Takefumi Horiuchi

Cryogenic properties of composite materials

For the support materials of superconducting magnet systems, specifically for the transportable cryostat used for nuclear magnetic resonance spectroscopy, three kinds of organic composite materials were investigated: carbon-fibre-reinforced plastics (CFRP), aluminium-oxide-fibre-reinforced plastics, and silicon-carbide-fibre-reinforced plastics. Considering the temperature dependence of the thermal conductivities of glass-fibre-reinforced plastics and CFRP, the authors recommend the use of a combination of these two materials. The thermal and mechanical properties of these organic composite materials and support-system requirements for superconducting magnet systems are presented.

A New Method of Photothermal Displacement Measurement by Laser Interferometric Probe -Its Mechanism and Applications to Evaluation of Lattice Damage in Semiconductors

A new, highly sensitive technique for measuring photothermal displacement using a laser heterodyne interferometric probe has been developed. This technique is based on the detection of phase changes in the probe beam and is very sensitive to the presence of lattice damage in semiconductors. It has been found that the phase change is caused by the thermal expansion of a sample surface induced by absorption of a modulated pump beam. The displacements of metals and semiconductors measured by this technique coincided with the results predicted by a thermal diffusion model. These displacements simply depended upon the ratio of the thermal expansion coefficient to the thermal conductlvity of a sample.

Mechanical Properties of High Manganese Steels at Cryogenic Temperatures

Structural steels for fusion magnet applications at any ambient temperatures have hitherto been austenitic stainless steels, such as SUS 304L and 316L. Although these steels have good fracture toughness even at liquid helium temperature, they also have several drawbacks: low yield strength and istability of the austenitic phase against deformation.1

ANALYSIS OF LATTICE DEFECTS INDUCED BY ION IMPLANTATION WITH PHOTO-ACOUSTIC DISPLACEMENT MEASUREMENTS

Subsurface lattice defects in silicon induced by ion implantation were studied by the use of the photo‐acoustic displacement (PAD) method based on the sensitive measurements of the surface displacement due to the absorption of laser‐light energy. A definite correlation between PAD and displaced atoms density (DAD) was found because PAD reflects the change in thermal conductivity associated with the net amount of displaced atoms in the crystal lattice beneath the surface. According to the linear dependence of 1/PAD on DAD, defects below a DAD of 1014/cm2 (corresponding to implant doses of 2×1011, 8×1010, and 6×1010 ions/cm2 for 100 keV B+, P+, and As+, respectively) were concluded to be point defects. After the DAD reached 1014/cm2, the PAD showed a gentle increase, and this can be attributed to the growth of point‐defect clusters. A marked dependence of the PAD on the DAD was not observed beyond a DAD of 1016/cm2. In this region, the presence of an amorphous layer was observed by cross‐sectional transmiss...

Training Behavior and Acoustic Emission of Superconducting Magnet

Recently, superconducting magnets have been increasing in size and stored energy. For stable operation of such large magnets, it is necessary to have monitoring systems to detect disturbances which would drive the magnet normal.

THE DEVELOPMENT OF A NITROGEN-STRENGTHENED HIGH-MANGANESE AUSTENITIC STAINLESS STEEL FOR A LARGE SUPERCONDUCTING MAGNET

Fusion Experimental Reactor, a big project following JT-60, needs a very large superconducting toroidal field magnet, of about 10-m diameter with a 12-T magnetic field. It is anticipated that the structural materials for the magnet should have a yield strength higher than 1,200 MPa and a fracture toughness exceeding 200 \( {\text{MPa}}\sqrt {\text{m}} \) at 4K in a very thick plate1.

Effect of cold working on precipitation behaviour in superconducting Ti-Nb alloys

Abstract The precipitation behaviour in thin foils and wire specimens of Ti-Nb alloys has been investigated mainly by means of small-angle X-ray scattering measurement but also by tramsmission electron microscopy. The small-angle diffuse scattering was observed in the later stage of ageing and it was concluded that α particles formed. The change of relative integrated intensity and Guinier radius was measured during ageing. The integrated intensity in the alloys aged after cold working was rather larger than that in the alloy without cold working, but in contrast the Guinier radius became smaller. The cold working was found to accelerate the precipitation of the α phase. The relative interparticle distance became constant after short term ageing for all specimens with various rates of cold working. A commercial quaternary alloy, Ti-27 at.% Nb-5 at.% Zr-5 at.% Ta, was also examined. Its precipitation behaviour was similar to that of the binary alloys and it produced the largest integrated intensity in smal...

Effects of ambient gas on photo‐acoustic displacement measurement by laser interferometric probe

The effect of gas adjacent to the sample surface on the photo‐acoustic displacement (PAD) measurement was studied by using an extremely sensitive laser interferometric probe with a sensitivity of 0.1 picometers. For silicon, the PAD signal measured at atmospheric pressure increased about 18% as compared to the signal obtained in vacuum, and varied by less than 0.7% for a change in pressure of 5% around 1 atm. It is shown, by a simple theoretical model, that the variation of the PAD is caused by a change in refractive index of the gas and the real PAD can be accurately obtained by correcting this effect.

Cryogenic Fe-Mn Austenitic Steels

It is taken for granted that structural materials used for superconducting magnets of a fusion reactor must combine strength, toughness and fatigue resistance especially in case of welded structures at cryogenic temperatures. Austenitic Fe-Mn alloys were expected to be promising candidates to meet the above requirements. However, they proved to have some problems to be overcome in terms of cryogenic brittle fractures. To optimize the mechanical properties, chromium, nickel, nitrogen and other elements were added to Fe-Mn alloy. Consequently, 22Mn-13Cr-5Ni steel has been developed as the candidate steel. Optimum manufacturing conditions have been established and the steel has been tested at cryogenic temperatures. The results show that the steel maintains satisfactory strength of more than 1.2 GPa and fracture toughness of more than 200 MPa\(\sqrt m \) at 4 K.

Role of α precipitates in flux pinning in a superconducting Ti-Nb-Ta-Zr quaternary alloy

Abstract The precipitation behaviour of the a phase in a Ti-27 at. %Nb-6 at. %Ta-6 at. %Zralloy has been investigated mainly by means of small-angle X-ray scattering measurements, by which the average size and number density of a precipitates were determined. The alloy was isothermally aged at 643 K after cold-rolling to various thicknesses. During ageing the average size of α precipitates increased but the number density decreased. The effect of cold-rolling was to increase the volume fraction of a precipitates. The superconducting critical current density was measured for the same specimens after the metallographical investigation. The specific pinning force produced by α precipitates, which corresponds to the global pinning force density divided by the number density of precipitates, was found to be proportional to the cube of the particle radius. It was found that the global pinning force density can be described using a scaling rule in terms of the volume fraction of α precipitates and the reduced ma...