Magnetic Generation Research Articles

Small Helical Magnetic Flux-Compression Generators: Experiments and Analysis

In order to gain experience in explosive pulsed power and to provide experimental data for modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency (FOI). The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high power pulses for various loads. This paper presents the design of, and tests with, this helical FCG. The generator had an initial inductance of 23 muH and was operated into a load of 0.2 muH. The generator is charged with 0.27 kg of high explosives (PBXN-5). Various types of diagnostics were used to monitor the operation of the generator, including current probes, optical fibers, and piezo gauges. With seed currents of 5.7 and 11.2 kA, final currents of 269 and 436 kA were obtained, corresponding to current amplification factors of 47 and 39. The peak of the current was reached about 30 mus after the time of crowbar. The two generators showed only small losses in terms of 2pi-clocking. Using signals from optical fibers, the deflection angle of the armature could be determined to be 10 deg in good agreement with hydrodynamic simulations of the detonation process and the detonation velocity to be 8.7 km/s in agreement with tabulated value.

Magnetoelectric effect in multilayer ferromagnetic-piezoelectric structures and its application in electronics

Magnetoelectric effect in multilayer structures composed of magnetostrictive and piezoelectric layers, materials and technologies used to fabricate these structures, and the results of studying the magnetoelectric effect in them are considered. Magnetic field sensors, transformers, and voltage generators based on the magnetoelectric effect in multilayer structures are described.

Exposure to alternating electromagnetic fields and effects on the visual and visuomotor systems

Acute effects on the visual and visuo-motor systems by exposure to electromagnetic fields (EMFs) at a frequency and amplitude similar to those produced by MR imaging gradient coils were assessed. 40 volunteers were exposed in random order to three, time varying, magnetic field gradients (0, 20 and 10 mT m(-1)r.m.s.). The waveform was 50 cycles of a 490 Hz sinusoidal waveform repeated every second with a total duration of 10 min for each trial. The EMFs were generated using an in-house designed and built magnetic gradient coil and waveform generator. During each trial, a test battery assessing the visual sensory (FACT) and visuo-motor (Pursuit Aiming II and visual tracking) neurobehavioral domains was completed by all volunteers. The sequence of these tests was assigned at random for each volunteer. Performance in these tests was analysed using linear mixed effects models adjusted for confounding factors collected in a pre-trial questionnaire. Variability of the estimates was assessed using a delete-1 jack-knife procedure. There was a trend for visuo-motor accuracy to be reduced (p = 0.06) by 1% during high exposure, but not at medium exposure. There was a weaker trend for visual contrast sensitivity to be improved by 12% and 21% during medium and high exposure, respectively, compared with the non-exposed sessions (p = 0.08). These effects did not reach 5% statistical significance within a population of 40 volunteers, but also the magnitude of these effects did not depend on single "extreme" observations.

Cable R&D for the LHC Accelerator Research Program

Future luminosity upgrades for the LHC at CERN will require higher field magnets than those presently being utilized. The US LHC Accelerator Research Program (LARP) is addressing this need with the design and fabrication of prototype quadrupole magnets utilizing Nb3Sn technology. A part of the R&D effort is the development of suitable cables for the magnet designs being considered by LARP. The cable parameter limits for LARP prototype and production cables are presented. At present the standard strand for LARP is the Restack Rod Processed (RRP) strand being produced by Oxford Superconducting Technologies. The present strand has 54 sub-elements and 53% Cu. An optimal cable for the first generation of LARP quadrupole magnets has a width of 10 mm, thickness of 1.26 mm, and a keystone angle of 1 degree. This work shows that if a strand is properly cabled a nearly degradation-free mechanically-stable cable can be produced.

Study on the Circular Fracture in MFCG Armatures Expansion

The high-speed photography and the non-linear dynamics engineering program LS-DYNA are conducted to research on Al and Cu armatures expansion process under the drive of pressed composite B explosive with the charge method of several-section. A special experiment system with an equipment of blocking off armature fracture flashing light is built, and a thorough analysis of the “circular fracture” phenomena has been made: For the air gap existence, there is a difference between the axial movement velocity of the armature joint position wall and other positions wall. The upward wall forces the joint position wall and then the “circular fracture” appears. Some conclusions about Magnetic Flux Compression Generator design are obtained.

