There are many dangerous processes in cryogenic experiment. Cryogenic operators must understand cryogen characteristics and the structure and functions of basic cryogenic equipments, such as storage dewars, transfer tubes and compressed gas cylinders and some other useful instruments, such as level meters, manometers, flow meters, which are installed in the cryostat for safety and effective handling of cryogen. Precooling using liquid nitrogen is effective for liquid helium saving. But, according to circumstances, cool-down by making good use of gaseous helium would be a better selection from the view point of safety and experimental efficiency.
Superconducting wires in a cable-in-coduit conductor are plated with chromium for reducing AC loss. However, the wire will suffer from fretting motion with a very small slip amplitude by the repeated pulse operation of the coil, which would damage the plated layer. In this paper, tribological properties, i.e., frictional and wear properties, of Cr-plated (NbTi)3SN wire were examined under a crossed-cylinder contact at mainly 4.2K in liquid helium. Coefficient of friction of Cr-plated wire ranged between 0.2 and 0.5, which was lower than that of copper. Wear volume was small at 4.2K, compared with those at 293 and 77K. The significant damage was the plastic deformation of the plated layer in the wear scar at 4.2K. However, the plated layer was broken and separated at the boundary of the wear scar. The cracks propagated from the circumferential cracks existing before the test. The particles formed were rectangular with a length of about 20μm. When the plated layer was broken, the coefficient of friction increased to that higher than unity.
Observation on the logarithmic decay of the levitation forces is important not only to evaluate the bearing performance, but also to clarify the influence of the flux creep on the force relaxation in YBCO superconductors. The study on magnetic bearings is pursued on the superconductors from the view of industrial-scale application. The levitation force and the magnetic flux density in the bearing gap are measured on two disc-type superconductors as a function of time at 77K. The measured results clearly show strong dependence of the loading hysteresis on levitational force relaxation. On applying the increased load on virgin state of the superconducting bearing, the gap length logarithmically decays and the absolute value is relatively large. Almost no change, however, is observed in the levitation forces when the load changes backward slightly from the last load value. Long term stability of the superconducting bearing will much more be improved if such an operational condition is adopted in practical use.
In order to measure the minimum propagating current of the composite superconductor, heater wire is usually wound around the superconductor to heat it up and make a small normal zone. Usually this heater wire is thermally insulated from helium by epoxy resin and these thermal insulation seems to affect on the steady state stability of the superconductor. A simple graphical scheme have been presented for incorporating heat transfer and heat generation characteristics into calculation of the stability of superconductive composite conductors with local thermal insulation, modifying the equal area method presented by Maddock et al. The effects of the local thermal insulation on the steady state stability of composite superconductors are discussed by this graphical scheme. It is shown that the minimum propagating current of the composite superconductor with the local thermal insulation is the same as that of the conductor without the thermal insulation but there are the thermal equilibrium current below the minimum propagating current.