Design Concepts of Optimized MRI Magnet

This paper describes a simple technique to design compact air core magnets for magnetic resonance imaging (MRI). The optimum geometry is obtained by a combination of analytical and numerical methods. The technique emphasizes achieving the required field homogeneity with a shorter magnet having a minimum amount of ampere turns. The code calculates the total inductance of the set of coils, total conductor length, force, and maximum field on each coil. The code also calculates the allowable geometrical tolerance to achieve the field uniformity. The code is flexible and can be used for various geometries. Here, we present three different cases to demonstrate the efficiency of the code. First, we present the design details of a 1.5 m long 1.5 T actively shielded magnet, where the stray field is reduced to less than 4 gauss outside the sphere of radius 3.65 m using a pair of shielded coils. A total of four pairs of coils provide a field uniform to within 5.2 parts per million (ppm) within a sphere of 50 cm diameter. Calculated values of all the higher order moments lie within a few ppm. Second, we describe optimized geometry of unshielded 1.5 T symmetric magnets having three pairs of coils. Third, we optimize the geometry of a 1.2 m long 1 T asymmetric magnet having five coils. Here, the good field region starts at 20 cm from one of the edges of the magnet, providing the attending physician better accessibility to the patient. But the ampere turns and computation time required are quite large compared to a symmetric magnet.      

Thermal stability of high current density magnets

The stabilization theories hitherto proposed for superconducting (SC) magnets are not fully developed for application to high current density magnets such as pulsed dipole magnets for a synchrotron. Hence, thermal stability in such high current density magnets is studied by obtaining a minimum energy of thermal disturbances which barely leads a magnet to quench. To find the minimum energy by calculation a dynamic simulation of temperature distribution along a conductor is carried out following an application of the disturbances on the conductor. The minimum energy is found to depend largely on time duration and spatial length of the disturbances. The values of the minimum energy given by calculation agree almost with the experimental results obtained for a coil which simulates a pulsed dipole magnet from the viewpoint of cooling. Discussion is also made in relation to the minimum energy on the performance of a pancake type solenoid magnet which has the same cooling as in the simulating coil.     

AC losses in the SSC high energy booster dipole magnets

The baseline design for the SSC (Superconducting Super Collider) high energy booster (HEB) has dipole bending magnets with a 50-mm aperture. An analysis of the cryogenic heat load due to AC losses generated in the HEB ramp cycle is reported for this magnet. Included in this analysis are losses from superconductor hysteresis, yoke hysteresis, strand eddy currents, and cable eddy currents. The AC loss impact of 2.5 μm vs. 6 μm filament conductor is presented. A 60-mm aperture design is also investigated     

A potential vector field solution in superconducting magnets, part II: Results of applications

The results obtained by applying the analytical method of calculation, described in Part I [1], to two large superconducting dc magnets, one composed of two equal cylindrical coils arranged along the same axis and the other composed of two saddle coils arranged so as to make a dipole are presented. Values and distributions of the flux density in the magnets are given and the influence exerted by the presence of an outer radial yoke and of possible ferromagnetic materials arranged perpendicularly to the magnet axis is shown. It is also shown how the proposed method gives values of the flux density components inside the thickness of the coils which may be very different from those obtained by applying other analytical methods in which the coils are represented by one or more current sheets. Finally the results obtained for a magnet are compared with those supplied for the same magnet using a finite-element numerical method.     

A pulsed dipole magnet made from a hollow composite superconductor with a circulatory refrigeration system

When a superconducting dipole magnetic field is limited by a value of about 2.5 T using a ‘window frame’ type dipole, the design of such a magnet can be essentially simplified, and the superconducting winding volume can be decreased. If the winding is made of hollow composite superconductor, the cryostat construction is simplified and it is easy to handle with superconducting magnets.In order to estimate the prospects of using ‘window frame’ type dipole magnets with a circulatory refrigeration system for the Nuclotron Accelerator project, a dipole magnet with a length of about 0.4 m, a 5.5 cm aperture and a magnetic field of up to 2.5 T has been constructed and tested at the High Energy Laboratory, JINR. The superconducting cable of the magnet consists of a cupro-nickel pipe with an outer diameter of 5 mm and a wall thickness of 0.5 mm on which multifilament superconductors are cabled. The magnet construction with a two-phase helium circulating refrigeration system is described. The dependences are presented of the critical current degradation and of ac losses on the magnetic field inhomogeneity and hydraulic resistance.     

