We can detect the indication of failures and prevent equipment from becoming damaged through visually monitoring the state inside cryogenic equipment. We investigated use of a flexible fiberscope as an endoscope. We also investigated use of a web camera owing to its low cost. We selected a flexible fiberscope without the function of remote control because of the lower heat invasion and higher voltage insulation. We set the fiberscope inside the cryogenic equipment using a rubber stopper to hold the vacuum. We confirmed that the inside was observable even for objects cooled to approximately 20 K. As for the web camera, we confirmed that it could be satisfactorily used adjusting the temperature to approximately 200 K under a high magnetic field of 5 T.
The Great East Japan Earthquake, which occurred on March 11, 2011, damaged a superconducting NMR magnet installed in the National Institute for Materials Science (NIMS) in Tsukuba, Japan. We have developed an instrument to visualize a material cooled in cryogenics to repair the magnet. A temperature controlled video scope works well in a helium gas environment and in the vicinity of a liquid helium surface. The instrument enables us to monitor the situation of a material in a low temperature state. Utilizing the instrument, we have succeeded in recovering the electric contact of an electric socket of a superconducting magnet covered with the contaminant of solid air. The socket was dug out using a drill while conducting monitoring using the instrument. Behaviors of solid air in cryogenic instruments for an injection of a helium gas were also visualized.
The Nb3Sn strands used for the fusion reactor of the ITER are made up of a typical composite material consisting of a brittle superconducting intermetallic compound. Thermally induced strain is inevitably generated in the composite due to different coefficients of thermal expansion and different moduli of elasticity among the constituent components. In order to evaluate the thermal strain exerted on superconductive filaments quantitatively, local strain measurements were carried out during heating and cooling using quantum beams. The stress versus strain curves of the Nb3Sn strand showed a typical elasto-plastic behavior, which could be numerically evaluated on the basis of the rule of mixture. The local strain exerted on superconductive filaments along the axial direction was compressive at room temperature and tensile at high temperatures. Recently, a numerical method to evaluate temperature dependence was proposed. The present paper reconfirms that the temperature dependence of the thermal strain can be reproduced well using the proposed numerical calculation.