High Field Resistive Magnets Research Articles

Deformation of Two Copper Matrix Conductors Under Cyclic Loading

High field resistive magnets use Cu matrix composites as conductors because composite conductors have high mechanical strength. The conductors are manufactured by cold deformation and heat treatment that introduce refined obstacle distance to resist dislocation motions. The increased density of these obstacles increases the mechanical strength of the conductors. Under cyclic loading, such as the loading condition in pulsed resistive magnets, the conductors may soften or harden depending on the interaction of the obstacles with the dislocations evolved during the loading. Understanding and predicting the performance of the conductors under cyclic loading help researchers to predict the life of the coils made from these conductors, to make efficient use of them in magnets, and to manufacture conductors to meet the requirements of the magnets, particularly when the magnetic stress is above the yield strength of the conductors. The goal of this research is to understand the fatigue properties of two composite conductors and to relate such properties to types of obstacles. The fatigue test loading is in displacement-controlled mode, which is like what occurs in a state-of-the-art pulsed magnet. This work sheds a light on the correlation between the tensile and fatigue properties in composite conductors by consideration of types of obstacle in composite conductors.

A Novel Clamping Method for Resistive Magnets

High field resistive magnets may be built by stacking hundreds of so-called Bitter disks to form a coil. Nowadays, these disks have strongly elongated cooling holes arranged in a staggered pattern. Near the outer diameter of the disks one finds larger holes, round or flattened, to accommodate a tie-rod. These tie-rods assist in the assembly of the coils and are usually pretensioned to ensure a net compressive clamping force on the coil. However, at high fields, the axial magnetic forces toward the center of the coil dominate over any pretension in the system, reducing the net resulting clamping force at the end plates to a very low value. This is of particular importance in hybrid magnets where the resistive insert coils experience the background magnetic field of a large superconducting magnet. Several different strategies have been reported to mitigate the effect. We describe a novel clamping method that employs a water-filled, pressurized bellow that exerts a compressive force to the different subcoils of the resistive coil assembly. In contrast to the tie-rods, the force exerted by the bellow will remain almost constant when the magnet is energized and contracts. This new clamping method will be a key element for the mechanical design of the resistive insert coils of the HFML 45 T hybrid magnet.

Full Three-Dimensional Multiphysics Model of High-Field Polyhelices Magnets

High field Resistive magnets for static field de- veloped at the Laboratoire National des Champs Magn etiques Intenses (LNCMI) are based on the so-called polyhelix technique. Their design relies on non-linear 3D multi-physic models. As the user demands for higher magnetic field or specific field profile are growing we have to revisit our numerical models. They need to include more physics and more precise geometry. In this context we have rewritten our numerical model in the frame of a collaboration with Institut de Recherche en Math ematique Avanc ee (IRMA). New models have been implemented with the finite element library Feel++. This paper gives a status of these developments and the new features available. Results are presented for a 14 polyhelices insert targeting 36 Tesla in a 34 mm bore.High field Resistive magnets for static field de- veloped at the Laboratoire National des Champs Magn etiques Intenses (LNCMI) are based on the so-called polyhelix technique. Their design relies on non-linear 3D multi-physic models. As the user demands for higher magnetic field or specific field profile are growing we have to revisit our numerical models. They need to include more physics and more precise geometry. In this context we have rewritten our numerical model in the frame of a collaboration with Institut de Recherche en Math ematique Avanc ee (IRMA). New models have been implemented with the finite element library Feel++. This paper gives a status of these developments and the new features available. Results are presented for a 14 polyhelices insert targeting 36 Tesla in a 34 mm bore.

A Reduced Basis Framework: Application to large scale non-linear multi-physics problems

In this paper we present applications of the reduced basis method (RBM) to large-scale non-linear multi-physics problems. We rst describe the mathematical framework in place and in particular the Empirical Interpolation Method (EIM) to recover an ane decomposition and then we propose an implementation using the open-source library Feel++ which provides both the reduced basis and nite element layers. Large scale numerical examples are shown and are connected to real industrial applications arising from the High Field Resistive Magnets development at the Laboratoire National des Champs Magn etiques Intenses.

Magnet Control and Monitoring System

The magnet control and monitoring system is one of the crucial parts for reliable and safe operation of the LNCMI (Laboratoire National des Champs Magnétiques Intenses) high field resistive magnets (up to 35 T), the 24 MW power supply and the cooling system. To take advantage of latest electronic components and real-time software, we decided to upgrade the entire system: network infrastructure, user consol, magnet safety system and power supply controller. In our contribution we will present the new setup. Our new control system is an assembly of in-house electronic and real-time software development. The new electronic components consist of different converters providing enhanced resolution (up to 24 bits) for a more precise measure of the field. We also developed tailored magnet coil voltage conditioning device used by the safety system. Software developments now provide enhanced process control between the different parts by using a real-time operating system. It also simplifies the network used to remotely control the power supply. In the new solution, it is based on RS485 communication over optical fiber. Finally we will show that the choice of Linux, customized with the real-time patch RTAI, is the main key to bring efficiency and flexibility for further extensions.

Design Study of High Field Resistive Magnets for Diffraction Experiments

The European Synchrotron Radiation Facility (ESRF), and the Institut Laue Langevin neutron facility (ILL) in collaboration with the Laboratoire National des Champs Magne¿tiques Intenses (LNCMI-CNRS) have performed during 2008 a design study for the implementation of continuous high magnetic fields suitable for diffraction studies. Two main designs have been studied: a horizontal field magnet suitable for back scattering and absorption experiments and a split magnet.

