Takashi Inaguchi

Numerical Simulations of Interaction between Stellar Wind and Interstellar Medium

Hydrodynamic interaction between supersonic spherical wind emitted from an astronomical object and a uniform streaming flow is simulated numerically assuming the flow to be axisymmetric, adiabatic and inviscid. Examples of such a phenomenon are a comet in the solar wind, and the solar wind or a stellar wind in an interstellar medium. Three cases of the incident flow, i.e., subsonic, supersonic and hypersonic flow, are considered. Discontinuities in the flow, i.e., a bow shock, a contact surface, an inner shock, a Mach disc and a slip surface are identified. The contact surface and the slip surface are found to be Kelvin- Helmholtz unstable. Other instabilities occurring near the stagnation region and the inner shock are also found.

Design of a large current-type fault current limiter with YBCO films

Superconducting fault current limiters (FCLs) with YBCO films are under development in a national project in Japan. In this project, Mitsubishi Electric Corporation is in charge of the development of a large current-type FCL with a target capacity of 1 kA/200 V. To develop the FCL, numerical simulations on the combined thermal and electrical behavior of the device have been carried out considering inhomogeneity. The thermal stress of the FCL has also been studied by FEM using the calculated temperature profile. Here, the assumed size of a YBCO film on a sapphire substrate with a metal shunt layer is 30 mm in width and 100 mm in length. The critical current (I c ) of the film is assumed as 210 A +20% or -20%. The main results are as follows: (1) 16 films are arranged as a series of two stacks each consisting of eight films in parallel. (2) A polygonal arrangement is adopted for the YBCO films to obtain uniform current distribution. (3) The maximum temperature is 150 K and the thermal sink on the film effectively reduces temperature increase. (4) The thermal stress caused by the temperature gradient is <10 MPa. Therefore, mechanical breakdown is not a serious problem with this device.

Helium Liquefaction by a Gifford-Mcmahon Cycle Cryocooler

This paper describes the principal design features and performance test results of a three-stage Gifford-McMahon cycle cryocooler by which the authors have achieved no-load temperature of 3.3 K and succeeded in helium liquefaction. The key point of the success is the selection of GdχEr1-χRh compounds as a regenerator material. The refrigeration capacity was 20 mW at 4.2 K and the helium liquefaction rate was 10 cm3/h. This is the first success that a Gifford-McMahon cycle cryocooler has liquefied helium without precooling.

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Abstract Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current ( I c ). (2) The current in a superconducting film was uniform when the current was much higher than I c . The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I c . (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.

Helium liquefaction by a Gifford-McMahon cycle cryogenic refrigerator

This article describes the principal design features and performance test results of a three‐stage Gifford–McMahon cycle cryogenic refrigerator by which the authors have achieved a no‐load temperature of 3.3 K and succeeded in helium liquefaction. The refrigeration capacity was 20 mW at 4.2 K and the helium liquefaction rate was 10 cm3/h. The key point of the success is the selection of GdxEr1−xRh compounds as a regenerator material. This is the first time that a Gifford–McMahon refrigerator has liquefied helium without the use of a Joule–Thomson stage.

4K Three-Stage Gifford-McMahon Cycle Refrigerator for MRI Magnet

The authors have succeeded in the development of a 4K Gifford- McMahon cycle cryogenic refrigerator capable of eliminating the evaporation of helium from a superconducting MRI magnet. We installed this refrigerator in a superconducting MRI magnet and established that it actually eliminated evaporation and was capable of stable operation.

Thermofluid Analysis of Shuttle Loss on Regenerative Cryocoolers

Shuttle loss is one of major losses of regenerative cryocoolers. In order to improve the efficiency of the cryocoolers, we have to estimate the shuttle loss precisely and reduce it. However, it has been difficult to calculate shuttle loss precisely because the hydrodynamic behavior of the working fluid has not been considered in the analyses and the temperature gradient of the working fluid was given as linear. Therefore, using a general purpose thermofluid analysis software STAR-CD1, we made a model in which the hydrodynamic behavior of the working fluid was considered. By using this model we did a thermofluid analysis which calculated the movement of the working fluid and the temperature change. From this, we calculated the shuttle loss. As a result of these calculations, the preciseness of the calculation model was confirmed by comparing with previous analyses. This means that acceptable analysis is possible with general purpose software, and various models. This paper reports on the analysis model and the results.

Cryogenic and electrical performance at a factory test of a rotor for 70 MW class slow-response excitation type superconducting generator

70 MW class superconducting generators are under development as a national project in Japan. This is an eleven-year program which commenced in 1988. The manufacturing of the slow response excitation type rotor was completed at the beginning of 1996, and performance test of the rotor was carried out in the factory at the beginning of 1997. The factory test has been completed successfully. This paper describes the factory test results of the cryogenic performance and the electrical performance.

Effects of Thermal Conductance in the Cooling Stage of a 4K-GM Refrigerator on Refrigeration Capacity

This paper describes the effects of thermal conductance in the cooling stage of a 4K-GM refrigerator on its refrigeration capacity. Calculation of the temperature difference between the gas and cooling stage in the expansion space of a 4K-GM refrigerator has shown that the thermal conductance between the gas and cooling stage is inadequate, and the heat transfer loss is too large to be ignored. Hence, we have made improvements in the shape and material of the cooling stage to increase thermal conductance, and have been successful in increasing refrigeration capacity at 4.2 K from 2.2 W to 3.0 W.

Dependence of contact weld force on arc energy supplied to contact surface in low voltage switches

This paper describes a mechanism of contact welding formed by bounce arc upon contact making, based on experiments, theoretic and analytic study. We measured the weld force after every contact making in a low-voltage condition (200 V-AC). The arc current was set to 140 to 500 A by using a reactor load, the arc duration time was within 0.22 to 2.37 ms, and the contact material was pure silver (99.99 %). In this condition, we clarified that the weld force increases depending on arc energy supplied to the contact surface. We also measured the melted area on contact surface and clarified the area dependence on the arc energy. Considering that the tensile strength of silver is 170 N/mm2, the measured melted area is too large as a welded area. Then we found out that the real welded area corresponds to about 25 % of the melted area. As a conclusion, we suggest that a vaporized area on the contact surface may not be involved in contact welding, from the analytical simulation of thermal conduction in the contact model. The real welded area is the region whose temperature is higher than the melting temperature and that remains without evaporation.