A Large Scale High-Tc Superconducting Shield and Its Cryostat

Abstract

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.

A magnetic shielding capacity of the superconducting shield for an external magnetic fields reaching 1.8 mT was found. This sufficed for biomagnetic measurements. The critical current density distribution of the superconductor was evaluated using a magnetic field source formed by a pair of small opposing coils placed near the wall of the cryostat. The mean critical current density of the superconductor was about 450 A/cm². External noise was attenuated by 160 dB within the shielded space. This kept the external noises below the internal noise of the SQUID magnetometer.