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.     

Experimental study of condensation heat transfer of R-404A in helically coiled tubes

In this research, the heat transfer coefficients of R-404A vapor condensation inside helically coiled tubes are studied, experimentally. The effects of different coil pitches and curvature radii at different vapor qualities and mass velocities are considered. The vapor is condensed inside the helically coiled tubes by transferring heat to the cooling water flowing in annulus. Results show that the coil diameter has significant effect on condensation heat transfer coefficient. By decreasing the coil diameter or increasing the Dean number, the heat transfer coefficient is increased as the highest value is obtained at curvature radius of 4.35 cm which is 45% greater than the corresponding figure of curvature radius of 7.65 cm at mass velocity of 125 kg m⁻² s⁻¹. Also, at low vapor qualities, the coil pitch effect is more pronounced. Finally, based on the results, a new correlation is developed to evaluate the condensation heat transfer coefficient of R-404A inside helically coiled tubes.     

Superconducting magnet for K-500 cyclotron at VECC, Kolkata

K-500 superconducting cyclotron is in the advanced stage of commissioning at VECC, Kolkata. Superconducting magnet is one of the major and critical component of the cyclotron. It has been successfully fabricated, installed, cooled down to 4.2 K by interfacing with LHe plant and energized to its rated current on 30th April, 2005 producing magnetic field of 4.8 T at median plane of cyclotron. The superconducting magnet (stored energy of 22MJ) consists of two coils (α and β), which were wound on a sophisticated coil winding machine set-up at VECC. The superconducting cable used for winding the coils is multi filamentary composite superconducting wire (1.29 mm diameter) having 500 filaments of 40 μm diameter Nb-Ti in copper matrix which is embedded in OFHC grade copper channel (2.794 mm × 4.978 mm) for cryogenic stability. The basic structure of coil consists of layer type helical winding on a SS bobbin of 1475 mm ID × 1930 mm OD × 1170 mm height. The bobbin was afterwards closed by SS sheet to form the LHe chamber. The total weight of the coil with bobbin was about 6 tonne and the total length of the superconducting cable wound was about 35 km. Winding was done at very high tension (2000 PSI) and close tolerance to restrict the movement of conductor and coil during energization. After coil winding, all four coils (two each on upper and lower half of median plane of cyclotron) were banded by aluminium strip (2.7 mm × 5 mm) at higher tension (20,000 PSI) to give more compressive force after cool down to 4.2 K for restricting the movement of coil while energizing and thereby eliminating the chances of quench during ramping of current.After completion of coil winding by October, 2003, cryostat assembly was taken up in house. The assembly of cryostat (13 tonne) with support links (9 Nos.) refrigeration port, instrumentation port, helium vapour cooled current loads, etc. was completed by June, 2004. Meanwhile assembly of magnet frame was taken up and the cryostat was positioned in the magnet frame with proper alignment by August, 2004. After installation of cryostat on magnet, the cryostat was connected to the helium refrigerator/liquefier, having refrigeration capacity of 200 W and 100 l/h in liquefier mode with LN2 pre-cooling. The cryogenic delivery system supplying the liquid helium and liquid nitrogen to the superconducting magnet was successfully commissioned in November, 2004. The cool down of the cryostat to 10 K took around 8 days following which the LHe was filled in the cryostat (300 l) on 15th January, 2005. Subsequently the superconducting coils (α and β) were energized by two DC current regulated power supplies (20 V, 1000 A, 10 ppm stability) with slow and fast dump resistors connected externally across the superconducting coils for protection of coils at the time of power failure and quench.The paper describes the intricacies involved in coil winding, winding set-up, assembly of cryostat, cooling down the superconducting coils, filling by LHe and energization to rated current. The paper also highlights the operating experience of superconducting magnet and related test results.     

A 10 tesla cryomagnetic system for neutron scattering experiments

A cryomagnetic system is described in which the first main component is a cryostat providing variable temperatures and the second a superconducting coil. The cryostat enables the coil to operate at either 4.2 K or 2.16 K, and allows a sample of diameter 10 mm, height 10 mm, to be brought to temperatures varying from 1.5 K to 300 K.The magnet is an asymmetrical split coil with a vertical magnetic axis. Aluminium windows provide access vertically over 15°, horizontally over 340° to a bore of useful diameter 32 mm. The superconducting magnet is wound from multifilamentary NbTi and Nb₃Sn wires and provides a central field of 8.7 T at 4.2 K and 10 T at 2.16 K.     