Design of superconducting magnets for full scale MHD generators

This paper describes the results of conceptual design studies and preliminary design work carried out relative to full-scale superconducting magnets for base-load size magnetohydrodynamic (MHD) generators. Conceptual layouts and design data were prepared for 6 T magnets of alternate configurations (circular-saddle coil and rectangular saddle coil) and for 5 T and 7 T variations. The major characteristics of the various designs are summarized and compared. Problem areas revealed during the design effort are identified and specific recommendations for future investigations and R & D effort in support of large MHD magnet technology are made.     

Upgraded coil configuration for ISABELLE magnets

Achievement of the design field of 5 T in the ISABELLE dipole magnets is turning out to be more arduous than expected and several avenues of improvement are being pursued. One possibility for improving training and peak field performance is discussed in this paper. It has been recognized that the inert spacers with their adjacent active turns in the cosine magnet windings can be replaced by a double thickness braid operating at approximately half-current density in 46 of the 190 turns. Since the high-field region occurs in the low current density turns near the poles, a performance improvement can be expected. It has been verified that the proposed coil configuration satisfies the field requirements and details thereof are given. Results from an experimental magnet in which superconducting spacer turns are used to simulate half-current density windings are presented. Construction of thick braid coils is being planned and the status of these magnets is reviewed.     

A cure against ‘training’ of superconducting magnets

Experiments with test coils show that ‘training’ of superconducting magnets is considerably reduced by loading and unloading the conductor at room temperature before winding the magnet. This preload treatment seems to be a simple way of coping with this problem.     

Design of epoxy-free superconducting dipole magnets and performance in both Helium I and pressurized Helium II

Three model superconducting dipole magnet 1m long, without iron, having a bore diameter of 76 mm have been built without epoxy resins or other adhesives and tested in He I and He II. The conductor is the 23-strand Rutherford-type cable used in the Fermilab Doubler Saver magnets, and is insulated with Mylar and Kapton. The two-layer winding is highly compressed by a system of structural support rings and tapered collets. Little "training" was required to reach quench currents greater than 95 percent of "short sample" in Helium I. The maximum quench current in He II is increased 20 to 30 percent, compared with He I operation at 4.4 K. Test results are given on cyclic losses, heater-induced quenches, and charge-rate effects.     

脉冲强磁体导体材料研究

在脉冲强磁体设计中,磁应力是我们面临的最大挑战,当磁场强度达到100T时,磁体绕组中的磁应力高达4GPa,这是目前任何实用导体材料都无法承受的,因此,脉冲强磁体的发展在很大程度上取决于磁应力的解决情况.文章从提高导体材料机械强度的角度出发,介绍了目前各种导体材料的加工过程和技术参数,包括铜、铜宏复合导体材料、铜微复合导体材料、多层绞线复合导体材料等.     

Finite element calculations on detailed 3D models for the superferric main magnets of the FAIR SIS100 synchrotron

The synchrotron SIS100 is one of the two basic accelerators of the future Facility for Antiproton and Ion Research (FAIR) at GSI in Darmstadt. This accelerator should provide high intensity U28+ and proton beams with a pulse repetition rate of 1 Hz (i.e. a ramp rate of 4 T/s). The magnetic system of the accelerator uses superferric 2.1 T dipoles of about 3 m length and 32 T/m quadrupoles of about 1 m length. The magnet coils are made of a hollow tube cable wrapped with Cu/NbTi composite wire cooled with two phase helium flow at 4.5 K. The bore dimensions were defined to 130 × 60 mm for the dipole and 135 × 65 mm for the quadrupole. We present the developed ANSYS models for different important aspects: AC loss, magnetic field quality and mechanical stability. Preliminary studies verified the approaches and these models were applied to calculate the effects for the coil, the yoke and the beam pipe structures. We outline further steps to fully describe the SIS100 magnets including mechanical and thermal properties.     

