The types of the oscillations of very low temperature circulation systems are reviewed. Despite the considerable success of the techniques which were developed in '60s to analyse the dynamical and transient behaviors of cryogenic systems, the incompetence thereof to predict exactly the modes of oscilllations and instabilities of the He systems operated around the critical point now becomes gradually apparent. The chief aim of this review is to clarify where they fail and suggest how we can save them. The evidences which show the abnormally swift diffusion of enthalpy along supercritical He channels are presented. The relationship between oscillation modes and inhomogenuities of temperature, density and velocity of He coolant is stressed.
This paper outlines the cooling by He II for the cryogenic stabilization of superconducting magnets. He II as a coolant is provided in order to achieve one or both two chief aims: 1) the enhancement of a generable magnetic field caused by the increment in the critical current density and in the upper critical field, and 2) the promotion of the stabilization attributed to improvements of thermal properties of the liquid helium. These will compensate the high cost of the refrigeration and the complexity of the dewar structure, which are encountered on lowering the temperature. And these will play a very important role in big projects which equip superconducting magnets with intensive magnetic fields such as toroidal field magnets of a fusion reactor. In such projects, compact-size magnets are particularly desirable because they fit into a limited space, leaving enough space for the reinforced structural materials. Only He II can attain this objective without reducing the stability of superconducting magnets. Furthermore, in a temperature region of He II more intensive magnetic fields than 10 Tesla can be brought about by the use of a ductile superconducting Nb-Ti alloy instead of a brittle intermetallic compound. In the text, we begin with a study on the characteristics of the superconductor in the temperature region of He II as compared with those at 4.2K. Next, we make reference to the superiorities of the thermal properties (heat transfer, heat transport and heat capacity) of He II in comparison with the normal boiling liquid helium at 4.2K. The transient effect of heat transfer and of the heat transport in He II is touched upon from the view point of stabilization against an instantaneous disturbance.
Theoretical studies of variations in the critical current Ic under bending conditions are carried. In Nb3Sn conductors produced by solid state diffusion process, the matrix (bronze) is subjected to a tensile stress, because bronze contracts stronger than Nb or Nb3Sn during cooling from the diffusion temperature to 4K. This tensile prestrain makes the stress-strain curve for the bronze be unbalanced about the origin of the σ-ε coordinates upon bending. Thus, during bending, the bronze on the tension side will reach the yield strain earlier than will the bronze on the compression side. This results in shifting of the neutral axis from the center of the conductor toward the center of curvature. The amount of the shiftng are theoretically calculated. Using the results, the critical current dependence on the bending strain and the maximum real tensile strains in Nb3Sn in bending test are estimated. Those results are compared with the experimental results and they are agreed each other well. It is also shown that the real tensile strain becomes larger than the simply added strain of the tensile one and the bending one when both strain work together.