This review paper outlines progress in the stabilization of superconducting magnets wound with alloy superconductors. Although the stabilization theories established in the 1960's have offered useful knowledge for understanding the phenomena and characteristics important to engineers, the manufacture of stable and reliable superconducting magnets remains elusive except fully stabilized ones. The paper also discusses previous leading themes in connection with the experimental results obtained by the author and his colleagues.
The flow characteristics of superfluid helium through the porous elements of a thermomechanical pump were experimentally studied placing special emphasis on the critical heat inputs which bound the ideal and turbulent flow regimes. The dependence of critical heat input on the temperature of superfluid helium and on the geometry of porous elements was investigated. It was found that temperature dependence arises from the density of the superfluid component of superfluid helium, the specific entropy. The flow characteristics of superfluid helium through the porous elements of a thermomechanical pump can be fully predicted by applying the critical heat input to the Gorter-Mellink mutual friction law. By combining the flow characteristics of superfluid helium through a phase separator with those through a porous element of a thermomechanical pump, a general description of superfluid helium flow through porous media is proposed in this study. This description suggests drawing of the flow characteristics of superfluid helium through porous media with various levels of permeability.
In this study gas displacement in a pulse tube is visualized with the use of a light porous ball. The ball was placed in a transparent pulse tube and its displacement recorded on a videotape. The diameter and weight of the ball were predetermined in order to maintain errors in measurement within 5%. This visualization method has following advantages: (1) Real pulsetube refrigerators, which have a sharp temperature gradient along the pulse tube, can be used for measurement; (2) long-term measurements are possible without the use of special means; and (3) the process doesn't require a great deal of skill. Disadvantages include: (1) It is inapplicable to basic pulse-tube refrigerators; and (2) the intricate flow stream out of the hot end of the tube is invisible. Experimental results verify that the ball displacement can approximatery visualize the gas displacement. An orifice pulse-tube refrigerator that utilizes air as the refrigerant was used for this experiment. Under the conditions of various valve-open angles, the gas displacement and phase shift in a double-inlet pulse-tube refrigerator were measured.