Optimizing the magnetic system parameters inside a measuring coil

Conclusions The magnetic core inside the coil of a magnetic measuring system has an optimal diameter to height ratio d_m/h_m for which the torque is maximal. The magnet diameter and height depend on the coil perimeter P, and their optimal ratio K_F is practically independent of the coil perimeter but depends on the alloy of which the magnet is made. Implementing optimal magnet dimensions makes possible fuller utilization of the energy of a permanent magnetic core inside the magnetic system coil. If the optimal ratio of magnet diameter to its height is known, calculation of the magnetic system parameters and the design of the entire measuring instrument becomes quite simple.Translated from Izmeritel'naya Tekhnika, No. 4, pp. 32–34, April, 1991.     

A high homogeneous field superconducting magnet for Argonne polarized proton target

We have designed and built a superconducting magnet for a large polarized proton target. The magnet consists of a pair of thick Helmholtz coils. Each Helmholtz coil consists of 9 step-subcoils balancing the ampere-turns around the Helmholtz line, thus providing an optimum for field uniformity without the help of Correction coils. This magnet generates 25 kilogausses with field uniformity one part in 10,000 over more than 5 cm diameter spherical target volume. It was designed to allow a large warm bore of 34.5 cm with axial aperture of 96° and between the Helmholtz pair, a wide warm separation of 12.7 cm with transverse aperture of 23°, thus allowing large accessibility to the proton target and for the scattering detectors in high energy scattering experiments.     

Design and Performance of the First Dual-Coil Magnet at the Wuhan National High Magnetic Field Center

The first 80 T dual-coil magnet was manufactured and tested at the Wuhan National High Magnetic Field Center (WHMFC). The inner coil consists of 8 layers of 2.8 mm × 4.3 mm CuNb microcomposite wire developed in China; the bore diameter is 14 mm and the outer diameter 135 mm. The outer coil was wound directly on the inner coil with 12 layers of 3 mm × 6 mm soft copper. Each conductor layer of both coils was reinforced by Zylon/epoxy composite. The inner and outer coil were driven by a 1.6 MJ/5.12 mF capacitor bank and by eight 1 MJ/3.2 mF modules, respectively. At the voltage of 14.3 kV for the inner coil and 22 kV for the outer coil, the inner and outer coils produced peak fields of 48.5 T and 34.5 T respectively, which gave a total field of 83 T. This was the first combined operation of the new capacitor banks installed at the WHMFC. We present details of the design, manufacture and test of the dual-coil magnet and discuss crucial material properties. Based on this experience, a second dual-coil magnet will be designed; the enhanced design will be discussed. With the total energy of 12.6 MJ, peak field up to 90 T is expected.     

11 T liquid helium-free superconducting magnet

An 11 T liquid helium-free superconducting magnet designed at 6 K in vacuum using high temperature superconducting current leads was developed. The coil was conductively cooled down from room temperature to 4.1 K in 40 h by two 4 K GM-cryocoolers. In a performance test, the coil temperature rose to 6.8 K for the inner Nb₃Sn coil and 5.9 K for the outer NbTi coil, while sweeping the field at 5 A min⁻¹. A central field of 10.7 T in a 52 mm room temperature bore was generated at an operating current of 149 A. Holding the field at 10.5 T was achieved continuously for 24 h at a constant coil temperature of 4.8 K.     

线性压缩机圆柱臂盘簧的设计及性能研究

弹簧组是线性压缩机的关键部件。为了优化线性压缩机的弹簧组,减小弹簧组的体积和重量,提高线性压缩机的工作效率,设计了一种易于加工的、大刚度的多线型圆柱臂盘簧。采用ANSYS有限元分析软件对圆柱臂盘簧进行刚度计算和模态分析,研究了圆柱臂盘簧中心线的基圆半径、盘簧线径、盘簧轴向高度等主要结构参数对其刚度和应力的影响。结果表明：随着基圆半径的增大,圆柱臂盘簧的轴向刚度、最大应力逐渐减小,径向刚度变化曲线近似为开口向上的二次函数曲线;随着线径的增大,圆柱臂盘簧的轴向刚度和径向刚度加速增大,最大应力基本呈线性增大。在设计圆柱臂盘簧时,将基圆半径设定在15~19 mm,有利于提高盘簧的性能;若线径为4~6 mm,应优先选择0 mm的轴向高度;若线径为6~8 mm,则优先选择10 mm的轴向高度。研究结果对提升线性压缩机的弹簧支撑技术具有重要的参考价值。     

Test operations of the VENUS superconducting magnet at KEK

The superconducting magnet of the VENUS detector was successfully operated with a central field of 0.75 T. A cryogenic system kept the coil temperature to below 4.5 K. When a coil quench was induced by built-in heaters, the stored energy of 11.7 MJ was safely extracted from the magnet to the outside dump resistor. The iron structure of the magnet yoke supported the magnetic force of about 230 t with a maximum elastic deformation of 0.4 mm. The maximum leakage field at the location of the barrel electromagnetic calorimeter was 33 G. The magnetic field was mapped in the solenoid bore by an NMR probe and by three-dimensional Hall probes with an accuracy of order 10⁻⁴. The field was confirmed to be uniform within 0.3% deviation in the spatial region of a central drift chamber.     