Interface properties of superlattices with artificially broken symmetry

We have used superlattices made of thin layers of transition metal oxides to design the so-called multiferroics, i.e. materials possessing simultaneously an electric polarization and a magnetic ordering. The polarization originates from the asymmetric stacking order accompanied by charge transfer effects, while the latter one also influences the magnetic properties of the interfaces. Due to the breaking of space and time-reversal symmetry by multiple ordering mechanism magnetic second harmonic generation is proven to be an ideal method to investigate the electric and magnetic properties of the superlattices.

Nonlinear processing of the output voltage a piezoelectric transformer

Reducing the size of power supplies raises the problem of new components that could be better candidates for integration. In this field, electromagnetic transformers may be replaced with significant benefit by piezoelectric transformers (PT). In a PT, the input electrical energy is transferred to the output by an acoustical means, using the direct and converse effects of piezoelectric materials. Its main advantages over an electromagnetic transformer are no magnetic noise generation, small size, high power density, and high efficiency. This paper deals with an innovative technique that produces a significant improvement of the power capability of piezoelectric transformers. This technique is based on a particular output voltage processing of the PT. Its effect is a vibration level reduction of the PT structure while keeping the output power practically constant. Vibration level is a critical parameter that determines the maximum power capability of a given PT. Thus, the new processing reduces significantly the losses of the PT. Both theoretical predictions and experimental results show that the increase of the power capability may reach 200%.

Perspectives of<tex>$rm Nb_3rm Sn$</tex>in Future Accelerator Magnets

Upgrades presently being planned for high-energy hadron colliders require magnet performance beyond the capabilities of NbTi superconducting wire. Recent improvements in the performance of Nb3 Sn material and in the capability to use it in practical magnets make it the logical choice for the next generation of accelerator magnets. We discuss recent results of R&D programs, some remaining issues and possible solutions. Recent progress in understanding and in technical details of design and assembly have made Nb3Sn magnets easier to fabricate and more reliable in performance

An Electrical-Explosion Opening Switch Powered from a Magnetic-Explosion Generator

The results from experiments with powering an electrical-explosion opening switch (EEOS) from a high-speed helical magnetic explosion generator are presented. The energy capacity of this generator is ∼1 kJ, the total operating time is 25 µs, and the characteristic current rise time is ∼2 µs. In experiments with thin copper wires used in EEOSs, voltage pulses with an amplitude of ∼180 kV and a rise time of ∼50 ns were formed across a 25-Ω resistive load connected in parallel to the EEOS.

Rail stator augmentation magnetic flux compression linear generator used in electric guns

In order to find an applicable power supply used in electric guns, there have been two types of the magnetic flux compression linear generator (MFCLG) that use the solenoids as the stators. That is to say, a piston armature or a solenoid armature is used within the stator. Although they can amplify the current theoretically, they can only compress the component of the magnetic flux. So their efficiency is low, and there exists some difficulties, which is difficult to solve in terms of the engineering technology. In addition, the use of the solenoid armature leads to the increment of the ineffective part of the solenoid. A kind of the rail stator augmentation MFCLG is presented here. It separates the combustion chamber-cylinder and the generating channel into two independent parts, and with an anti-recoil device. The combustion chamber is extended as a cylinder. Within it the piston pushes the stator-armature package to move forwards through the connecting rod along the generating channel consisted of the rails. This can compress the magnetic flux to generate the large electric current to be used by electric guns. If we fill the integrated fuel into the combustion chamber, we can realize the successive pulse operation. The separation of the combustion chamber-cylinder and the channel can avoid the high temperature burning gases from entering the channel to cause ablation. The basic mathematic model of this type of the generator is given and its physical properties are analyzed in this paper. It is found that it can compress the magnetic flux perpendicularly and has higher efficiencies for compression and conversion. It can produce large current with sufficient pulse width to supply the electric guns. Besides, a self-excitation method for this type of generator is also put forward, which can omit the seed power source. Instead, it uses the chemical energy that drives and compresses the magnetic flux to generate the electric current to produce the initial seed magnetic flux used by it.

