Recent progress in magnetic refrigeration has been reviewed. Adiabatic demagnetization refrigerators (ADR) are under development at NASA for large deployable reflectors and X-ray spectrometers. The refrigerators provide temperatures of 2K at 0.027W for 165s and 0.1K at 0.01W for 13h using gadolinium gallium garnet and paramagnetic salt crystals. The Gas-Gap heat switch is used for the isothermal portion of a Carnot cycle. Active magnetic regenerator (AMR) cycles have the advantage of the Carnot cycle for larger temperature span and refrigeration capacity, in particular, above 20K. A test apparatus of the AMR demonstrated that the refrigeration capacity of 1.5-3.5W was obtainable at the temperature spans of 13-25K with a Carnot efficiency of 42%. Practical applications of magnetic refrigeration are also described.
Quantum nucleation is considered a general mechanism to explain first-order phase transition at sufficiently low temperatures. In order to obtain unambiguous evidence of the phenomenon, many attempts have been made both experimentally and theoretically. After giving a general survey of investigations with liquid helium, we present the results of our experimental studies related to the nucleation phenomenon in supersaturated 3He-4He mixtures of the dilute phase.
Aluminum stabilizer, which is used in composite superconductors, in external fields at 4.2K prvides a remarkable enhancement of magnetoresistance as compared to predictions by conventional theory. In order to study the cause of this anomalous behavior, we measured the electric resistivities of copper-clad aluminum conductors having rectangular cross-sections in various magnetic fields at 4.2K. The measured values depend on the magnitude and orientation of the applied magnetic field with respect to the sample; the resistivity being several times larger for a parallel field. We calculated the current distributions in the conductors by solving Maxwell's equations, taking the Hall effect into consideration for evaluating resistivities. The predicted resistivities were consistent with our experimental results, leading us to conclude that the current caused by the Hall effect is the main origin of the anomalous magnetoresistance and its anisotropy. Finally, we discussed a method to reduce the anomalous resistivity.
Wire motion in a superconducting coil has been simulated dynamically by means of solving the equation of motion of each winding with time in order to make quantitative analyses of the relationships between the winding conditions and the quench current of a superconducting coil, especially between winding tension and quench current. Using this method, each wire position, wire velocity and frictional heating and stress in the coil during excitation could be calculated dynamically. It was found that not only quench current but also the position where quench initiates changed with the winding tension and frictional coefficient. Wire motion after quench was also calculated. It was clarified that, due to the friction force, the wire did not return to its initial position resulting in misalignment of the wire. It was found that the method developed is useful for revealing the characteristics of a superconducting coil during operation. The method can be applied to the design of stable super conducting magnets.
The heat transfer characteristics of a fountain effect pump in a He II flow loop have been investi gated by experiments and numerical analyses. Temperatures at several points in the loop, pressure difference between both sides of the porous plug and mass flow rate of He II were measured in the experiments. The fountain effect is generated by heat-input at one side of the porous plug. In a closed-loop fountain effect pump, the temperature on the opposite side of the plug also increases even though heat is not input to that side. In this study, we show that a part of the heat-input is transported to the upstream side of the porous plug due to supercompnent flow (zero entropy flow) in the plug. Taking account of the distribution of the heat-input, heat transfer and the pressure drop in the loop and fountain effect at the plug, a model is proposed. From analyses based on the model, the temperature distribution in the loop, pressure difference between both sides of the porous plug and He II mass flow rate can be estimated accurately in the case of laminar flow in the plug.