Recent progress of superconducting magnet system design for Large Helical Device (LHD) of National Institute for Fusion Science (NIFS) is described. LHD is one of large scale nuclear fusion devices which will be completed in the 20th century, and fully superconducting coil system to create magnetic field. LHD is composed of a plasma vacuum vessel, superconducting helical and poloidal coils, cryogenic supporting structures, and an outer vacuum vessel, and surrounded by utilities of coil power supply, helium refrigerator and nitrogen supply, and vacuum pumping system. Cryogenic mass of the system is estimated as 860tons including coil and supporting structure. Cooling method of the superconducting coil is decided as bath cooled of liquid helium for the helical coil, and force cooled of supercritical helium for the poloidal coil. Major and minor radii of the helical coil are 3.9m and 0.975m, respectively. The conductor material of Nb-Ti alloy is used for both helical and poloidal coils from the point of strain effect. Conductor maximum field will be 6.9T for the plasma center field of 3T at the first phase with saturated boiling helium and 9.2T for 4T at the second phase with superfluid helium, Electromagnetic force and electrical insulation of the coil are the most important items to design the coil. Research and development work is intensively continued now, including conductor stability, helical and poloidal winding, elasticity of the coil, and cryogenic handling.
In measuring strain and displacement at cryogenic temperatures, the strain-gauge measurement system is particularly noted for high accuracy and ease of handling. When measuring at cryogenic temperatures, it is important to grasp in advance the fundamental characteristics of a strain gauge, such as the thermal effects on the apparent strain and gauge factor. There are a variety of trial strain-gauge instruments under examination for measuring displacement. They are roughly classified either in the rod type or the clip-on type, which respectively meet specific measurement purposes. Their fundamental characteristics depend on the strain gauges being used. They serve in various fields to measure displacement by taking advantage of distinctive strain gage features.
Detailed knowledge on the steady-state and transient heat transfer from solid surfaces in He I and He II is important as a database for the analysis of the influence of local thermal disturbances on the stability of He I or He II cooled large superconducting magnets. In this paper, an overview of the transient heat transfer characteristics on solid surfaces in He I and He II caused by various large stepwise heat inputs, such as the quasi-steady nucleate boiling with a certain lifetime in He I and the quasi-steady Kapitza conductance heat flux with a certain lifetime in He II, are presented in comparison with their steady-state heat transfer characteristics.
The superconductivity technology has been receiving a great concern in engineering application. It enables to produce high magnetic field because of very high Jc density, and this high magnetic field can be utilize in various engineering field. However, as magnets become larger and larger with high current, many engineering problems have rised. Among all problems, mechanical disturbance is one of the most important problem to give strong effects on stability of superconducting magnets. Hence, the analysis of the mechanical behavior of the winding structure of superconducting magnets is of great importance, In this report, the strain behavior was measured on coils where the magnetic force by excitation was simulated by the distributed forces along the inside surface. The result is that the strain distribution showed some anomalies compared with theoretical result. The strain of the outer turn was smaller than that by theoretical analysis. It is because initial misfit (non-uniform distribution of winding of remaining stress, micro crack or spallings of binder epoxy between each windings). The misfit became larger and larger with increase of load cycle. This misfit prevented the stress transmission from inside to outside turn. It is also found that the mechanism of the degradation of coil rigidity can be a resultant of these processes.
A large superconducting magnet for fusion reactor, which is one of the composite structure, have initial imperfections caused by the complicated residual stress distribution, irregular winding situation, cracking and delamination of GFRP insulator during fabrication and cooling down, respectively. This initial imperfection propagates by thermal cycling. The intial imperfection and its subsequent propagation by thermal cycling causes a significant decrease in the coil rigidity. We detected the initial imperfection and its propagation in the GFRP insulator non-destructively by using the SPATE 9000 stress analyzer, Taking the result, we evaluated the relationship between the coil rigidity, the initial imperfection and its propagation quantitatively.
Temperature measurement is one of the most fundamental techniques to proceed cryogenic experiment. Especially, information on temperature distribution is quite useful to know flow pattern of nuclear boiling fluid. To carry out the distribution measurement, a large number of sensors are required accompanying with huge number of data transfer wires. However, the complex wirings always lead to poor reliability of the measurement system. This paper proposes a new method for temperature distribution measurement in cryogenic fluid with Si diodes matrix as thermo-sensors. The data scanning method is introduced to solve huge wirings problem. In the paper, 64 points (8×8) diodes matrix distrubution sensor is manufactured as a prototype system, which is operated in cryogenic GN2 flow to assess its availability.
Two-stage screw compressors incorporating oil injection systems are extensively applied to helium refrigeration systems used in cooling superconducting equipment. The investigation of the optimum condition and the reliability of the two-stage oil injection type helium screw compressor have been carried out, in order to determine ways of enhancing compressor performance with the high reliability. In result, it was found that the compressor volumetric efficiency is increased by reducing the mass flow ratio of the helium gas/lubricating oil while accelerating the compressor revolution speed. And it was proved that the compressor volumetric efficiency is significantly influenced by the sound speed in the mixture of gas and lubricating oil. Furthermore, authors were able to derive a formula for predicting the change of the compressor volumetric efficiency, when the mass flow ratio of gas/lubricating oil and the revolution speed are changed. It was also found that the shaft power is decreased by setting the ratio of the high- and low-stage theoretical volumetric flow rates to 1:4 and utilizing a low viscosity lubricating oil. In result, the advanced experimental machine achieved an isothermal efficiency level of 60% without any trouble.
The influence of an intermediate cold isostatic pressing (C. I. P.) on the superconducting properties of Bi-Pb-Sr-Ca-Cu-O superconducting bulk has been investigated, and heat input per a pair of the bulk between 4.2K and 77K has been evaluated. The critical current density (Jc) was improved by the effect of C. I. P. and sintering after initial sintering. The maximum transport Jc at 77K under zero magnetic field was indicated as 580A/cm2, and that at 4.2K under zero magnetic field was indicated as about 3, 500A/cm2. The reasons for these improvements are the formation of the high Tc single phase with a fraction of low Tc phase and the higher density increasing of microstructure. The heat input per a pair of the bulk between 4.2K and 77K has been indicated to be about 60mW and also considered to be of nearly one-third lower than the level of 100A DC condition for the optimum designed current leads with helium vapor cooling.
Stabilities and heat flux in liquid helium in a high gravitational field up to 2, 000G at a rotating speed of 3, 600rpm, are studied to establish the fundamental design criterion of field windings of superconducting generators. It is clarified that magnet stability is greatly improved in a rotational field compared with stationary state, because of the enhancement of natural convection for the cooling of the coils caused by a high gravity. The heat transfer coefficient measured from a copper surface to liquid helium is 0.75×104Wm-2K-1 at about 1, 000G, and it is identical to the calculated value based on the normal zone propagation velocity under the same condition.
A helium blower that can pre-cool superconducting magnets with helium gas at a temperature of 80K was developed. The blower incorporates self-acting gas journal bearings, Rayleigh step-type thrust bearings, a variable-speed induction-motor drive, and a backward impeller. The performance of the blower was measured in a vacuum chamber containing a tank of liquid nitrogen and two heat exchangers. Tests results show that pressure ratio is more than 1.2 and adiabatic efficiency is 61% at design point (mass flow rate of 0.005kg/s and rotational speed of 130, 000rpm). Moreover, for testing bearing reliability, the blower shaft could be successfully driven at up to 148, 300rpm and a start-stop test was done more than one hundred times, but there were no troubles with the shaft or bearings.