Application of ferromagnetic nano-wires in porous alumina arrays for magnetic force generator

Magnetic chromatograph is a techniques that use magnetic force to separate small particles. It is interesting because the magnetic column can be used repeatedly and does not produce secondary wastes. In order to increase the efficiency of the separation, the magnetic force is desired to be as large as possible. The magnetic force increases in proportion to the magnetic field gradient, and when the source of the magnetic force is a wire, the magnetic field gradient increases in inverse proportion to the diameter of the wire. Therefore, it is estimated that an assembly of ferromagnetic nano-wires can produce large magnetic force. Anodized porous alumina has ordered and uniform nanometersized hole arrays. Various materials including ferromagnets, such as Fe, Ni and Co, can be easily electrodeposited from the bottoms of the holes, and the holes can be filled with nano-wires. These ferromagnetic wires have nano-meter sized diameters. Therefore, they are assumed to be able to produce strong magnetic force. We propose to apply ferromagnetic nano-wires in porous alumina arrays to magnetic force generators for magnetic chromatograph.

リラクタンスネットワーク解析による永久磁石発電機の特性算定

This paper presents a method for calculating the operating characteristics of permanent magnet (PM) generators based on the reluctance network analysis (RNA) method proposed by the authors. The PM generator used in the discussion has a distributed winding and an overhang structure in which the permanent magnet is longer than the stator's stack in the axial direction. In the analysis, the stator and rotor, gap, and the permanent magnet including the overhang are expressed as a simplified three-dimensional RNA model. The RNA model of the PM generator is combined with an external electric circuit and motion calculation blocks on a general-purpose circuit simulator called “SPICE.” We can readily calculate the characteristics of the PM generator using the combined model, including its transient state. The calculated values agree well with the measured ones.

Quantification of Ohmic and Intrinsic Flux Losses in Helical Flux Compression Generators

Helical magnetic flux compression generators (MFCGs) are the most promising energy sources with respect to their current amplification and compactness. They are able of producing high current pulses required in many pulsed power applications with at least one order of magnitude higher energy density than capacitive storage with similar discharge characteristics. However, the main concern with MFCGs is their intrinsic flux loss that limits severely their performance and which is not yet well understood. In general, all flux losses have a differing degree of impact, depending on the generator's volume, current and energy amplification, size of the driven load, and angular frequency of armature-helix contact point. Although several computer models have been developed in the open literature, none of them truly quantify, starting from basic physics principles, the ohmic and intrinsic flux losses in helical MFCGs. This paper describes a novel method that provides a separate calculation of intrinsic flux losses (flux that is left behind in the conductors and lost for compression) and ohmic losses, being especially easy to implement and fast to calculate. We also provide a second method that uses a simple flux quantification, making a mathematical connection between the intrinsic flux losses, quantified by the first method, and the intrinsic flux losses observed in the generators. This second method can also be used to a priori estimate the MFCG performance. Further, we will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.

Design of the Next Generation of Florida-Bitter Magnets at the NHMFL

The National High Magnetic Field Laboratory in Tallahassee, Florida designs, builds and operates the world's highest field dc resistive magnets, providing fields up to 33 T in purely resistive systems and up to 45 T in resistive-superconducting hybrids. The next generation of magnets is presently being designed and used technology developed for our hybrid to upgrade the field in our various resistive magnets. Coil designs are presented for the following 20 MW dc systems: 1) a new 50 mm bore magnet expected to provide 32 T, 2) a new 32 mm bore magnet expected to provide 35 T, and 3) a new high-homogeneity magnet expected to provide 30 T with inhomogeneity of 50 ppm or less over a 10 mm diameter spherical volume.

