Teruo Kato

Mechanical Properties of Organic Insulators for Superconducting Magnets after Low Temperature Irradiation

Organic insulators will be used as electric insulators and thermal insulators in superconducting magnets since ductile materials are required for coil windings. When superconducting magnets are used in fusion reactors, the magnet components will be exposed to fast neutron and gamma irradiation produced by fusion reaction. The magnet construction also undergoes high mechanical stresses produced by electromagnetic force. Therefore, it is necessary to study the effect of low temperature irradiation on the physical and mechanical properties of insulators at cryogenic temperature. Very recently, some experiments which study the effect of low temperature irradiation on organic insulators have been reported 1~10. Effects of low temperature irradiation on the electrical properties, such as electrical resistivity, dielectric constant and electrical breakdown, were relatively small even when the mechanical properties showed drastic changes by irradiation2,10. Therefore, measurements of the mechanical strength after low temperature irradiation are important. Plastics, for example epoxies, are extremely sensitive to irradiation. However, by using glass fiber to reinforce plastics it has been possible to improve the mechanical properties.

Fabrication and Test of a 60‐kA HTS Current Lead for Fusion Magnet System

A 60‐kA HTS current lead was fabricated and tested in the frame of the R&D work for large fusion magnets such as the ITER magnet. The design of the current lead was characterized by its safe thermal protection for the current decay of about ten seconds after quench as well as lower electrical power consumption for its cryogenic equipment. The 60‐kA current lead is composed of a lower temperature HTS part using HTS and a high temperature copper part using a conventional copper cable. The HTS part consists of 48 HTS units installed in the cylindrical array into the grooves provided on the outer surface of a stainless steel cylinder with a diameter of 146 mm. The unit was composed of six Bi2223/Ag‐10at%Au tapes and its cross‐sectional dimension is 6.5 mm × 2.7 mm. The stainless steel cylinder and Ag alloy sheath have a share in thermal protection for quench. The current lead was tested under the cooling condition that the bottom of the HTS part was immersed into 4.2 K liquid helium, and the bottom of the cop...

The influence of electron irradiation on superconductivity of YBa2Cu3Ox

Abstract The change of superconductive property by electron irradiation was studied in YBa 2 Cu 3 O x , x =6.9, 6.6 and 6.4. By the irradiation the transition temperatures measured by electrical resistivity shift from 87 to 83 K, 58 to 36 K, and 22 to below 10 K for x =6.9, 6.6 and 6.4, respectively. The critical current densities increase in x =6.9 after irradiation, but a slight decrease is found in both x =6.6 and 6.4. Time-dependent magnetic relaxations are measured and activation energies for flux motion are calculated as a function of temperature. The increase of J c and activation energies for x =6.9 are attributed to the small defects produced by electron irradiation.

THERMAL STABILITY OF HELIUM BUBBLES IN NANOCRYATALLINE GOLD PREPARED BY GAS DEPOSITION METHOD

Specimens of nanocrystalline pure gold were prepared by the gas deposition method. The formation of helium bubbles in the specimens and their annealing behavior were studied in order to confirm their effect on thermal stabilities of grain size and mechanical properties. The specimens with 10-25nm mean grain size were analyzed by transmission electron microscopy and X-ray diffraction methods. Spherical helium bubbles, about 5nm in diameter, were formed, the same as in the case of helium ion implantation. After annealing at 573K for 1h in vacuum, most of the bubbles had not disappeared and some were trapped at the grain boundaries. Larger numbers of bubbles were trapped at grain boundaries in the specimens with high thermal stability than for low thermal stability specimens. Helium bubbles trapped at grain boundaries can be considered as local barriers to grain growth and to contribute to thermal stability of mechanical properties.

X-ray diffuse scattering of quenched copper single crystals

Abstract X-ray diffuse scattering was measured for quenched copper. The diffuse scattering near the (111) and (222) reflections was observed at room temperature for as quenched and successively annealed samples from 200 to 800°C. A sharp and strong subpeak accompanied by a thermal diffuse scattering was appeared on a smaller angle side of the (111) Bragg angle. The subpeak was disappeared after annealing at 400°C. A characteristic feature of this scattering is from vacancy type dislocation loops, of which the mean radius is estimated to be about 1 nm, associated with a caustic scattering.

Electron Irradiation Effects on Y1Ba2Cu3Oy Superconductors

3 MeV electton irradiations are performed in the broad range of electton dose, up to 2.0 x 1018 e- /cm2 , to investigate the effect of irradiation-induced defects on the superconducting transition for oxygen-controlled specimens. The electron irradiations depress Tc at the rate of 0.5 ∿ 3.5 K/1018 e- /cm2 depending on the specimen treatments and the probe currents. A specimen with the lower contents of oxygens is more sensitive to the irradiation. The detailed analysis on the transition region reveals the complicated behaviors. The three typically different phenomena are found in the three kinds of specimens(y=6.6, 6.7 ∿ 6.8, 6.8 ∿ 6.9). In a specimen with y=6.8 ∿ 6.9, the two stage transition is observed, and only the transition at the lower temperature is shifted to the lower temperature region with increasing probe currents. The electron irradiations do not influence on this feature. In a specimen with y=6.7 ∿ 6.8, the single transition is observed. The single transition is shifted to the lower temperature region by the irradiation without changing the sharpness of the transition. Contrary to the above two cases, in a specimen with y=6.6, the two stage broad transition is found, and the transition at the lower temperature depends on the probe current. The irradiation affects both of the peaks. With the increase of the electron dose, the transition at the higher temperature is broadened, but the transition at the lower temperature is sharpened with the shift to the lower temperature region.

Superconductivity of Bi-Sr-Ca-Cu-O Prepared from CuO or Cu 2 O, and Doped with Ag Ions

Superconductivity in Bi compounds is examined in samples prepared from CuO and Cu 2 O powders, and also in samples doped with Ag. In BiSrCaCu x O (x=2,3,4,5) prepared from CuO powder, T c decreases with an increase of Cu content, while in samples prepared from Cu 2 O the opposite trend, an increase of T c with Cu content, is observed. From Ag doping, BiSrCaCu 2 O y +Ag x (x=0, 1.0, 1.5, 2.0), the volume ratio of high to low T c phase is found to be a maximum at a Ag content between 1.5 and 2.0. From these experiments, a strong correlation between the high T c phase and Cu + valence state is suggested.