Use of Helmholtz coils for magnetic measurements

Helmholtz coils can be used for the measurement of open-circuit magnetization of most permanent magnet materials. The author describes the physics of the measurement, lists the materials that can be measured, derives the coil constant, and derives a correction factor for the measurement of arc magnets. A measurement is made by placing the magnet at the center of the coils, and zeroing the integrating voltmeter or fluxmeter. The magnet is then removed from the coil, parallel to the coil axis, to a distance such that the sample has no influence on the reading, typically 75 to 100 cm. The open-circuit magnetization of the sample is related to the time-integrated voltage     

Liquid helium cryostat for SQUID-based MRI receivers

We describe a liquid helium cryostat, developed to cool SQUID-based receivers in low field MRI systems. The cryostat has a 4 L liquid helium capacity, a hold time of over 3 days and accommodates 10 cm diameter receiver coils. New vacuum insulation methods reduce the noise level by at least an order of magnitude compared to existing commercial designs. The minimum detectable field at 425 kHz, with a 5 cm diameter circular coil, was estimated to be 0.018 fT/Hz^1/2 from Q-factor measurements and 0.035 fT/Hz^1/2 by direct measurement with a SQUID amplifier. Further measurements indicated that most of this field noise probably originates with dielectric losses in the cryostat’s fibreglass shells.     

Low resistance splices for HTS devices and applications

This paper discusses the preparation methodology and performance evaluation of low resistance splices made of the second generation (2G) high-temperature superconductor (HTS). These splices are required in a broad spectrum of HTS devices including a large aperture, high-field solenoid built in the laboratory to demonstrate a superconducting magnetic energy storage (SMES) device. Several pancake coils are assembled in the form of a nested solenoid, and each coil requires a hundred meters or more of 2G (RE)BCO tape. However, commercial availability of this superconductor with a very uniform physical properties is currently limited to shorter piece lengths. This necessitates us having splices to inter-connect the tape pieces within a pancake coil, between adjacent pancake coils, and to attach HTS current leads to the magnet assembly. As a part of the optimization and qualification of splicing process, a systematic study was undertaken to analyze the electrical performance of splices in two different configurations suitable for this magnet assembly: lap joint and spiral joint. The electrical performance is quantified in terms of the resistance of splices estimated from the current-voltage characteristics. It has been demonstrated that a careful application of this splicing technique can generate lap joints with resistance less than 1 nΩ at 77 K.     

Intense polarized atomic deuteron source

An intense polarized deuteron gas jet, to be used as a target in an electron storage ring, is described.With a gas flow of 0.3 1 Torr/s through a nozzle, the flux of polarized atoms from a separation magnet is Q = 1.5 × 10¹⁶ atoms/s. At a distance of 45 cm from the magnet the jet width, at half-height, is 6 mm and the target thickness t = 2 × 10¹¹ atoms/cm². The degree of tensor polarization is P_zz = ?0.75 ± 0.10 for the 3 → 5 transition and P_zz = 0.80 ± 10 for the 2 → 6 transition.     

Microstructure controls and their effects on the properties of Bi2Sr2Ca1Cu2Ox superconductors

This paper reports our recent progresses in the development of Bi₂Sr₂Ca₁Cu₂O _x /Ag tape conductors for the applications of magnetic field generation in liquid helium or around 20 K, using a refrigerator. We have carried out extensive work to optimize the processing parameters, investigating the relationship between the microstructure and transportJ _c. We have found that the partial melting in oxygen atmosphere is effective to have large transportJ _c with good reproducibility. The pre-annealing and intermediate rolling (PAIR) process has been successfully applied to the multilayer conductors to improve the grain alignment and transportJ _c. TheJ _c of 5×10⁵A/cm² at 4·2 K and 10 T has been achieved, which is the highest value reported so far. Two magnets fabricated by using different types of Bi-2212/Ag conductors were tested. One is a magnet designed as an insert magnet for a 18 T-class large bore Nb-Ti/Nb₃Sn superconducting magnet. The conductor of this magnet was multifilamentary tape processed by powder-in-tube method. TheI _c was 98 A in the backup field of 18 T, which generated the self field of 1·79 T. A large pancake coil was fabricated with multilayer conductor and tested under the operation of cryocooler system. The coil was stably operated up to theJ _c of the coil at the temperatures below 30 K.     