Superconducting magnets for the CBA project

The superconducting magnets that were designed and tested for the BNL colliding beam accelerator are described, including dipoles, quadrupoles and trim coils. The dipoles had an effective length of 436 cm, a good field aperture of 8.8 cm diameter, and were designed for an operating field of 5.28 T in a temperature range between 2.6 K and 3.8 K (provided by supercritical helium). The quadrupoles had the same aperture, an effective length of 138.5 cm, and were designed to operate in series with the dipoles, with a gradient of 70.8 T/m. The dipoles incorporated internal sextupole, octupole, and decapole trim coil windings; the quadrupole trim coils consisted of dipole, quadrupole, and dodecapole windings. The design, construction, and performance (training, field quality, quench protection characteristics) of prototype magnets are discussed in considerable detail.     

Development of a cryogenic test bed for high amperage critical current measurement on high temperature superconductor wire

Many areas of research have benefited from the application of conduction-cooled superconducting magnet technology. The middle and small-scale magnets immersed in the liquid helium will be replaced by the easy-operating conduction-cooled superconducting magnet due to convenient operation, lower operating cost and easy for user. For the goal of superconducting magnet applications in the advanced testing for high temperature superconducting (HTS) wire and sample coils, a wide bore conduction-cooled superconducting magnet with available warm bore of ?186 mm and center field of 5-6 T for background magnetic field applications was designed, fabricated and tested. The system allows measurements to be performed in a repeatable and reliable fashion. In order to support the high stress in magnet, the detailed finite element (FE) analysis with electro-plastic model is proposed. The sample cryostat is designed with cryofree. It includes two GM cryocoolers. The detailed design, fabrication and thermal analysis are presented in the paper.     

DEALS: a demountable superconducting magnet system for fusion reactors

A new type of superconducting magnet system (DEALS) for large fusion reactors is described. Instead of winding large planar or multi-axis coils, as has been previously proposed in fusion reactor designs, the demountable superconducting coils would be made by joining together several prefabricated conductor sections. Conductor types, fabrication processes, and joining schemes are described. The magnet sections would be made at a central factory and shipped to the reactor site for assembly.The magnetic forces on the conductors would be transmitted to an external room temperature support structure via low thermal conductivity bearing blocks. This reduces conductor tensile stresses to very low levels. Differential and mechanical thermal movements between the magnet coil and the external support structure would be accommodated by the use of moveable joints between magnet sections. These pressure type contact joints carry current during magnet operation, and do not carry tensile loads.Finite element analyses on the magnet and its support structure are presented together with analyses of magnet cooling requirements. Results of experiments on small movable pressure type joints at liquid helium temperatures are described.These indicate that adequately low joint losses should be achievable in large magnet systems. Current carrying capcity is not affected by relative motion, and friction coefficients are reaonable. Based on these results and the analyses, the DEALS concept appears feasible for fusion magnet systems.     

Acoustic emission monitoring results from a fermi dipole

Acoustic emission (AE) techniques have been used to monitor a Fermi dipole magnet during quench current experiments. The dipole data show that conductor motion is the principal source of AE and the primary cause of quenches, as is the case for most high-performance superconducting magnets. The middle of the dipole was found to be more susceptible to quenches with motion than either end. This result is consistent with the field distribution along the magnet. There is good correlation between quench current and AE history. The experimental data can be satisfactorily explained by a model relating AE energy and frictional motion. Acoustic emission was used in conjunction with voltage data to quantify the average frictional dissipation in the windings, estimated to be ≈ 10 μW cm⁻³.     