Florida-Helix Resistive Magnets

A new technology is being developed at the National High Magnetic Field Laboratory in Tallahassee, Florida, USA for generating very high DC fields. The technique preserves the best features of the Florida-Bitter technology that has become the international standard for resistive magnets while eliminating the slit inherent in all disk solenoids. Thus, higher current densities and average stresses can be supported. While continuous helices for high field resistive magnets have been built at various laboratories, the new Florida-Helix shows potential to surpass today's state-of-the-art for field-to-power ratios.

Noise reduction performance of a YBCO cylinder in high-field resistive magnets

The field stability of high-field resistive magnets is generally inferior to that of superconducting magnets. The main reason for this is the low inductance of resistive magnets. High-precision measurements, such as those conducted using nuclear magnetic resonance, are very difficult by using high-field resistive magnets due to lack of field stability and ripples in the magnetic fields. However, recent progress in the area of high-temperature bulk superconductors indicates that it is possible to drastically improve these measurements. We used a YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) bulk cylinder made by the Nippon Steel Corporation. Its dimensions were 5 mm (inner diameter)/spl times/10 mm (outer diameter)/spl times/10 mm (height). A pickup coil was inserted into the center of the cylinder to measure ripples in the magnetic field. The bulk cylinder with the pickup coil was then inserted into a resistive magnet of the Tsukuba Magnet Laboratory. At a certain field, the bulk superconductor was cooled down from room temperature to the superconducting state. The signals of the pickup coil before and after transition to the superconducting state were compared.

Diamagnetism and magnetic force: a new laboratory for granular materials and chaotic/deterministic dynamics

We report studies on the dynamics of macroscopic particles in a low gravity “magnetic levitation” environment. Particle ensembles are held in a weak confining potential due to diamagnetic forces in a high-field resistive magnet. In such a case, the kinetic energy is not zero, and assemblies of particles undergo ergodic processes to find the lowest configurational ground state. Examples of such processes, as well as consideration of chaotic versus deterministic motion for single-particle orbits, will be presented.

New opportunities in science, materials, and biological systems in the low-gravity (magnetic levitation) environment (invited)

We discuss new opportunities that present themselves with the advent of very high magnetic field resistive magnets with appreciable central bore access. A detailed description of the parameters of the magnetic force environment for the case of diamagnetic materials in a water-cooled Bitter-type resistive magnet is provided for the reader who may have an interest in low-gravity experiments. We discuss emerging research activities involving novel uses of magnetic forces in high field resistive magnets at the National High Magnetic Field Laboratory. Particular attention is given to the area of diamagnetic materials that allow a low or “zero” gravity state, i.e., magnetic levitation. These include studies involving plant growth, protein crystallization, and dynamics of single particles and granular materials. In the latter case, unique aspects of the magnetic force environment allow low gravity experiments on particulates that cannot be performed on the Space Shuttle due to the lack of a weak confining potential in space.

Special purpose high field resistive magnets

Since 1994 the National High Magnetic Field Laboratory in Tallahassee, FL has built numerous high field DC magnets for condensed matter physics and other research. Our user community is now requesting various special purpose high field magnets that are at various stages of design and development. Concepts under consideration include the following: 1) AC modulation (0.1 T AC to 100 Hz) at 27 T DC for dc Haas van Alphen; 2) AC gradient (0.05 T/cm) at 27 T DC for force magnetometry, 3) >20 T radial access magnet for two angle rotation, 4) 18 cm bore high field*field gradient magnet for levitation of large samples and 5) 50 mm bore magnet with uniform field*field gradient for precision levitation. Design concepts. field magnitude and quality, stresses, power densities and construction status will be presented for these various systems.

Test results and potential for upgrade of the 45 T hybrid insert

Construction of the NHMFL hybrid magnet is complete. The resistive insert was tested to full current without the background field from the superconducting magnet on May 17, 1999. Tests of the combined system have been scheduled for October 1999. The resistive insert fits into the 616 mm bore of the 14 T outsert superconducting magnet. The insert consists of a five coil, axially cooled Florida-Bitter design. The two innermost coils are electrically in parallel and this pair is in series with the other four coils. The magnet design uses Florida-Bitter disks made of Cu-Ag, Cu-Be, Cu-Zr and Cu sheet and is heavily based upon the high field resistive magnets previously built at the NHMFL. Details of the coil design, construction and testing are presented.

Material and cooling requirements for poly-Bitter resistive magnets and hybrid inserts generating continuous fields up to 50 T

The new National High Magnetic Field Laboratory (NHMFL), equipped with a 40 MW DC power supply, will design and construct the next generation of high field resistive magnets and hybrid inserts generating DC fields up to 50 T. We present a study on the required materials and the necessary cooling characteristics, these magnets need. The configuration selected for this study consists of a combination of thin poly-Bitter and thick Bitter coils optimized in dimensions and power under constraint of maximum design stress and heat removal to obtain maximum field. The study shows that each design requires a different optimum ratio of conductor strength to electrical conductivity and that efficient cooling is only advantageous if strong copper alloys are used. For efficient use of the available power the development of new high strength, high conductivity materials will be necessary. Equally important are improvements in the heat transfer characteristics of these high power density magnets. >