The influence of low temperature neutron irradiation on superconducting magnet systems for fusion reactors

The components (insulation, stabilization material and the superconductor) of a superconducting coil system for a fusion reactor are exposed to fast neutrons, probably with a dose of about 10¹⁸n/cm²during the lifetime of a reactor, and with an energy spectrum comparable with the one of a fission reactor. Experimental investigations were made on the influence of low temperature neutron irradiation on the stability of a superconductor and of the superconducting properties of NbTi wires with different initial jc and pre-treatment. The influence on jcand Tcof Nb3Sn was investigated.     

Safety and reliability in superconducting fusion magnet systems

Superconducting toroidal field coils for future commercial Tokamak reactors must operate safely and reliably for periods of up to 30 years or more with minimal interruption for maintenance. This paper represents an initial attempt to consider safety and reliability issues for superconducting fusion magnets. Goals and approaches for safety and reliability are characterized. Possible accident initiators are identified, and the effect of mechanical, thermal, and electrical factors are examined, by using some early Tokamak reactor concepts as illustrations. Principal factors in magnet instrumentation and control are identified, and the role of engineered-safety features are discussed.     

Development of a high rigidity superconducting magnet for the Maglev system

A high rigidity type superconducting magnet (SCM) is developed and tested by an electromagnetic vibration simulator which simulates the actual running conditions. To reduce the increase in heat load due to the vibration of the cryostat, several kinds of design concepts are chosen. It is recognized from the experiment result that the most effective design concept is to increase the characteristic frequency in twisting mode of the inner vessel with the high rigidity outer vessel.     

A superconducting magnet system of a polarized proton target

This paper presents the experimental studies and main operational characteristics of a superconducting magnet system for a polarized proton target. The system was used in pion photoproduction from polarized protons. The characteristics described meet the requirements of the dynamic proton polarization method and the kinematic conditions of photoproduction experiments.     

A superconducting magnet system for a very high intensity magnetic mineral separator

The basic cryogenic engineering requirements for building a superconducting magnet system for the magnetic separation of minerals are considered. The different construction problems for solenoids and non-circular coils are considered. The solution adopted is epoxy impregnation with high density glass-fibre inserted to control the difference in expansion coefficient between epoxy resins and the winding material of the superconducting magnet. The short hold-time cryostat required for preliminary measurements is described and proposals made for a small scale pilot plant machine with continuous refrigeration.     

Development of a variable quench pressure relief valve for superconducting magnet system

A new variable quench pressure relief valve (VQRV) for a superconducting magnet system has been developed at the High Energy Accelerator Research Organization (KEK). The VQRV is designed that the setting of the blowout pressure can be freely controlled and be maintenance-free for long-term operation.A prototype VQRV was tested under a high radiation environment up to 2.5 MGy. The heat load of 1.5 W at 4.2 K and a seat leakage rate of 4.5 × 10⁻⁷ kg/s at 4.2 K of the VQRV were confirmed. It has enough performances for the cryogenic system operation in the Japan Proton Accelerator Research Complex (J-PARC) neutrino beam line.The design and test results of the VQRV are described in this technical note.     

Cluster test facility for superconducting tokamak toroidal magnet system development

As the step of development of superconducting tokamak toroidal magnet system, a Cluster Test system is being constructed. The test facility has two coils, called cluster test coil (CTC), in a sector position, which provides back-ground field to a test module coil (TMC). The oval-shaped TMC has a mean width of 1.5 m and a mean height of 2.0 m. And it is designed to operate at a peak field of 8 T and an average current density of 3 KA/cm²on the winding space when CTC, operated with rated current, has 7 T as a peak field with current density of 3 KA/cm². Experience on the Cluster Test will provide fabrication techniques and verifications of computer codes for future toroidal coil design principles.     

Electromagnetic behavior of lap-joints for fusion magnet system

The joint between superconducting Cable-In-Conduit-Conductors (CICC) is a key technology in a magnetic confinement fusion apparatus. Several hundreds of joints are involved in one apparatus generally. DC resistance of the lap-joint is typically designed less than several n-ohms and the allowable joule loss is several watts. AC loss due to external magnetic field is also limited to less than several watts. Reduction of the AC loss and low joint resistance are required simultaneously and those are conflicting trade-off.The lap-joint had been examined under both self-field and external-transverse-field experimentally. In this study, we established a numerical model for the joint and analyzed for electromagnetic behavior of it numerically. In the simulation, modeling of contact resistances between twisted strands is important. Circuit constants, e.g., conductance between strands, were determined to reproduce the experimental results; those are the circuit constants and the DC joint resistance. The relation between the joint resistance and the AC loss was discussed. Constitution of the joint does not only influence on the joint resistance and the AC loss but also current distribution in the cable. Non-uniform current distribution among the strands is reported to result in the degradation of the stability. We successfully simulated mentioned phenomena and found our numerical model was useful in joint design to find a good compromise.     

19.3 T with a superconducting magnet

The magnet system HOMER has been used to generate a magnetic field of 19.3 T. Nb₃Sn insert coils were used in conjunction with the NbTi background magnet.     

Strain gauges for superconducting magnet testing

A survey of commercially available strain gauges potentially suitable for testing of superconducting magnets is presented. Strain measurement errors caused by magnetic fields and temperature changes are discussed. Previously unpublished data for the magnetic field induced error for weldable half-bridge strain gauges are given, along with the results of a test for the temperature dependence of the magnetorestistance of modified Karma alloy foil gauges in the liquid helium temperature range. Apparent strain-vs-temperature curves for the weldable half-bridge gauges and metal foil gauges are shown. Errors caused by magnetic fields are in general repeatable from gauge to gauge, whereas errors due to temperature effects exhibit considerable variation from gauge to gauge in a lot of nominally identical gauges.     

