Kumiko Imai

A Large Scale High-Tc Superconducting Shield and Its Cryostat

We have developed a cryostat for a large scale high-Tc superconducting shield enclosing a human body to measure biomagnetism. The cryostat has an interior space of ϕ 0.8 m × L2.8 m and cools a high-Tc superconducting shield ϕ 0.87 m × L2.4 m. The cryostat is made of non-magnetic materials to avoid internal magnetic noise. It also includes other provisions to achieve a very low magnetic noise environment. We investigated the optimal design of the radiation shields for the cryostats using liquid nitrogen and concluded that the one radiation shield has sufficient performance for the thermal insulation. We designed the liquid nitrogen reservoir to keep the superconductor at 77.3 K until the reservoir is nearly emptied.

High Sensitivity SQUID Damage Detector in a High-Tc Superconducting Shield

A non-destructive evaluation (NDE) system for material damages of metals has been developed. The system enables very low magnetic field measurements by using a SQUID magnetometry system in a high-Tc superconducting shield. The shield has large internal dimensions of D0.87 m × L2.4 m, so that large specimen of up to 50 cm × 10 cm can be inspected. Size and location of magnetization from various material damages were contact less estimated with a Nb-based multi-channel SQUID magnetometer system which was placed directly in the shield.

Relaxation in High Transport Current Bi-Sr-Ca-Cu-O System

The relaxation of a persistent current circulating in a toroid made of Bi2Sr2Ca1Cu2O8+x superconductor has been measured. The relaxation is described of the form of t-a and is explained by a equation deduced from a power-law characteristic of the I–V property, which is originated from a 2-dimensional coupled Josephson junction array. It is further revealed that the power-law can be applied to the superconductors over very wide ranges of voltages from 0.1mV to 0.1pV.

High Tc Superconducting Shielded Biomagnetometer System

Superconductors are expected to achieve perfect magnetic shielding, which will be useful for studies on neural activities of the brain especially in the lower frequency range down to DC. Operating in the environment of other noisy instruments and/or interference such as subways, cars, etc. will be available. Instead of gradiometers, higher sensitivity magnetometers are effective in the low noise space, which will allow detections of signals from deeper and/or weaker magnetic sources with a higher S/N ratio.

Increased Critical Temperature in Bi-Sr-Ca-Cu-O Oxide System

The change in critical temperature Tc of Bi2Sr9CaCu2Ox has been investigated. Tc changes with oxygen content which is determined by annealing condition. The critical temperature reaches a maximum value of 95K at an oxygen content about X=8.25.

A Deterioration Mechanism on Critical Current Densities of High Temperature Superconductors

Deterioration of oxide superconductors has been investigated by evaluating the critical current densities at 77K and the resistivity at room temperature. The oxide superconductors were exposed to moisture or heat cycle between 77K and room temperature. The experimental results indicate that the external substance penetrates into the superconductors and brings about chemical reactions to form non-superconductive products so that it involves volume expansion at the site to cause micro-cracks. An analysis of the relation between the critical current density and the conductivity has been carried out and we have estimated the mechanism of the deterioration by the analysis.