Superconducting magnets in fusion research

For several years there has been a concerted effort worldwide towards developing magnets for fusion. The progress made has been impressive and the technology now seems forthcoming, at an appropriately high level, to build further large-scale fusion devices with superconducting magnets. This Paper attempts to summarize the goals and the present major areas of research.     

Superconducting magnets

Although small superconducting magnets have been in regular use in many laboratories for several years, it is only recently that the technical difficulties preventing the use of superconductors in large magnet systems have been overcome. A better understanding of the transient thermal and magnetic behavior of such devices has led to various methods of reducing or eliminating the degradation effects present in earlier magnets. The use of large amounts of very high conductivity normal metal in conjunction with the superconductor has resulted in "stabilized" magnets whose behavior is completely predictable and which will not quench except under unusual conditions. Other techniques have been developed to improve "unstable" magnets so that very high field coils can be constructed. These developments are dramatically altering the equipment used in high-energy physics and controlled thermonuclear experiments, and their effect on other areas of technology is already beginning to be felt.     

Superconducting magnets for large accelerators

A significant milestone in the application of superconducting magnets for large accelerators was reached on June 2nd 1983 when the Tevatron at Fermilab transported the first beam in its giant superconducting ring. This machine, which extends over 6 km and contains 1000 superconducting magnets, is the result of 10 years of intensive development work and has opened a new era for high energy physics. Recently, the construction of a similar ring has been started for the HERA collider at DESY, Hamburg (FRG) and development of a 3 TeV synchrotron is going on at Serpukhov (USSR). Even more powerful machines are already being studied such as the SSC in the USA and the LHC collider in the LEP tunnel at CERN. A review of these applications is presented, showing the main technological achievements they represent and the worldwide activity they have given rise to. In addition to accelerator magnets, beam lines and detectors utilize sophisticated superconducting magnets, and are worthy of mention.     

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.     

Superconducting magnets for accelerators and detectors

Accelerators and superconductivity have been good companions for 40 years. This paper is a review of the characteristics of present superconducting magnets for particle accelerators and particle detectors, in particular the ones used for high energy physics, emphasizing similarities and differences. Discussions on material characteristics and coil structure for accelerator magnets beyond 10 T are presented.     

Superconducting magnets in the world of energy,especially in fusion power

Industrial applications of superconducting magnets are only feasible in the near future for superconducting homopolar machines and possibly MHD generators. In both cases there are no longer any major problems with the superconducting windings or cryogenic systems, but technological problems connected with other components are governing the chances of large scale applications.For superconducting synchronous machines, after the successful operation of machines in the MVA range, a new phase of basic investigations has started. Fundamental problems which could not be studied in the MVA machines, but which influence the design of large turbo-alternators, must now be investigated.Fusion power by magnetic confinement will probably be the largest field of application for superconducting magnets in the long run. The present research programmes require large superconducting magnets by the mid-1980s for the experimental reactors envisaged at that time.In addition to dc windings, pulse-operated superconducting windings are required in some systems, such as Tokamak. The high sensitivity of the overall plant efficiency and the active power demand of the pulsed windings require great efficiency from energy storage and transfer systems. Superconducting energy storage systems would be suitable for this, if transfer between inductances could be provided with sufficient efficiency. Basic experiments gave encouraging results.In power plant systems and electric machines an extremely high level of reliability and availability has been achieved. Less reliability will not be accepted for systems with superconducting magnets. This requires great efforts during the development work.     

Superconducting magnets for accelerators, beam lines and detectors

After twenty years of intensive R & D, superconducting magnets are now widely used in High Energy Physics laboratories. Large detectors and spectrometers have been the first area of successful application. New giant accelerators under construction or planned are definitely based on superconducting components whilst the use for beam lines appears more restricted to special purpose elements. A review of these various applications is presented.     

Superconducting magnets for very high fields: status in Japan

Three projects on superconducting magnets for very high fields are now in progress in Japan. The Japan Atomic Energy Research Institute constructed TMC-1 having an inner diameter of 600 mm which could produce 11.1 T by using a multifilamentary Nb₃Sn conductor. The National Research Institute for Metals has developed a multifilamentary Nb₃Sn wire with Ti doping and fabricated a high-stability superconducting magnet which will produce a magnetic field of 15 T in an 180 mm bore. Combined with this magnet, a new superconducting magnet system, which may break the field record in the superconducting state, is under construction. The High Field Laboratory for Superconducting Materials at Tohoku University constructed a hybrid magnet, in which a Nb₃Sn superconducting magnet was combined with a polyhelix type water-cooled magnet and produced a total magnetic field of 29.3 T in a 32 mm bore, where 10 T was produced by the superconducting magnet with an inner bore of 430 mm. The present status of these projects is briefly described.     

First-wall coatings for Tokamak fusion reactors

The coating of the first wall of a fusion reactor must withstand an intense neutron flux, the impact of energetic charged and neutral particles, and photons, all of which escape from the plasma. Penetration of material eroded from the first-wall coating into the plasma will have a detrimental effect on the plasma stability and energy balance. Therefore material selection for the first-wall coating of a fusion reactor represents one of the critical problem areas for the achievement of fusion power.Theoretical and experimental studies which have been conducted on the many facets of the problem are reviewed with special emphasis placed on the more critical aspects. Possible directions of future research in this area are outlined.     

Cermet coatings for magnetic fusion reactors

Cermet coatings consisting of SiC particles in an aluminum matrix were produced by a low pressure chamber plasma spray process. Properties of these coatings are being investigated to evaluate their suitability for use in the next generation of magnetic confinement fusion reactors. Although this preliminary study has focused primarily upon SiC-Al cermets, the deposition process can be adapted to other ceramic-metal combinations. Potential applications for cermet coatings in magnetic fusion devices are presented along with experimental results from thermal tests of candidate coatings.     