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.     

Direct digital control of a superconducting magnet

A microcomputer-based controller and its input/output interfaces have been developed for an inductor-converter unit (IC unit) which consists of a six-pulse Graetz-type converter and a solenoidal superconducting magnet (SM). The SM current is feedback-controlled through the following operations: detection of the SM current, comparison with the reference input for generation of the error signal, determination of the firing angle by the digital operation of compensation, and generation of the trigger pulses. The algorithms for digital compensation have been studied by experiments as well as computer simulations. Proportional-integral-derivative (PID), first-order exponential response with transport delay (Dahlin algorithm), and finite time settling control (FTSC) algorithms have been simulated and experimented by using the IC unit. Accurate agreements between the results of simulations and experiments have been obtained for the PID and Dahlin algorithms. The FTSC algorithm has shown the best response of the three methods.     

Long term experience on the superconducting magnet system for the CELLO detector

A large superconducting magnet system has been installed and is currently operated at the PETRA storage ring (Desy/Hamburg) for the CELLO detector. This system has been carried out as a joint collaboration of CEN/, Saclay and ITP/Karlsruhe and includes as main components a large "thin wall" solenoid cooled by force flow in a pipe bonded to the winding, two bath-cooled compensating solenoids, a 450 W liquifier-refrigerator unit and all connecting lines and appropriate control equipment designed for long-term automatic operation. The system has been in operation for nearly two years with continuous runs exceeding three months and has provided considerable technical experience.     

Sweeping a persisting superconducting magnet with a transformer

A method for sweeping a persisting superconducting magnet is described. The field sweep is achieved by including in the superconducting loop of the magnet a coil which acts as the secondary coil of a transformer. Variation of the current in the primary coil of the transformer, controlled from outside the cryostat, causes the field-sweeping action through flux-linking with the superconducting loop. Compared by the ratio of the magnet's inductance to the transformer's inductance. The advantages of using an all-metal vacuum-tight superconducting feedthrough are discussed.     

传导冷却超导磁体系统真空问题的试验研究

成功研制了6T NbTi 传导冷却超导磁体系统,制冷机为二级GM-制冷机.磁体冷却到4 K需用74 h左右,目前磁体系统已分别完成了115 A(6T),45 h和95 A,264 h的无间断运行实验.制冷机可在长达15天(359 h)的时间里,对系统抽真空并维持系统真空在1.5×10-2 Pa左右,其间不再使用扩散泵对超导磁体系统抽真空.     

40T混合磁体外超导磁体杜瓦真空系统设计

根据40 T稳态混合磁体外超导磁体杜瓦对真空的要求,对杜瓦真空系统进行了设计计算,计算结果如下:杜瓦真空系统包括粗抽泵机组和主泵机组,其中粗抽泵机组由两套ZJ-70罗茨泵和2XZ-15直联式旋片真空泵组成,可将杜瓦真空抽至1 Pa;主泵机组由两套F250/1500涡轮分子泵和2X-30旋片式机械泵串联组成,可将杜瓦真空...     

Minimum heat pulse to quench a superconducting magnet

The minimum energy required to quench a fully impregnated superconducting winding has been measured at a constant field of 5 T for various currents. Great care has been taken to match the experimental conditions with those presumed in the minimum propagating zone (MPZ) treatments of the situation. In particular the winding has been designed so that the MPZ is smaller than the heat source thus satisfying the requirement for a point disturbance. The adiabatic requirement has been met with an inductive heating technique in place of the usual resistive heating. As a consequence of these features the minimum quench energies are much smaller than those obtained in previous experiments and agree well with Wilson's theoretical treatment for a point disturbance.     

35 kJ高温超导磁储能(SMES)的热输运实验研究

研制了中国首台高温超导磁储能直接冷却系统,该系统不使用低温液体(液氦、液氮).在10-3Pa的真空度下,高温超导磁体线圈由1台单级GM制冷机从室温293 K冷却到19 K,Bi2223电流引线由另一台制冷机冷却到77 K以下.整个系统在通140 A直流电流的时候产生了4.5 T的磁场.系统连续运行480 h(20 d),磁体和低温系统各参数动态特性良好.实验研究表明,控制系统的漏热,优化磁体内部导冷结构,有效减少热传导部件的接触界面热阻是制冷机直接冷却高温超导磁体的关键技术.     

Lumped-Element Dynamic Electro-Thermal model of a superconducting magnet

Modeling accurately electro-thermal transients occurring in a superconducting magnet is challenging. The behavior of the magnet is the result of complex phenomena occurring in distinct physical domains (electrical, magnetic and thermal) at very different spatial and time scales. Combined multi-domain effects significantly affect the dynamic behavior of the system and are to be taken into account in a coherent and consistent model.A new methodology for developing a Lumped-Element Dynamic Electro-Thermal (LEDET) model of a superconducting magnet is presented. This model includes non-linear dynamic effects such as the dependence of the magnet’s differential self-inductance on the presence of inter-filament and inter-strand coupling currents in the conductor. These effects are usually not taken into account because superconducting magnets are primarily operated in stationary conditions. However, they often have significant impact on magnet performance, particularly when the magnet is subject to high ramp rates.Following the LEDET method, the complex interdependence between the electro-magnetic and thermal domains can be modeled with three sub-networks of lumped-elements, reproducing the electrical transient in the main magnet circuit, the thermal transient in the coil cross-section, and the electro-magnetic transient of the inter-filament and inter-strand coupling currents in the superconductor. The same simulation environment can simultaneously model macroscopic electrical transients and phenomena at the level of superconducting strands.The model developed is a very useful tool for reproducing and predicting the performance of conventional quench protection systems based on energy extraction and quench heaters, and of the innovative CLIQ protection system as well.