Design of a full-scale magneplane vehicle

The design of a full-scale 140-passenger Magneplane vehicle is described, emphasizing its cryogenic aspects. The vehicle has an overall length of 50m and maximum width of 3.8m. Its gross weight is just under 45,000 kg. The cross-sectional profile and structure of the fuselage are modeled after modern commercial Jet aircraft such as the Boeing 707. Sixteen elliptic superconducting pancake coils are placed in the bottom 120° arc of the vehicle with a center-to-center spacing of 2.75m. With all coils energized at 1.25×10⁶A-turns each, a lift force of 5×10⁵N will be produced at high speeds for a separation distance of 25cm between the surfaces of the coils and the levitation strips. A hollow conductor is proposed for the superconducting coils. It is a niobium-titanium multifilament, aluminum stabilized and stainless-steel reinforced composite with a rectangular cross-section, having overall dimensions of 2.5cm by 1.25cm, with an equivalent 1-cm diameter hole. Supercritical helium is circulated through the coils to keep them in a cryogenically stable condition. Each coil is enclosed in its own vacuum envelope together with its thermal shield, persistent switch and necessary structural support. The coolant passes continuously through hollow passages in a number of parallel paths, maintaining a set temperature. The stability of the conductor during its normal excursion has been analyzed to choose a coolant flow rate which allows a certain portion of the coil to become resistive, and remain so for a limited duration of time, without forcing the entire coil to quench. The behavior of the normal-zone propagation as a function of coolant flow rate, as well as pressure drop and frictional heat generation through the conductor, have been studied for the present geometry.     

Design and optimization of the 12.5 T EFDA dipole magnet

The aim of this paper is to provide an overview of a recent study carried out—within the framework of the European Fusion Program—to design a 12.5 T superconducting dipole. By focusing on the CICC based design option, the overall design procedure is presented. In particular, the 2D optimization of the dipole cross section is described including the magneto-static analysis of the winding and iron yoke, the mechanical analysis of the conductor jacket, insulation and outer cylinder, the conductor hot spot analysis, etc. As far as the thermo-hydraulic design is concerned, simulations of nominal as well as offset operating conditions (e.g., magnet quench) are presented with emphasis on their role played in the overall magnet design. For example, diagrams reporting the helium heat removal capabilities, pressure drop, mass flow, etc. are shown and their usefulness as guidance for the magnet designer described.     

A 1.5 t superconducting magnet with closed cooling system for spin-imaging: an outline

In the field of NMR-imaging superconducting magnets will become important. At higher field strengths proton-imaging can be achieved faster and with better resolution.In this paper a 1.5 T superconducting magnet system based on a double pair coil configuration is described. Some details are given on the construction of the coils. Also the use of a small cryogenerator for cooling radiation shields and precooling the magnet system is described.     

A critical current-margin design criterion for high performance magnet stability

A ‘critical-current-margin. (CCM) design criterion for superconducting magnet stability is presented. It is applicable to high-performance magnets that operate at high current densities; these magnets have cooled conductor surfaces, but their calculated heat fluxes are well beyond recovery values. For stability of a magnet based on this CCM design criterion, disturbances must be limited in size but need not be eliminated nor localized.The CCM criterion is compared with the MPZ/Cold-End Theory. It is shown that this criterion offers a much higher confidence level of stability than the MPZ/Cold-End Theory. Furthermore, and unique to this theory, stability is independent of current density in the stabilizer.The CCM criterion is based on two magnet winding elements: heat transfer and composite conductor. In this criterion heat transfer must always remain in the nucleate boiling region throughout the operation of a magnet. The design emphasis is on more superconductor and less stabilizer. It is also demonstrated that the CCM theory works particularly well with conductors such as cabled conductors that have high ratios of cooled perimeter to cross-section.Experimental results are presented supporting this CCM design criterion.     

Lessons learned from twenty-year operation of the Large Helical Device poloidal coils made from cable-in-conduit conductors

The Large Helical Device (LHD) superconducting magnet system consists of two pairs of helical coils and three pairs of poloidal coils. The poloidal coils use cable-in-conduit (CIC) conductors, which have now been adopted in many fusion devices, with forced cooling by supercritical helium. The poloidal coils were first energized with the helical coils on March 27, 1998. Since that time, the coils have experienced 54,600 h of steady cooling, 10,600 h of excitation operation, and nineteen thermal cycles for twenty years. During this period, no superconducting-to-normal transition of the conductors has been observed. The stable operation of the poloidal coils demonstrates that a CIC conductor is suited to large-scale superconducting magnets. The AC loss has remained constant, even though a slight decrease was observed in the early phase of operation. The hydraulic characteristics have been maintained without obstruction over the entire period of steady cooling. The experience gained from twenty years of operation has also provided lessons regarding malfunctions of peripheral equipment.