Generation of the Highest Continuous Magnetic Fields

The generation of the highest magnetic fields is a true challenge to the magnet designer and a stimulus for the materials scientist. Record fields are achieved through a successful interplay between the precise understanding of the physics of magnets, the detailed knowledge and conscious use of the available materials, and a design that optimizes all the different parameters. We describe three technologies essential for the generation of the highest magnetic fields: coils wound from high-temperature superconductors (HTS) operated at liquid helium temperatures in a background field; high-power resistive magnets; and hybrid magnets, which combine the advantages of resistive and superconducting magnets. HTS show high current densities at elevated fields confirmed so far by measurements up to 33 T. In spite of the strain limitation of the conductor, insert coils up to 3.2 T have been built at different laboratories for a total field of 23.5 T. Recently a record field of 25 T using a 5 T Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ superconducting insert coil has been achieved at the NHMFL. For resistive magnets, dramatic improvements in magnet design, such as the Florida-Bitter magnet, and new materials, such as the microcomposite CuAg, have made it possible that magnetic fields can now be generated that exceed the fields that hybrid magnets produced 10 years ago. Today, 33 T are provided on a routine basis with resistive magnets. The highest continuous magnetic field ever of 45 T is obtained with a new generation of hybrid magnets. It is available for scientific research in our user facility.

Conceptual design for a periodic permanent magnetic ring magnetic flux compression generator

In a lot of single-shot applications and experiments at remote locations where helical magnetic flux compression generator (HMFCG) provides the most practicable power, it is impracticable or inconvenient to use a capacitor bank to be the initial energy source. Homogenous permanent magnet has been used to be the initial energy source to drive HMFCG, but the energy density stored in a single permanent magnet is very low. The possibility of using periodic permanent magnetic ring structure(PPMR) to be initial energy source of an MFCG was analyzed theoretically and numerically. It was concluded that 1kJ initial energy can be provided to an HMFCG by the PPMR which weights less than 30kg and the outside radius less than 30 cm. In this paper, an axially detonated PPMR HMFCG structure is put forward and studied numerically, and its current amplification expression is reduced. Meanwhile some numerical examples for such an HMFCG are also provided. It is concluded that an experimental investigation of such an HMFCG design is worthwhile.

Phenomenology of magnetic second harmonic generation from low symmetry surfaces and interfaces

Low dimensional magnetic structures show interesting and novel phenomena such as oscillatory magnetic coupling and giant magnetoresistance. Magnetic second harmonic generation (MSHG) can provide unique information on magnetic surfaces and interfaces because, within the dipole approximation, broken space-inversion symmetry at the surface or interface of centrosymmetric media, and broken time-reversal symmetry arising from the magnetization, are both required in order to observe a magnetic-field-dependent second harmonic response. However, the additional reduction in symmetry arising from the magetization produces many non-zero susceptibility tensor components, particularly in the case of vicinal, stepped surfaces of 1m symmetry, and care is needed in designing experiments that will produce readily interpretable results. Phenomenological expressions for the MSHG response from systems of 1m symmetry are presented, where combinations of input and output polarizations and magnetic field orientations allow the essential physics of these systems to be explored, particularly in relation to distinguishing terrace and step contributions to the magnetization from vicinal surfaces and interfaces. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-current plasma opening switch fed by a magnetic explosion generator

A plasma opening switch fed by a helical magnetic explosion generator is developed. A plasma sheath with an axial length of ≈4 cm and an electron density of ∼1017 cm−3 is produced in the electrode gap of the switch by six coaxial gaseous-plasma injectors. A series of explosion experiments shows that the system developed allows one to study the switch at currents of about 2 MA.

New magnetic implant material for interstitial hyperthermia

Magnetic properties and heat generation characteristics of a bio-compatible ceramic Mg1+xFe2–2xTixO4 system have been investigated as an implant material for the magnetic induction hyperthermia. Curie temperature (Tc) of the ceramic decreases with increasing x; and become Tc, 350 K at x ç0:35 and, 315 K at x ç 0:38; which is suitable Curie temperature for implant material. The temperature of ceramic as a function of time under the high frequency alternating magnetic field is self-controlled at Tc.The surface temperature of a powder injection sphere cancer model, which was a mixture of the agar phantom and the ceramic powder implant, and the temperature distribution around the sphere set in the pure agar phantom matrix have been measured. The result is in good agreement with calculation using a finite element method (FEM). It was found that the temperature distribution inside of the sphere and the minimum quantity of Mg1+xFe2–2xTixO4 necessary for hyperthermia could be estimated by the FEM calculation.

Japanese hybrid tech moves centre stage

The motor vehicle is evolving fast and powder metallurgy has a key role to play in forming the new generation of magnets that help drive hybrid and fuel-cell vehicles. Toyota claims a world-leading role in the technologies that lie behind the innovations…