Theoretical and experimental investigation of magnetic field related helium leak in helium vessel of a large superconducting magnet

The helium vessel of the superconducting cyclotron (SCC) at the Variable Energy Cyclotron centre (VECC), Kolkata shows a gradual loss of insulation vacuum from 10⁻⁷ mbar to 10⁻⁴ mbar with increasing coil current in the magnet. The insulation vacuum restores back to its initial value with the withdrawal of current. The origin of such behavior has been thought to be related to the electromagnetic stress in the magnet. The electromagnetic stress distribution in the median plane of the helium vessel was studied to figure out the possible location of the helium leak. The stress field from the possible location was transferred to a simplified 2D model with different leak geometries to study the changes in conductance with coil current. The leak rate calculated from the changes in the leak geometry was compared with the leak rate calculated from the experimental insulation vacuum degradation behavior to estimate the initial leak shape and size.     

Simplified method of porosity prediction in directionally solidified Al–4·5 wt-%Cualloy

Abstract

An experiment was carried out to verify a simplified theory of porosity formation during the unidirectional solidification of Al–4·5 wt-%Cu alloy in cylindrical moulds 1·5 and 2·4 cm in diameter. For specimens 1·5 cm in diameter, a higher thermal gradient G and a lower solidus velocity V_s at low drawing speeds are measured compared with the specimens 2·4 cm in diameter, while this difference tends to be reversed at high drawing speeds. The experimental results also conjirm the roles of interdendritic fluid flow and surface tension effects in the formation of porosity. The porosity content tends to increase with increasing local solidification time or a decrease in G^0·4/V_s^1·6when this parameter is less than about 1·0 K^0·4min^1·6cm^?2.

MST/3122     

Wide Variety of Experiments Using a Cryogen-Free 27.5 T Hybrid Magnet and a Cryogen-Free 18.1 T Superconducting Magnet

A cryogen-free hybrid magnet without liquid helium for operation, generating 27.5 T in a 32 mm room temperature bore of an 8 MW water-cooled resistive insert magnet in an 8.5 T background field of a cryogen-free superconducting outsert magnet, is being operated for basic research at low temperatures down to 17 mK in combination with a dilution refrigerator. In addition, we are developing functional materials using a differential thermal analysis DTA at high temperatures up to 1473 K in high fields up to 27 T. This cryogen-free hybrid magnet will be upgraded to generate 29 T by improving the outer superconducting magnet. A cryogen-free 18.1 T superconducting magnet with a 52 mm room temperature experimental bore, consisting of a Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) insert coil, has been developed using a GM-JT cryocooler. Recently, bronze-tape-laminated Bi2223 has revealed excellent irreversible stress tolerance of 250 MPa at 77 K. In addition, the critical current properties for recent Bi2223 tapes are largely improved from 200 to 400 A/cm-width at 77 K in a self-field. Therefore, the stainless steel reinforcement tape incorporated for the previous Bi2223 insert coil is no longer needed for a new Bi2223 one. A new Bi2223 insert coil with almost the same size as the existing insert coil can generate two times higher fields at the elevated operation current from 162 to 191 A. An upgraded cryogen-free superconducting magnet can offer a long-term experiment at the constant magnetic field of 20 T for an in-field heat-treatment investigation.     

Cryogen-Free 23 T Superconducting Magnet Employing an?YBa2Cu3O7 Coated Conductor Insert

We have successfully constructed an 18.1 T superconducting magnet conductively cooled by a GM/JT cryocooler. The double-pancake insert using Ag-sheathed Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) tape with stainless steel reinforcement tape generated 2.5 T in a 15.6 T background magnet. In order to develop a cryogen-free high-field superconducting magnet producing over 20 T, an YBa₂Cu₃O₇ (Y123) coated conductor insert is intended for upgrading the cryogen-free 18 T superconducting magnet. The Bi2223 insert, whose size is 176 mm outer diameter, 90 mm inner diameter, and 252 mm coil height, is now excited at 162 A operation current, and will be replaced by a new Y123 insert. We have already confirmed excellent mechanical properties of 1000 MPa hoop stress tolerance for Y123 coated conductor tape with Hastelloy substrate. This means that we no longer need stainless steel reinforcement for the insert. As a result, an Y123 insert with almost the same size as the Bi2223 insert is designed to generate 7.5 T at 187 A, because the number of turns can be improved extremely. A cryogen-free 23 T superconducting magnet can sufficiently be developed for a long-term experiment at a constant high magnetic field.     

Structured coils for NMR applications

An approach to the optimized design of high-homogeneity coil systems of the type used in nuclear magnetic resonance (NMR) applications is presented. The approach effectively minimizes parameters such as the coil size in a solenoid magnet, or maximizes the gap/pole diameter ratio in a vertical gap NMR magnet, while preserving uniformity in an exceptionally large volume, resulting in novel coil geometries that have never been tried before. The approach can be applied to cylindrically symmetric coil systems with or without an active shield, as well as to long magnets of the type used in particle accelerators. Several practical examples are presented