Superconducting toroidal combined-function magnet for a compact ion beam cancer therapy gantry

A superconducting, combined-function, 5 T, 90°, toroidal magnet with a large bore is described in this paper. This magnet is designed to be the last and most difficult part of a compact superconducting magnet-based carbon gantry optics for ion beam cancer therapy. The relatively small size of this toroidal magnet allows for a gantry the size of which is smaller or at least comparable to that of a proton gantry. The gantry design places the toroidal magnet between the scanning magnets and the patient, that is the scanning magnets are placed midway through the gantry. By optimizing the coil winding configuration of this magnet, near point-to-parallel optics is achieved between the scanning magnets and the patient; while at the same time there is only a small distortion of the beam-shape when scanning. We show that the origin of the beam-shape distortion is the strong sextupole components, whose effects are greatly pronounced when the beam is widely steered in the magnet. A method to correct such an undesirable effect is suggested and demonstrated by a numerical particle tracking through the calculated three-dimensional magnetic field.     

Superconducting magnet development for fusion at JAERI

The development of superconducting magnets for fusion at the Japan Atomic Energy Research Institute (JAERI) is described. The objective of the project is the Fusion Experimental Reactor (FER) which will be constructed with superconducting toroidal and poloidal coils. For toroidal coils, JAERI has already developed the 8 T Japanese LCT coil and five other large coils (one NbTi and four Nb₃Sn coils, from 7 T to 11 T) for the Cluster Test Programme. For poloidal coils, JAERI has developed three 30–50 kA pulsed conductors. In addition to coil development, cryogenic technology and structural material development are also in progress.     

Radiation effects on insulators for superconducting fusion magnets

Fibre-reinforced plastics (FRPs) are candidate materials for the insulation of superconducting magnet coils in future fusion reactors. This paper reports on a test programme performed to assess the mechanical properties of these materials and to obtain information on the damage and fracture mechanisms. Different types of FRPs (epoxies and poly- and bismaleimides as resins; two- and three-dimensional E-, S- or T-glass fabrics as reinforcements) were irradiated at room temperature by 2 MeV electrons and ⁶⁰Co-gamma rays up to 1.8 × 10⁸ Gy and by different reactor spectra up to a neutron fluence of 1 × 10²³ m⁻² (E > 0.1 MeV) at room temperature, 80 K or 5 K. Mechanical tests in tension as well as in the intralaminar crack opening and shear mode were carried out on the irradiated samples at 77 K. After low temperature irradiation, half of the samples were subjected to a warm-up cycle to room temperature before testing at 77 K. Results on the influence of different radiation sources and annealing cycles on the mechanical properties of these materials will be discussed.     

Stress analysis of toroidal field magnets of the HL-1 Tokamak ofChina

The simultaneous analysis of the magnetic field and stress of the real structure of HL-1 Tokamak of China is presented. Considering the discrete location and varied cross section of the TF coils, the Biot-Savart law is used for the toroidal magnetic field computation. In the FEM stress analysis, 8-20 nodes nonconforming isoparametric elements are adopted, the constraint of the central cylinder is analyzed, and the anisotropy due to layer winding is considered. The magnetic field, stress, and deformation distribution of the TF coils are obtained     

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.     

聚变堆用结构材料的研究现状与进展

介绍了在聚变堆中有较好应用前景的9种候选结构材料的研究现状和进展,主要包括聚变堆用结构材料的安全使用性能、机械性能、热物理性能、腐蚀性和与冷却剂量的相容性以及辐照对聚变堆用结构材料性能的影响等.给出了这些候选结构材料的优缺点,提出了一些要解决的关键问题和将来的研究方向.     

ZnO crystal as a potential damage-recoverable window material for fusion reactors

Hydrothermal-grown bulk zinc oxide (ZnO) single crystals are investigated before and after 1-keV hydrogen-ion and deuterium-ion plasma irradiation. A decrease in optical transmittance, red-shifting of ultraviolet (UV) emission peaks, and shortening of UV emission lifetimes are observed from the bulk crystals immediately after irradiation. The results are attributed to the radiation-induced defects formed by hydrogen implantation. Although these defects immediately affect the optical properties of the single crystals, the optical transmittances and photoluminescence emissions recover days after irradiation. Our observations show the potential properties of a bulk ZnO single crystal as a damage-recoverable window material in high-flux-radiation environments such as those of fusion reactors.     

An overview of D-3He fusion reactors

D-³He fusion reactors offer a number of potentially attractive features including low radiation damage levels and low radioactivity. There have been a number of reactor studies, which will be reviewed. The emphasis would be on the tandem mirror reactor, due to its potential for converting much of the fusion power by direct conversion of charged particles with high conversion efficiency. It can also operate at relatively high β to maximize fusion power density. The ³He fuel supply from various sources will be examined. The safety and waste disposal issues will be discussed. The cost of electricity, including the fuel cost, will be estimated.     

Cryogenic fracture and adiabatic heating of austenitic stainless steels for superconducting fusion magnets

This paper discusses the fracture and the adiabatic heating of austenitic stainless steels that may be used in cryogenic structures for the superconducting magnets of magnetic fusion reactors. Elastic–plastic fracture toughness tests were performed on compact specimens in liquid helium at 4 K. Adiabatic heating was detected by measuring the internal temperature. The effect of test specimen size and side grooving on the cryogenic fracture toughness, and the temperature rise during dynamic crack growth are examined. A size independent toughness parameter results in the case of side-grooved specimens. The effect of inclusion content on the fracture mechanics parameters is also discussed.