS coil, we made a SMES system set-up in which HTS coils were serially connected to 3 LTS coils of the SMES. The SMES including the HTS coil was connected to Imajuku substation's power system, to made operational tests of compensation for load fluctuation at the 6 kV power line. The test results lead to the feasibility of the HTS SMES for practical use in future power systems.